Instructor’s Manual
Table of Contents
for
Biology 10e by Sylvia Mader
1
A View of Life
PART I
THE CELL
2
3
4
5
6
7
8
Basic Chemistry
The Chemistry of Organic Molecules
Cell Structure and Function
Membrane Structure and Function
Metabolism: Energy and Enzymes
Photosynthesis
Cellular Respiration
PART II
GENETIC BASIS OF LIFE
9
10
11
12
13
14
The Cell Cycle and Cellular Reproduction
Meiosis and Sexual Reproduction
Mendelian Patterns of Inheritance
Molecular Biology of the Gene
Regulation of Gene Activity
Biotechnology and Genomics
PART III
EVOLUTION
15
16
17
18
19
Darwin and Evolution
How Populations Evolve
Speciation and Macroevolution
Origin and History of Life
Systematics and Phylogeny
PART IV
MICROBIOLOGY AND EVOLUTION
20
21
22
Viruses, Bacteria, and Archaea
Protist Evolution and Diversity
Evolution and Diversity of Fungi
PART V
PLANT EVOLUTION AND BIOLOGY
23
24
25
26
27
Plant Evolution and Diversity
Flowering Plants: Structure and Organization
Flowering Plants: Nutrition and Transport
Flowering Plants: Control of Growth and Responses
Flowering Plants: Reproduction
1
PART VI
ANIMAL EVOLUTION AND DIVERSITY
28
29
30
Invertebrate Evolution
Vertebrate Evolution
Human Evolution
PART VII COMPARATIVE ANIMAL BIOLOGY
31
32
33
34
35
36
37
38
39
40
41
42
Animal Organization and Homeostasis
Circulation and Cardiovascular Systems
Lymph Transport and Immunity
Digestive Systems and Nutrition
Respiratory Systems
Body Fluid Regulation and Excretory Systems
Neurons and Nervous Systems
Sense Organs
Locomotion and Support Systems
Hormones and Endocrine Systems
Reproductive Systems
Animal Development
PART VIII BEHAVIOR AND ECOLOGY
43
44
45
46
47
Animal Behavior
Population Ecology
Community and Ecosystem Ecology
Major Ecosystems of the Biosphere
Conservation of Biodiversity
2
The Instructor's Manual
The Instructor's Manual is designed to help you coordinate the various ancillaries and aids that accompany Biology.
The manual is aligned with the text. There is a chapter in the manual for each chapter in the text. Each chapter
includes the following:
The Chapter Outline
The concepts as presented in the text are outlined under each major heading in each chapter. The outline provides a
basis for an organized lecture or discussion class, and the outline format is kept direct and concise to help students
who have not developed good outlining skills to learn them. While it is critical that students meaningfully internalize
distinct biology concepts, biology majors must also organize this huge body of information as they pursue advanced
work and specialize. Note-taking in an organized manner is an important aspect of understanding the intellectual
content of biology. An instructor who presents material in a clear and logically organized manner helps to set the
student’s understanding of the chapter concepts. The importance of organizing biology concepts by outline cannot
be overstated.
Tenth Edition Changes
Significant changes between the ninth and tenth editions are described. This will assist you in revising your lecture
notes if you used the previous edition of this textbook.
Lecture Enrichment Ideas
Every chapter in this Instructor's Manual includes from 3–20 enrichment ideas. These provide ideas for discussions,
techniques to relate the concepts to students meaningfully within their realm of experiences, suggested questioning
strategies, and additional applications of the chapter concepts.
Critical Thinking Questions
Sample critical thinking questions are supplied for use in testing or to stimulate class discussion.
Acknowledgments
Sylvia Mader wrote the edition changes and the textbook chapter questions, and of course provided the basis on
which the chapter outline is condensed. It is therefore her voice that shows through most of the content outlines.
Margaret Horn worked closely to get the finished product coordinated with the text and ancillary materials and get it
to press.
3
1 INTRODUCTION
CHAPTER
1
A VIEW OF LIFE
The text opens with a description of the characteristics of life, followed by a discussion of the human species’
integration into the highly-diverse biosphere. Taxonomic classification, the system by which all organisms are
categorized, is discussed. The steps of the scientific method are outlined. Two specific types of scientific
experiments—the controlled study and the field study—are described in detail.
Chapter Outline
1.1 How to Define Life
A. Living Things Are Organized
1. Organization of living systems begins with atoms, which make up basic building blocks called
elements.
2. The cell is the basic structural and functional unit of all living things.
3. Different cells combine to make up tissues (e.g., myocardial tissue).
4. Tissues combine to make up an organ (e.g., the heart).
5. Specific organs work together as a system (e.g., the heart, arteries, veins, etc.).
6. Multicellular organisms (each an “individual” within a particular species) contain organ systems (e.g.,
cardiovascular, digestive, respiratory, etc.).
7. A species in a particular area (e.g., gray squirrels in a forest) constitutes a population.
8. Interacting populations in a particular area comprise a community.
9. A community plus its physical environment is an ecosystem.
10. The biosphere is comprised of regions of the Earth’s crust, waters, and atmosphere inhabited by
organisms.
11. Each level of organization is more complex than the level preceding it.
12. Each level of organization has emergent properties due to interactions between the parts making up the
whole; all emergent properties follow the laws of physics and chemistry.
B. Living Things Acquire Materials and Energy
1. Maintaining organization and conducting life-sustaining processes require an outside source of energy,
defined as the capacity to do “work.”
2. Metabolism is all the chemical reactions that occur in a cell.
3. The ultimate source of energy for nearly all life on Earth is the sun; plants and certain other organisms
convert solar energy into chemical energy by the process of photosynthesis.
4. All organisms must maintain a state of biological balance, or homeostasis. Temperature, moisture
level, pH, etc. must be maintained within the tolerance range of the organism. Organisms have intricate
feedback and control mechanisms to maintain homeostatic balance.
C. Living Things Respond
1. Living things interact with the environment and with other living things.
2. Response often results in movement of the organism (e.g., a plant bending toward the sun to capture
solar energy, a turtle withdrawing into its shell for safety, etc.).
3. Responses help ensure survival of the organism and allow the organism to carry out its biological
activities.
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4. The collective responses of an organism constitute the behavior of the organism.
D. Living Things Reproduce and Develop
1. Reproduction is the ability of every type of organism to give rise to another organism like itself.
2. Bacteria, protozoans, and other unicellular organisms simply split in two (binary fission).
3. Multicellular organisms often unite sperm and egg, each from a different individual, resulting in an
immature individual that develops into the adult.
4. The instructions for an organism’s organization and development are encoded in genes.
5. Genes are comprised of long molecules of DNA (deoxyribonucleic acid); DNA is the genetic code in
all living things.
E. Living Things Have Adaptations
1. Adaptations are modifications that make organisms suited to their way of life.
1.2 Evolution, the Unifying Concept of Biology
A. Organizing Diversity
1. Taxonomy is the discipline of identifying and grouping organisms according to certain rules.
2. Taxonomic classification changes as more is learned about living things, including the evolutionary
relationships between species.
3. From smaller (least inclusive) categories to larger (more inclusive), the sequence of classification
categories is: species, genus, family, order, class, phylum, kingdom, and domain.
4. The species within one genus share many specific characteristics and are the most closely related.
5. Species in the same kingdom share only general characteristics with one another.
B. Domains and Kingdoms
1. Biochemical evidence suggests that there are three domains: Bacteria, Archaea, and Eukarya.
2. The domains Bacteria and Archaea contain unicellular prokaryotes; organisms in the domain Eukarya
have a membrane-bound nucleus.
3. The prokaryotes are structurally simple but are metabolically complex.
4. Archaea can live in water devoid of oxygen, and are able to survive harsh environmental conditions
(temperatures, salinity, pH).
5. Bacteria are variously adapted to living almost anywhere (water, soil, atmosphere, in/on the human
body, etc.).
6. The domains Archaea and Bacteria are not yet categorized into kingdoms.
7. Eukarya contains four kingdoms: Protista, Fungi, Plantae, and Animalia.
8. Protists (kingdom Protista) range from unicellular forms to multicellular ones.
9. Plants (kingdom Plantae) are multicellular photosynthetic organisms.
10. Fungi (kingdom Fungi) are the molds and mushrooms.
11. Animals (kingdom Animalia) are multicellular organisms that ingest and process their food .
C. Scientific Name
1. A binomial name is a two-part scientific name: the genus (first word, capitalized) and the specific
epithet of a species (second word, not capitalized).
2. Binomial names are based on Latin and are used universally by biologists.
3. Either the genus name or the specific epithet name may be abbreviated.
D. Evolution is Common Decent with Modification
1. Natural selection is the process by which species become modified over time.
2. In natural selection, members of a species may inherit a genetic change that makes them better suited
to a particular environment.
3. These members would be more likely to produce higher numbers of surviving offspring.
1.3 How the Biosphere is Organized
A. Levels of Complexity
1. The biosphere is the zone of air, land, and water where organisms exist.
2. A population consists of all members of one species in a particular area.
5
3.
4.
B.
C.
1.4
A.
B.
C.
D.
A community consists of all of the local interacting populations.
An ecosystem includes all aspects of a living community and the physical environment (soil,
atmosphere, etc.).
5. Interactions between various food chains make up a food web.
6. Ecosystems are characterized by chemical cycling and energy flow.
7. Ecosystems stay in existence because of a constant input of solar energy and the ability of
photosynthetic organisms to absorb it.
The Human Population
1. The human population modifies existing ecosystems which can upset their natural nutrient cycles,
causing harm to human populations and disrupting the ecosystem’s natural energy flow.
Biodiversity
1. Two biologically diverse ecosystems, rain forests and coral reefs, are severely threatened by the
human population.
2. Destruction of healthy ecosystems has unintended effects including: loss of food, medicine, raw
materials, and extinction of organisms.
3. Biodiversity is the total number of species, their variable genes, and their ecosystems.
4. Extinction is the death of a species or larger group; perhaps 400 species become extinct every day.
5. The continued existence of the human species is dependant on the preservation of ecosystems and the
biosphere.
The Process of Science
Scientific Method
1. Biology is the scientific study of life, and it consists of many disciplines.
2. The scientific process differs from other ways of learning in that science follows the scientific
method, which is characterized by observation, development of a hypothesis, experimentation and
data collection, and forming a conclusion.
Observation
1. Scientists believe nature is orderly and measurable, and that natural laws (e.g., gravity) do not change
with time.
2. Natural events, called phenomena, can therefore be understood from observations.
3. Scientists also use the knowledge and experiences of other scientists to expand their understanding of
phenomena.
4. Chance alone can sometimes help a scientist get an idea (e.g., Alexander Fleming’s discovery of
penicillin).
Hypothesis
1. Inductive reasoning allows a person to combine isolated facts into a cohesive whole.
2. A scientist uses inductive reasoning to develop a possible explanation (a hypothesis) for a natural
event; the scientist presents the hypothesis as an actual statement.
3. Scientists only consider hypotheses that can be tested (i.e., moral and religious beliefs may not be
testable by the scientific method).
Experiments/Further Observations
1. Testing a hypothesis involves either conducting an experiment or making further observations.
2. Deductive reasoning involves “if, then” logic to make a prediction that the hypothesis can be
supported by experimentation.
3. An experimental design is proposed to test the hypothesis in a meaningful way.
4. An experiment should include a control group which goes through all the steps of an experiment but
lacks (or is not exposed to) the factor being tested.
5. Scientists may use a model (a representation of an actual object) in their experiments.
6. Results obtained from use of a model will remain a hypothesis in need of testing if it is impossible to
test the actual phenomenon.
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E. Data
1.
2.
3.
Data are the results of an experiment, and are observable and objective rather than subjective.
Data are often displayed in a graph or table.
Many studies rely on statistical data which, among other things, determines the probability of error in
the experiment.
F. Conclusion
1. Whether the data support or reject the hypothesis is the basis for the conclusion.
2. The conclusion of one experiment can lead to the hypothesis for another experiment.
3. Scientists report their findings in scientific journals so that their methodology and data are available to
other scientists.
4. The experiments and observations must be repeatable or the research is suspect.
G. Scientific Theory
1. The ultimate goal is to understand the natural world in scientific theories, which are speculative ideas
that join supported, related hypotheses, and are supported by a broad range of observations,
experiments, and data.
2. Some basic theories of biology are:
a. Cell: all organisms are made of cells.
b. Homeostasis: the internal environment of an organism stays relatively constant.
c. Gene: organisms contain coded information that dictates their form, function, and behavior.
d. Ecosystem: organisms are members of populations which interact with each other and the physical
environment.
e. Evolution: all living things have a common ancestor.
3. A principle or a law is a theory that is generally accepted by most scientists.
H. Using the Scientific Method
1. A controlled study ensures that the outcome is due to the experimental (independent) variable, the
factor being tested.
2. The result is called the responding (dependent) variable because it is due to the dependent variable.
3. The Experiment
a. Observation: Nitrogen fertilizer in the short run exchanges yield and increases food supplies.
b. Hypothesis: pigeon pea/winter wheat rotation will increase winter wheat production as well as or
better than the use of nitrogen fertilizer.
c. Prediction: wheat biomass following the growth of pigeon peas in the soil will surpass wheat
biomass following nitrogen fertilizer treatment.
d. Control group: winter wheat that receives no fertilizer.
e. Test groups: winter wheat treated with different levels of fertilizer; winter wheat grown in soil into
which pigeon pea plants had been tilled.
f. Environmental conditions and watering were identical in control and test groups.
g. Results: all test groups produced more biomass than control group, but high level of nitrogen
fertilizer produced more biomass than pigeon pea test group. Thus, hypothesis is not supported.
4. Continuing the Experiment
a. To test the hypothesis that pigeon pea residues will build up over time and will increase winter
wheat production compared to nitrogen fertilizer, the study is continued for another year.
b. The fertilizer-only treatment no longer exceeded biomass production with the use of pigeon peas;
biomass in the pigeon pea-treated test group was highest.
c. Conclusion: at the end of two years, the yield of winter wheat is better in the pigeon pea-treated
test group. Hypothesis supported.
d. Continuation of the study for another year showed that the soil was continuously improved by the
pigeon peas compared to the nitrogen fertilizer test groups.
e. Results were reported in a scientific journal.
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I.
J.
A Field Study
1. Hypothesis: aggression of the male mountain bluebird varies during the reproductive cycle.
2. Prediction: aggression will change after the nest is built, after the first egg is laid, and after hatching.
3. To test the hypothesis, a male bluebird model was placed near the nest while the male was gone and
observations were made upon his return.
4. Control: a model of a male robin placed near certain nests.
5. Results: resident male bluebirds did not bother the control model but were aggressive toward the
male bluebird model depending on the stage in the reproductive cycle.
6. Conclusion: hypothesis is supported.
7. Study was reported in scientific journal with evolutionary interpretation.
The Benefits and Limitations of Statistical Studies (Science Focus box)
1. Benefits of Statistical Studies
a. Benefits of statistical studies include: Basis for supporting null or alternative hypotheses, gaining
information and insights into problems scientists are trying to solve.
2. Limitations of Statistical Studies
a. One of the main limitations to statistical studies is that data collected supports correlations, not
causations.
b. There are always additional details that could be identified and quantified. There is no scientific
study that has collected the perfect quantity and quality of data.
Lecture Enrichment Ideas
Experience Base: [As teachers, we make assumptions about the common experiences shared with our students and
thus the meaningfulness of the vocabulary we use. Questions that solicit student feedback establish if those
experiences are adequate and the concepts are being understood. Students who relate their experiences and
understanding in turn help classmates learn the concepts.] Lecturers new to a college or university may wish to
confer with veteran teachers about the state’s high school biology and other science requirements, and the proportion
of students likely to come from farm or urban backgrounds. Most high school biology textbooks address basic
properties of life and the five kingdom system. Only the 1998 texts onward mention domains. Most entering college
undergraduates do not have genuine experience with open-ended and purposeful science research.
1.
People from all backgrounds and educational levels make distinctions between living and non-living things.
Even if they have not read the material in the text, students can be led through a common sense discussion of
“what is life?” List the common traits of living things: growth, reproduction, response to the environment,
metabolism, etc. Ask whether any one of these aspects alone distinguishes a living organism from a nonliving
substance. Is it necessary, for example, that every individual within a species be able to reproduce? Select
specific species from various kingdoms and describe some of the responses and some of the adaptations those
species have. Discuss how those responses and adaptations allow those species to survive in their environment.
2.
Scientists are continuing attempts to confirm whether there is or ever has been life on Mars. What phenomena
should they look for? What is required for us to know for sure that there is or was life on Mars?
3.
Read job descriptions for biologists (e.g., biochemist, anatomist, population ecologist, etc.) available from
current journals and newspapers and ask students what level of biological organization the scientist studies.
4.
List organisms with which students should be familiar; ask them to place the organisms into the correct
kingdom based on their known characteristics. List unfamiliar organisms and their characteristics; ask students
to determine the kingdom to which they belong.
5.
Discuss the evolutionary relatedness of mammals. Ask students to consider Australian marsupials that fit into
the various niches filled by other mammals in other parts of the world. Note how marsupials are more closely
related to one another but have diversified to fill many niches.
6.
Discuss the theory that proposes that all life is a result of the development from unicellular ancestors with the
same basic chemical structures and metabolism.
8
7.
Discuss how organisms diversify due to the effects of mutation and selection by the environment.
8.
Caution students not to confuse use of the terms “name,” “identify,” and “classify.” “Classify” only involves
grouping. Only a specialist who describes and publishes a new species “names” the species. In most cases, we
are merely “identifying” organisms that are already known to science, originally “named” by taxonomists and
“classified” by systematists.
9.
Ask students to design an experiment, following the scientific method, in which they test a weight loss drug.
Ensure that the experiments have the proper controls, are conducted logically and realistically, that the data
(made up by the instructor for the sake of this experiment) are interpreted correctly, and that a proper
conclusion is drawn.
10. Ask students for examples of scientific method in their everyday lives, such as fixing dinner, determining how
to dress for the day’s weather or activities, finding their way around a strange area, or dealing with a
malfunctioning car.
11. Have students search a week’s newspapers for examples of using the scientific method in the news—such as
testing consumer goods or reports on medical research—and discuss them in class. Bring in a tabloid newspaper
making fabulous claims and discuss why it does not fulfill scientific standards.
12. Discuss the difference between scientific observations of the natural world and superstitions such as those
associated with Friday the thirteenth and black cats. Scientific examination of examples involving water
dousing, spontaneous human combustion, crop circles, and other modern misbeliefs are given in the Skeptic and
Skeptical Inquirer magazines.
13. Discuss why it is possible to prove a hypothesis or theory false but not to prove it absolutely true. (This Karl
Popper concept of “falsifiability” has limited usage in science and is not applicable in all areas of biology.
However, deep discussion of this concept is best left to upper level philosophy of science courses where the
students will have had some actual direct experiences with genuine research.)
14. Ask students if they believe the world is spherical. Have them cite evidence to support their belief. However,
unless they have flown in the Concorde, they have no direct observational evidence to prove that the Earth is in
fact spherical. Emphasize the point that even though they have no direct, personal evidence that fits the
spherical Earth model, science frequently relies on reasoning and models as well as observational data.
Critical Thinking
Question 1. Assume that you found an object that you think might be “alive.” What would be several things you
would need to observe about this object to determine if it was, in fact, alive?
Answer: You could subject the object to environmental stimuli (heat, pH changes, physical pressure, etc.) to see if it
responded to any of those stimuli. The object should have a number of physical characteristics (adaptations) that
allow it to survive in a particular environment. The object would have a metabolism. At the biochemical level, the
object would have DNA and certain molecules in common with all known life on Earth. The object may or may not
be able to reproduce: it could be a sterile individual of a species or it could be a sexually-reproducing organism, in
which case a “mate” would be needed!
Question 2. Arguments about teaching evolution in the public school classroom continually attract public attention
as elected school boards (in certain states) meet to approve new science teaching curricula. Issues raised in the
public sector (not from scientists) include arguments about evolution being “just a theory,” giving religious
creationism “equal time,” and replacing the definition of science as study of natural phenomena with science as a
logical construct. Why do scientists reject these arguments?
Answer: Although school boards must vote on policy, scientists do not “vote” on science. The nature of “theory” in
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this context means considerably more than the street vernacular “it’s just a theory.” Science does not provide “equal
time” to non-science explanations that do not lend themselves to observation, experimentation, and the development
of new research. And, although many academic fields are based on “logic” (most notably mathematics), science is
defined and uniquely tethered to concepts that are in agreement with the workings of the natural world.
“Supernatural” explanations simply do not allow a researcher to formulate tests and expand understanding of the
“natural world.”
Question 3. Ecology experiments can be separated into two general categories. Some ecologists prefer to bring an
organism into the laboratory and isolate it in a chamber where they can keep the setup simple and where they
determine all the environmental conditions. Other ecologists prefer to study organisms in the wild and attempt to
determine why they occur in certain natural habitats. Using terms for scientific methodology, explain the benefits of
both systems.
Answer: The natural environment is so complex that only simple lab settings can eliminate complex factors and
show how an organism responds to just one variable. A controlled chamber allows the scientist to set up a control
group that lacks just the experimental variable. On the other hand, a natural environment is closer to what the
organism actually encounters. There may be many factors in a natural environment that are major and important
influences on an organism but which a researcher has not considered in a controlled lab setting experiment. While
two “equal” field sites can be used, one as a treatment and one as a control, the ecologist cannot be certain that
another uncontrolled factor in nature is not involved or that chance variation is not a contributing factor. Ecologists
often integrate the results of both types of research to formulate their conclusions.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
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PART
I
THE CELL
Because the cell is the basic unit of life, cell structure, function, and metabolism are major
concepts necessary to understand life. Cellular metabolism is simplified and cellular processes
are described in preparation for an understanding of genetic control, the origin of the cell, and
adaptation to the environment.
2
3.
4
5.
6.
7.
8.
Basic Chemistry
The Chemistry of Organic Molecules
Cell Structure and Function
Membrane Structure and Function
Metabolism: Energy and Enzymes
Photosynthesis
Cellular Respiration
CHAPTER
2
BASIC CHEMISTRY
Living organisms and non-living substances are composed of matter. The basic unit of matter is the atom. This
chapter presents a study of atomic structure, which allows us to see how atoms interact by ionic or covalent bonds to
form molecules. All living things are composed of 70-90% water; as such, a thorough knowledge of the chemical
and physical properties of water, as well as those properties of acids and bases, is critical to understanding the
chemistry of life. The chemical and physical properties of water are presented in detail, as is the concept of acids
and bases.
Chapter Outline
2.1 Chemical Elements
1. Matter is defined as anything that takes up space and has mass.
2. Matter exists in three states: solid, liquid, and gas.
A. Elements
1. All matter (both living and non-living) is composed of 92 naturally-occurring
elements.
2. Elements, by definition, cannot be broken down to simpler substances with different
chemical or physical properties.
3. Six elements (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur—acronym
CHNOPS) make up 95% of the body weight of organisms.
B. Atoms
1. Elements consist of tiny particles called atoms.
2. An atom is the smallest unit of an element that displays the properties of the element.
3. One or two letters (e.g., H, Na) create the atomic symbol of the element.
4. Atoms contain specific numbers of protons, neutrons, and electrons.
5. Protons are positively charged particles; neutrons have no charge; electrons are
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C.
D.
E.
F.
negatively charged particles located in orbitals outside the nucleus.
6. Protons and neutrons are in the nucleus of an atom; electrons move around the
nucleus.
Atomic Mass and Atomic Number
1. The atomic mass of an atom depends on the presence of certain subatomic particles.
2. Atomic mass is the sum of protons and neutrons.
3. Protons and neutrons have one atomic mass unit (amu of weight); electrons have zero.
4. All atoms of an element have the same number of protons, called the atomic number
of the element.
The Periodic Table
1. The periodic table shows how various characteristics of atoms of elements recur.
2. Groups are the vertical columns in the table, periods are the horizontal rows; atomic
mass increases as you move down a group or across a period.
3. The atomic number is above the atomic symbol and the atomic mass is below the
atomic symbol.
Isotopes
1. Isotopes are atoms of the same element that differ in the number of neutrons (and
therefore have different atomic masses). For example, carbon-12 has 6 protons and 6
neutrons, carbon-14 has 6 protons and 8 neutrons.
2. A carbon atom with 8 rather than 6 neutrons is unstable; it releases energy and
subatomic particles and is thus a radioactive isotope.
3. Because the chemical behavior of a radioactive isotope is the same as a stable isotope
of a particular element, low levels of the radioactive isotope (e.g., radioactive iodine
or glucose) allow researchers to trace the location and activity of the element in living
tissues; these isotopes are called tracers.
4. High levels of radiation can destroy cells and cause cancer; careful use of radiation
can sterilize products and kill cancer cells.
Electrons and Energy
1. Electrons occupy orbitals within various energy levels (or electron shells) near or
distant from the nucleus of the atom. The farther the orbital from the nucleus, the
higher the energy level.
2. An orbital is a volume of space where an electron is most likely to be found; an
orbital can contain no more than 2 electrons.
3. When atoms absorb energy during photosynthesis, electrons are boosted to higher
energy levels. When the electrons return to their original energy level, the released
energy is converted into chemical energy. This chemical energy supports all life on
Earth.
4. The innermost shell of an atom is complete with 2 electrons; all other shells are
complete with 8 electrons. This is called the octet rule.
5. Atoms will give up, accept, or share electrons in order to have 8 electrons in an
electron shell.
2.2 Compounds and Molecules
1. When atoms of two or more different elements bond together, they form a compound
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(e.g., H2O).
2. A molecule is the smallest part of a compound that has the properties of the
compound.
3. A formula tells you the number of each kind of atom in a molecule (ex., Glucose,
C6H12O6).
4. Electrons possess energy, and bonds that exist between atoms in molecules therefore
contain energy.
A. Ionic Bonding
1. An ionic bond forms when electrons are transferred from one atom to another atom.
2. By losing or gaining electrons, atoms fill outer shells, and are more stable (the octet
rule).
3. Example: sodium loses an electron and therefore has a positive charge; chlorine gains
an electron to give it a negative charge. Such charged particles are called ions.
4. Attraction of oppositely charged ions holds the two atoms together in an ionic bond.
5. A salt (e.g., NaCl) is an example of an ionically-bonded compound.
B. Covalent Bonding
1. Covalent bonds result when two atoms share electrons so each atom has an octet of
electrons in the outer shell (or, in the case of hydrogen, 2 electrons).
2. Hydrogen can give up an electron to become a hydrogen ion (H+) or share an electron
with another atom to complete its shell with 2 electrons.
3. The structural formula of a compound indicates a shared pair of electrons by a line
between the two atoms; e.g., single covalent bond (H–H), double covalent bond
(O=O), and triple covalent bond
(N = N). Each line between the atoms represents a pair of electrons.
C. Nonpolar and Polar Covalent Bonds
1. In nonpolar covalent bonds, sharing of electrons is equal, i.e., the electrons are not
attracted to either atom to a greater degree.
2. With polar covalent bonds, the sharing of electrons is unequal.
a. In a water molecule (H2O), sharing of electrons by oxygen and hydrogen is not equal; the oxygen
atom with more protons attracts the electrons closer to it, and thus domin ates the H2O
association.
b.
Attraction of an atom for electrons in a covalent bond is called the electronegativity of the atom;
an oxygen atom is more electronegative than a hydrogen atom.
c. Oxygen in a water molecule, more attracted to the electron pair, assumes a partial negative charge.
2.3. Chemistry of Water
1. The shape of water and of all organic molecules is necessary to the structural and
functional roles they play in living things.
2. A hydrogen bond is the attraction of a slightly positive hydrogen to a slightly
negative atom in the vicinity.
A. Hydrogen Bonding
1. A hydrogen bond is a weak attractive force between the slightly positive charge of
the hydrogen atom of one molecule and slightly negative charge of another atom
13
(e.g., oxygen, nitrogen) in another or the same molecule.
2. Many hydrogen bonds taken together are relatively strong.
3. Hydrogen bonds between and within complex biological molecules (e.g., DNA,
proteins) help maintain their proper structure and function.
B. Properties of Water
1. Water has a high heat capacity.
a. The temperature of liquid water rises and falls more slowly than that of most other
liquids.
b. A calorie is the amount of heat energy required to raise the temperature of one
gram of water 1º C.
c.
Because the hydrogen bonds between water molecules hold more heat, water’s temperature falls
more slowly than other liquids; this protects organisms from rapid temperature changes and helps
them maintain homeostatic temperature.
2. Water has a high heat of evaporation.
a. When water boils, it evaporates, or vaporizes into the environment.
b. Hydrogen bonds between water molecules require a relatively large amount of
heat to break.
c.
d.
This property moderates Earth’s surface temperature; permits living systems to exist.
When animals sweat, evaporation of the sweat removes body heat, thus cooling the animal.
3. Water is a solvent.
a.
b.
c.
d.
Water dissolves a great number of substances (e.g., salts, large polar molecules).
A solution contains dissolved substances called solutes.
Ionized or polar molecules attracted to water are hydrophilic (“water loving”).
Nonionized and nonpolar molecules that cannot attract water are hydrophobic (“water fearing”).
4. Water molecules are cohesive and adhesive.
a.
b.
c.
Cohesion allows water to flow freely without molecules separating.
Adhesion is the ability to adhere to polar surfaces; water molecules have positive and negative
poles.
Water rises up a tree from roots to leaves through small tubes.
1) Adhesion of water to walls of vessels prevents water column from breaking apart.
2) Cohesion allows evaporation from leaves to pull water column from roots.
d. Water has a high surface tension and is relatively difficult to break through at its
surface.
1) This property permits a rock to be skipped across a pond surface, and supports insects
walking on surface.
5. Unlike most substances, frozen water (ice) is less dense than liquid water.
a. Below 4º C, hydrogen bonding becomes more rigid but more open, causing expansion.
b.
c.
d.
2.4.
Because ice is less dense, it floats; therefore, bodies of water freeze from the top down.
If ice was heavier than water, ice would sink and bodies of water would freeze solid.
This property allows ice to act as an insulator on bodies of water, thereby protecting aquatic
organisms during the winter.
Acids and Bases
1. When water ionizes or dissociates, it releases a small (1 x 107 moles/liter) but equal
number of hydrogen (H+) ions and hydroxide (OH-) ions; H – O –H → H+ + OH-.
2. Acid molecules dissociate in water, releasing hydrogen (H+) ions: HCl → H+ + Cl-.
14
3. Bases are molecules that take up hydrogen ions or release hydroxide ions. NaOH →
Na+ + OH-.
4. The pH scale indicates acidity and basicity (alkalinity) of a solution.
a.
b.
pH is the measurement of free hydrogen ions, expressed as a negative logarithm of the H +
concentration (-log [H+]).
pH values range from 0 (1 x 100 moles/liter; most acidic) to 14 (1 x 10-14 moles/liter; most basic).
1) One mole of water has 1 x 10-7 moles/liter of hydrogen ions; therefore, has neutral pH of 7.
2) An acid is a substance with pH less than 7; a base is a substance with pH greater than 7.
3) Because it is a logarithmic scale, each lower unit has 10 times the amount of hydrogen ions as
next higher pH unit; moving up pH scale, each unit has 10 times the basicity of previous unit.
5. Buffers keep pH steady and within normal limits in living organisms.
a. Buffers stabilize pH of a solution by taking up excess hydrogen (H +) or hydroxide (OH-) ions.
b. Carbonic acid helps keep blood pH within normal limits: H 2CO3 → H+ + HCO3-.
A. The Harm Done by Acid Deposition (Ecology Focus box)
1. Acid deposition is rain or snow with pH < 5.0, and dry acidic particles that settle on the Earth from
the atmosphere.
2. The burning of fossil fuels increases the amount of acid deposition that falls from the atmosphere to
the Earth.
3. Impact of Lakes
a. Aluminum is leached from the soil converts mercury deposits to methyl mercury, which is toxic
and accumulates in fish.
4. Impact on Forests
a.
Acid rain damages plant leaves so they cannot conduct photosynthesis.
b.
Acid rain stresses plants and they are then more susceptible to disease and pests.
c. When toxins are leached into the soil, the toxins can kill vital soil fungi that help roots
absorb nutrients.
5. Impact on Humans and Structures
a.
Inhaling dry acidic particles can increase the chance of respiratory illnesses such as asthma.
Lecture Enrichment Ideas
Experience Base: In many states, fewer than half of high school graduates have taken a chemistry course although
students majoring in biology are more likely to have such background. An instructor cannot assume that students
have previously been exposed to many chemical concepts in a meaningful way.
1.
Students can examine the periodic table and locate the elements that are common in living organisms. Have
them determine how many electrons are in the outer shell by examining the atomic numbers.
2.
Describe how carbon14 breaks down into nitrogen14 and what the differences in protons and neutrons would be
between these elements. Discuss why some isotopes are radioactive and others are not.
3.
“Radioactive isotopes” are often confused with “radiation,” the energy or particle given off by the radioactive
isotopes. Keep explanations clear and use words carefully to distinguish the two. Isotopes with very low levels
of radiation can be concentrated in one tissue, such as iodine in the thyroid, and the radiation can be used to
damage local cells. Discuss the medical use of this.
4.
Discuss the formation of ions and how electrons are lost or gained in such formation. Make sure students
understand that an ion can be a combination of atoms, and that even large molecules (e.g., H3PO4) may become
ionized at different locations within the molecule.
5.
Consider the protons and electrons of an atom as similar to the ends of a bar magnet, with positive and negative
charges attracting each other, and electrons at greater distances from the nucleus having more energy because
they are not as closely attracted to the positive charges of the protons.
15
6.
Adhesion, cohesion, and surface tension are difficult concepts for some students. The ability of a water strider
to support itself on the water’s surface can be described based on water molecules pushing in all directions; lack
of water above the surface provides an upward surface tension “support.” Water droplets rolling across the hood
of a newly waxed car illustrate cohesion, and supporting a water droplet between your fingers viewing with an
overhead projector can illustrate both adhesion (to fingers) and cohesion (of water).
Critical Thinking
Question 1. Water drawn through trees by transpiration evaporates into the air above forests and generates updrafts
of lighter weight moist air! Glider pilots know that they can gain lift from these currents by flying above forests.
Consider that a molecule of water displaces a molecule of nitrogen or oxygen gas. Why then is air lighter when we
add moisture to it?
Answer: The atomic mass of one oxygen and two small hydrogen atoms is less than the mass of two nitrogen or two
oxygen atoms. Therefore, when an H2O displaces an O2 or N2, a volume of air is lighter because the mass of atoms
is lighter.
Question 2. Detergent soaps reduce the surface tension of water, therefore making water “wetter” and better able to
penetrate dirt on dishes, etc. A water strider is an insect that “skates” across the surface of fresh water streams and
ponds to locate food. Disregarding any chemical reactions that might be toxic to the insect, what would happen to
the water strider if detergent was added to the surface of that stream or pond?
Answer: Because the water strider relies on the surface tension of the water to stay on the surface, the wetting effect
of the detergent would keep the insect’s feet from staying on top of the water’s surface.
Question 3. The discovery of liquid water under the frozen surface of a distant moon in our solar system has caused
scientists to speculate on the possibility of life on that moon. Researchers hold no hope of any life form existing on
any planet or moon in the absence of water. Why?
Answer: The metabolic pathways in a cell are the chemical reactions for life and all are dependent upon water as a
solvent. Water also moderates the environment, serves as a transport medium, supplies H + or OH- ions, and has
hydraulic properties for capillary action.
Question 4. Why does water freeze at a temperature lower than 0oC when salt is added to it?
Answer: The salt disrupts the water’s ability to form rigid hydrogen hydrogen bonds as the temperature lowers. The
water thus remains liquid at a temperature where, without the salt, the water would form the solid state ice.
Technology from WCB/McGraw-Hill
Life Science Animations Video: Video #1 Chemistry, The Cell, and Energetics
Formation of an Ionic Bond (#1) The atomic structure of sodium and chlorine is reviewed
before these atoms are seen reacting to form sodium chloride.
16
CHAPTER
3
THE CHEMISTRY OF ORGANIC MOLECULES
Organic molecules are a diverse group of chemicals that contain carbon and hydrogen bonded to other atoms. In the
living organism, four types of organic molecules, or biomolecules, exist: carbohydrates, lipids, proteins, and nucleic
acids. The unique chemistry of carbon makes these biomolecules highly diverse in structure and function. This
chapter presents a detailed description of these biomolecules and of the macromolecules built from them; various
types of chemical reactions involving these organic substances are described. Many examples are presented to
illustrate the diversity and biological importance of these molecules of life.
Chapter Outline
3.1 Organic Molecules
Organic molecules contain carbon and hydrogen atoms bonded to other atoms.
1. Four types of organic molecules (biomolecules) exist in organisms: carbohydrates, lipids, proteins,
and nucleic acids.
2. Organic molecules are a diverse group; even a simple bacterial cell contains some 5,000 organic
molecules.
A. The Carbon Atom
1. The chemistry of the carbon atom allows it to form covalent bonds with as many as four other elements
(generally with the CHNOPS elements).
2. Hydrocarbons are chains of carbon atoms bonded exclusively to hydrogen atoms; hydrocarbons can
be branched and they can form ringed (cyclic) compounds.
3. Carbon atoms can form double or triple bonds with certain atoms (carbon, nitrogen).
B. The Carbon Skeleton and Functional Groups
1. The carbon chain of an organic molecule is called its skeleton or backbone.
2. Functional groups are clusters of specific atoms bonded to the carbon skeleton with characteristic
structure and functions.
a. As an example, the addition of an –OH (hydroxyl group) to a carbon skeleton turns the molecule
into an alcohol.
b. Ethyl alcohol (ethanol) is hydrophilic (dissolves in water) because the hydroxyl group is polar.
c. Nonpolar organic molecules are hydrophobic (cannot dissolve in water) unless they contain a
polar functional group (ex., ethane), while hydrophilic compounds (such as ethanol) can dissolve
in water because the –OH functional group is polar.
d. Depending on its functional groups, an organic molecule may be both acidic and hydrophilic. An
example is a hydrocarbon that contains a carboxyl group; carboxyl groups ionize in solution by
releasing hydrogen ions, becoming both polar and acidic.
e. Because cells are 70–90% water, the degree to which an organic molecule interacts with water
affects its function.
3. Isomers are molecules with identical molecular formulas but different arrangements of their atoms
(e.g., glyceraldehyde and dihydroxyacetone).
C. The Biomolecules of Cells
1. Carbohydrates, lipids, proteins, and nucleic acids are called biomolecules because certain foods are
known to be rich in them.
2. Cellular enzymes carry out dehydration reactions to synthesize biomolecules. In a dehydration
reaction, a water molecule is removed and a covalent bond is made between two atoms of the
monomers.
3. Hydrolysis (“water breaking”) reactions break down polymers in reverse of dehydration; a hydroxyl
(— OH) group from water attaches to one monomer and hydrogen (— H) attaches to the other.
17
4.
3.2
A.
B.
C.
D.
3.3
A.
Enzymes are molecules that speed up chemical reactions by bringing reactants together; an enzyme
may even participate in the reaction but is not changed by the reaction.
5. The largest biomolecules are called polymers, constructed by linking many of the same type of small
subunits, called monomers. Examples: amino acids (monomers) are linked to form a protein
(polymer); many nucleotides (monomers) are linked to form a nucleic acid (polymer).
a. In a dehydration reaction, a hydroxyl (— OH) group is removed from one monomer and a
hydrogen (— H) is removed from the other.
b. This produces water, and, because the water is leaving the monomers, it is a dehydration reaction.
Carbohydrates
Monosaccharides: Ready Energy
1. Monosaccharides are simple sugars with a backbone of 3 to 7 carbon atoms.
a. Most monosaccharides of organisms have 6 carbons (hexose).
b. Glucose and fructose are hexoses, but are isomers of one another; each has the formula (C 6H12O6)
but they differ in arrangement of the atoms.
c. Glucose is found in the blood of animals; it is the source of biochemical energy (ATP) in nearly all
organisms.
2. Ribose and deoxyribose are five-carbon sugars (pentoses); they contribute to the backbones of RNA
and DNA, respectively.
Disaccharides: Varied Uses
1. Disaccharides contain two monosaccharides joined by a dehydration reaction.
2. Maltose is composed of two glucose molecules; it forms in the digestive tract of humans during starch
digestion.
3. Sucrose (table sugar) is composed of glucose and fructose; it is used to sweeten food for human
consumption.
4. Lactose is composed of galactose and glucose and is found in milk.
Polysaccharides: Energy Storage Molecules
1. Polysaccharides are polymers of monosaccharides. They are not soluble in water and do not pass
through the plasma membrane of the cell.
2. Starch, found in many plants, is a straight chain of glucose molecules with relatively few side
branches. Amylose and amylopectin are the two forms of starch found in plants.
3. Glycogen is a highly branched polymer of glucose with many side branches. It is the storage form of
glucose in animals.
Polysaccharides: Structural Molecules
1. Cellulose is a polymer of glucose which forms microfibrils, the primary constituent of plant cell walls.
a. Cotton is nearly pure cellulose.
b. Cellulose is indigestible by humans due to the unique bond between glucose molecules.
c. Grazing animals can digest cellulose due to special stomachs and bacteria.
d. Cellulose is the most abundant organic molecule on Earth.
2. Chitin is a polymer of glucose with an amino group attached to each glucose.
a. Chitin is the primary constituent of the exoskeleton of crabs and related animals (lobsters, insects,
etc.).
b. Chitin is not digestible by humans.
3. Peptidoglycan is a polymer of glucose derivatives and is found in bacteria.
Lipids
Lipids are varied in structure.
1. Lipids are hydrocarbons that are insoluble in water because they lack polar groups.
2. Fat provides insulation and energy storage in animals.
3. Phospholipids form plasma membranes and steroids are important cell messengers.
4. Waxes have protective functions in many organisms.
Triglycerides: Long-Term Energy Storage
1. Fats and oils contain two molecular units: glycerol and fatty acids.
2. A fatty acid is a long hydrocarbon chain with a carboxyl (acid) group at one end.
a. Most fatty acids in cells contain 16 to 18 carbon atoms per molecule.
b. Saturated fatty acids have no double bonds between their carbon atoms.
c. Unsaturated fatty acids have double bonds in the carbon chain where there are less than two
18
B.
C.
D.
3.4
A.
B.
hydrogens per carbon atom.
3. Glycerol is a water-soluble compound with three hydroxyl groups.
4. Triglycerides are glycerol joined to three fatty acids by dehydration reactions.
5. Fats contain saturated fatty acids and are solid at room temperature (e.g., butter).
6. Oils contain unsaturated fatty acids and are liquid at room temperature.
7. Animals use fat rather than glycogen for long-term energy storage; fat stores more energy.
Phospholipids: Membrane Components
1. Phospholipids are constructed like neutral fats except that the third fatty acid is replaced by a polar
(hydrophilic) phosphate group; the phosphate group usually bonds to another organic group
(designated by R).
2. The hydrocarbon chains of the fatty acids become the nonpolar (hydrophobic) tails.
3. Phospholipids arrange themselves in a double layer in water, so the polar heads face toward water
molecules and nonpolar tails face toward one other, away from water molecules.
4. This property enables phospholipids to form an interface or separation between two solutions (e.g., the
interior and exterior of a cell); the plasma membrane is a phospholipid bilayer.
Steroids: Four Fused Rings
1. Steroids have skeletons of four fused carbon rings and vary according to attached functional groups;
these functional groups determine the biological functions of the various steroid molecules.
2. Cholesterol is a component of an animal cell’s plasma membrane, and is the precursor of the steroid
hormone (aldosterone, testosterone, estrogen, calcitriol, etc.).
3. A diet high in saturated fats and cholesterol can lead to circulatory disorders.
Waxes
1. Waxes are long-chain fatty acids bonded to long-chain alcohols.
2. Waxes have a high melting point, are waterproof, and resist degradation.
3. Waxes form a protective covering in plants that retards water loss in leaves and fruits.
4. In animals, waxes maintain animal skin and fur, trap dust and dirt, and form the honeycomb.
Proteins
Protein Functions
1. Metabolic enzymes are proteins that act as organic catalysts to accelerate chemical reactions within
cells.
2. Support proteins include keratin, which makes up hair and nails, and collagen fibers, which support
many of the body’s structures (e.g., ligaments, tendons, skin).
3. Transport functions include channel and carrier proteins in the plasma membrane, and hemoglobin
that transports oxygen in red blood cells.
4. Defense functions include antibodies that prevent infection.
5. Hormones are regulatory proteins that influence the metabolism of cells. For example, insulin
regulates glucose content of blood and within cells.
6. Motion within cells and by muscle contraction is provided by the proteins myosin and actin.
Peptides
1. A peptide bond is a covalent bond between two amino acids.
2. Atoms of a peptide bond share electrons unevenly (oxygen is more electronegative than nitrogen).
3. The polarity of the peptide bond permits hydrogen bonding between different amino acids in a
polypeptide.
4. A peptide is two or more amino acids bonded together.
5. Polypeptides are chains of many amino acids joined by peptide bonds.
6. A protein may contain more than one polypeptide chain; it can thus have a very large number of
amino acids.
a. The three-dimensional shape of a protein is critical; an abnormal sequence will have the wrong
shape and will not function normally.
b. Frederick Sanger determined the first protein sequence (of the hormone insulin) in 1953.
Amino Acids: Proteins Monomers
1. Amino acids contain an acidic group (— COOH) and an amino group (—NH2).
2. Amino acids differ according to their particular R group, ranging from single hydrogen to complicated
ring compounds.
3. The R group of amino acid cystine ends with a sulfhydryl (— SH) that serves to connect one chain of
amino acids to another by a disulfide bond (— S— S—).
19
4. There are 20 different amino acids commonly found in cells.
C. Shape of Proteins
1. Protein shape determines the function of the protein in the organism; proteins can have up to four
levels of structure (but not all proteins have four levels).
2. The primary structure is the protein’s own particular sequence of amino acids.
a. Just as the English alphabet contains 26 letters, 20 amino acids can join to form a huge variety of
“words.”
3. The secondary structure results when a polypeptide coils or folds in a particular way.
a. The  (alpha) helix was the first pattern discovered.
1) In a peptide bond, oxygen is partially negative, hydrogen is partially positive.
2) This allows for hydrogen bonding between the C=O of one amino acid and the N—H of
another.
3) Hydrogen bonding between every fourth amino acid holds the spiral shape of an  helix.
b. The  (beta) sheet was the second pattern discovered.
1) Pleated  sheet polypeptides turn back upon themselves.
2) Hydrogen bonding occurs between extended lengths.
c. Fibrous proteins (e.g., keratin) are structural proteins with helices and/or pleated sheets that
hydrogen bond to one another.
4. Tertiary structure results when proteins are folded, giving rise to the final three-dimensional shape of
the protein. This is due to interactions among the R groups of the constituent amino acids.
a. Globular proteins tend to ball up into rounded shapes.
b. Strong disulfide linkages maintain the tertiary shape; hydrogen, ionic, and covalent bonds also
contribute.
5. Quaternary structure results when two or more polypeptides combine.
a. Hemoglobin is globular protein with a quaternary structure of four polypeptides; each polypeptide
has a primary, secondary, and tertiary structure.
D. Protein Folding Diseases
1. As proteins are synthesized, chaperone proteins help them fold into their correct shapes; chaperone
proteins may also correct misfolding of a new protein and prevent them from making incorrect shapes.
2. Certain diseases (e.g., the transmissible spongiform encephalopathies, or TSEs) are likely due to
misfolded proteins, called prions.
3.5 Nucleic Acids
1. Nucleic acids are polymers of nucleotides with very specific functions in cells.
2. DNA (deoxyribonucleic acid) stores the genetic code for its own replication and for the amino acid
sequences in proteins.
3. RNA (ribonucleic acid) allows for translation of the genetic code of DNA into the amino acid
sequence of proteins; other functions for RNA in the cell exist.
4. Some nucleotides have independent metabolic functions in cells.
a. Coenzymes are molecules which facilitate enzymatic reactions.
b. ATP (adenosine triphosphate) is a nucleotide used to supply energy for synthetic reactions and
other energy-requiring metabolic activities in the cell.
A. Structure of DNA and RNA
1. Nucleotides are a molecular complex of three types of molecules: a phosphate (phosphoric acid), a
pentose sugar, and a nitrogen-containing base.
2. DNA and RNA differ in the following ways:
a. Nucleotides of DNA contain deoxyribose sugar; nucleotides of RNA contain ribose.
b. In RNA, the base uracil occurs instead of the base thymine. Both RNA and DNA contain adenine,
guanine, and cytosine.
c. DNA is double-stranded with complementary base pairing; RNA is single-stranded.
1) Complementary base pairing occurs where two strands of DNA are held together by
hydrogen bonds between purine and pyrimidine bases.
2) The number of purine bases always equals the number of pyrimidine bases.
3) In DNA, thymine is always paired with adenine; cytosine is always paired with guanine.
Thus, in DNA: A + G = C + T.
d. Two strands of DNA twist to form a double helix; RNA does not form helices.
B. ATP (Adenosine Triphosphate)
20
1.
2.
3.
4.
5.
ATP (adenosine triphosphate) is a nucleotide in which adenosine is composed of ribose and adenine.
Triphosphate derives its name from three phosphate groups attached together and to the ribose.
ATP is a high-energy molecule because the last two phosphate bonds release energy when broken.
In cells, the terminal phosphate bond is hydrolyzed, leaving ADP (adenosine diphosphate); energy is
released when this occurs.
The energy released from ATP breakdown is used in the energy-requiring processes of the cell, such as
synthetic reactions, muscle contraction, and the transmission of nerve impulses.
Lecture Enrichment Ideas
Experience Base: This chapter utilizes a substantial amount of terminology for organic molecules that are not in
most students’ direct experience unless they have had exposure to organic chemistry. For students with limited
chemistry background, it is important to stress that the various acronyms stand for complex molecules and are not a
stand-alone chemical alphabet (i.e., ADP is not a molecule found in the center of NADPH).
1.
Describe specific macromolecules (e.g., carbohydrates, proteins) that differ in various life forms. Why are
certain macromolecules virtually identical in highly diverse life forms (i.e., humans and bacteria) but other
macromolecules are very different?
2.
Discuss why life is based on carbon. Note the characteristics of carbon that make it the basis of organic
molecules. Early researchers probing the possibility of life on other planets speculated that life might be based
on another atom besides carbon—perhaps silicon. Ask students to consider the chemical properties of silicon
(note its position on the periodic table) compared to carbon, and speculate on how this might make life different
or impossible as we know it.
3.
Describe the structures and characteristics of functional groups that are important in the biochemistry of living
organisms.
4.
Explain the different isomeric forms of the hexose sugars glucose, fructose, and galactose, and reasons for their
different characteristics. Describe how the structures fit into enzymes and why different enzymes would be
needed to interact with different isomers.
5.
Ask if condensation synthesis and hydrolysis reactions would be exact opposites of each other. Point out that
these reactions are not generally one-step processes but require several steps and several enzymes to carry out
the complete reaction.
6.
Speculate what would happen if genetic engineering gave humans the ability to directly digest cellulose. Would
this be useful or not? Ask students to consider the structure of our digestive tracts and the use of cellulose as
“roughage” or “bulk.”
7.
Describe how denaturation affects the different levels of a protein’s structure. Which levels would be most
affected by denaturation? Contrast denaturation caused by heat (e.g., frying an egg) as opposed to mild
denaturation caused by reversible pH changes. Would freezing affect the levels of a protein’s structure?
8.
Compare the different types of subunits of the carbohydrates, lipids, proteins, and nucleic acids, and the types of
bonds that link these subunits. Prepare the students for the link between the genetic information in DNA, RNA,
and protein synthesis that will be discussed later.
9.
Explain how the amount of ATP used in one day exceeds by many times the amount of ATP in the human body.
Critical Thinking
Question 1. For every molecule that life forms can synthesize, such as cellulose or chitin, there are bacteria that can
digest that particular macromolecule into sugar and other smaller molecules. What would happen if a tree or insect
could build a complex molecule that no organism or natural process could decompose?
21
Answer: This would be a disastrous situation. Without organisms that can break down cellulose, chitin, or other
large organic molecules, these molecules would accumulate. Not only would we be flooded with, say, dead tree
trunks and insect shells, eventually these tissues would capture all of the cycling carbon and bring life to a halt.
Question 2. Many insects feed exclusively on one narrow type of food molecule such as plant sap (sugar), blood
proteins, or starch. Yet these organisms are themselves made of a wide range of molecules. Where does this
molecular diversity come from?
Answer: Each species of organism has its unique enzyme combinations that allow it to feed on certain foods and
convert those molecules to the other molecules necessary for its structure and physiology. Organisms that lack the
enzymes to build necessary molecules must take in those molecules intact in their diet. The food preferences of
caterpillars and grubs, for example, actually reflect the enzyme and metabolic abilities of the insects’ cells.
Question 3. Why is it somewhat true to say that ultimately, biology is the study of proteins?
Answer: While some other types of molecules are important for standard functions in energy storage, structure, etc.,
the great majority of variation among organisms is due to unique proteins (e.g., structural, enzymes, etc.) and the
resultant unique biochemistry. By understanding the protein chemistry (e.g., its enzymatic activity, its structural
proteins, etc.) of an organism, we can begin to understand how the organism lives. And, of course, different proteins
mean different genes and therefore uniqueness in DNA.
22
CHAPTER
4
CELL STRUCTURE AND FUNCTION
This chapter presents an overview of the cell theory. A description of the various types of microscopes and their
uses precedes a detailed study of cell structure and function. The organelles and their activities are discussed for
both prokaryotic and eukaryotic cells. Various human diseases associated with organellar dysfunction are
mentioned.
Chapter Outline
4.1 Cellular Level of Organization
1. Detailed study of the cell began in the 1830s; some of the scientists contributing to the understanding
of cell structure and function were Robert Brown, Matthais Schleiden, Theodor Schwann, and Rudolph
Virchow.
2. The cell theory states that all organisms are composed of cells, that cells are the structural and
functional unit of organisms, and that cells come only from preexisting cells.
A. Cell Size
1. Cells range in size from one millimeter down to one micrometer.
2. Cells need a surface area of plasma membrane large enough to adequately exchange materials.
3. The surface area to volume ratio requires that cells be small.
a. As cells get larger in volume, surface area relative to volume decreases.
b. Size limits how large the actively metabolizing cells can become.
c. Cells needing greater surface area utilize membrane modifications such as folding, microvilli, etc.
B. Microscopy Today (Science Focus box)
1. Compound light microscopes use light rays focused by glass lenses.
2. Transmission electron microscopes (TEM) use electrons passing through specimen and focused by
electromagnetic lenses.
3. Scanning electron microscopes (SEM) use electrons scanned across metal-coated specimen;
secondary electrons given off by metal are collected by a detector.
4. Magnification is a function of wavelength; the shorter wavelengths of electrons allow greater
magnification than the longer wavelengths of light rays.
5. Resolution is the minimum distance between two objects at which they can still be seen as separate
objects.
6. Contrast is the difference in the shading of an object compared to its background.
7. Phase contrast and differential interface contrast microscopy uses fluorescent antibodies to reveal
proteins in cells.
8. Confocal microscopy uses laser beam to focus on a shallow plane within the cell; this forms a series
of optical sections from which a computer creates a three dimensional image.
9. Video-enhanced contrast microscopy accentuates the light and dark regions and may use a computer
to contrast regions with false colors.
10. Bright-field, phase contrast, differential interference, and darkfield are different types of light
microscopes.
4.2 Prokaryotic Cells
1. Prokaryotic cells lack a nucleus and are smaller and simpler than eukaryotic cells (which have a
nucleus).
2. Prokaryotic cells are placed in two taxonomic domains: Bacteria and Archaea. Organisms in these two
domains are structurally similar but biochemically different.
A. The Structure of Prokaryotes
1. Prokaryotes are extremely small; average size is 1–1.5 μm wide and 2–6 μm long .
2. Prokaryotes occur in three basic shapes: spherical coccus, rod-shaped bacillus, and spiral spirillum (if
rigid) or spirochete (if flexible).
3. Cell Envelope
a. In bacteria, the cell envelope includes the plasma membrane, the cell wall, and the glycocalyx.
23
4.3
A.
B.
C.
4.4
A.
The plasma membrane is a lipid bilayer with imbedded and peripheral proteins; it regulates the
movement of substances into and out of the cell.
b. The plasma membrane can form internal pouches called mesosomes, which increase the internal
surface area of the membrane for enzyme attachment.
c. The cell wall maintains the shape of the cell and is strengthened by peptidoglycan.
d. The glycocalyx is a layer of polysaccharides on the outside of the cell wall; it is called a capsule if
organized and not easily removed or a slime layer if it is not well-organized and is easily
removed.
4. Cytoplasm
a. The cytoplasm is a semifluid solution containing water, inorganic and organic molecules, and
enzymes.
b. The nucleoid is a region that contains the single, circular DNA molecule.
c. Plasmids are small accessory (extrachromosomal) rings of DNA; they are not part of the bacterial
genetic material.
d. Ribosomes are particles with two RNA- and protein-containing subunits that synthesize proteins.
e. Inclusion bodies in the cytoplasm are granules of stored substances.
f. Cyanobacteria (also called blue-green bacteria) are bacteria that photosynthesize; they lack
chloroplasts but have thylakoids containing chlorophyll and other pigments.
5. Appendages
a. Motile bacteria usually have flagella; the filament, hook, and basal body work to rotate the
flagellum like a propeller to move through fluid medium.
b. Fimbriae are small, bristle like fibers that attach to an appropriate surface.
c. Conjugation pili are tubes used by bacteria to pass DNA from cell to cell.
Introducing Eukaryotic Cells
Origin of the Eukaryotic cell
1. According to the endosymbiotic theory, energy-related organelles, such as mitochondria and chloroplast,
arose when a eukaryotic cell engulfed prokaryotic cells.
2. Eukaryotic cells are members of the domain Eukarya, which includes the protists, fungi, plants, and
animals.
3. A membrane-bounded nucleus houses DNA; the nucleus may have originated as an invagination of the
plasma membrane.
4. Eukaryotic cells are much larger than prokaryotic cells, and therefore have less surface area per
volume.
Structure of Eukaryotic Cells
1. Some eukaryotic cells (e.g., plant cells) have a cell wall containing cellulose; plasmodesmata are
channels in a cell wall that allow cytoplasmic strands to extend between adjacent cells.
2. Eukaryotic cells are compartmentalized; they contain small structures called organelles that perform
specific functions.
3. The nucleus communicates with ribosomes in the cytoplasm.
4. The organelles of the endomembrane system communicate with one another; each organelle contains ts
own set of enzymes and produces its own products, which move from one organelle to another by
transport vesicles.
5. The energy-related mitochondria (plant and animal cells) and chloroplasts (plant cells) do not
communicate with other organelles; they contain their own DNA and are self-sufficient.
6. The cytoskeleton is a lattice of protein fibers that maintains the shape of the cell and assists in
movement of the organelles.
Cell Fractionation and Differential Centrifugation (Science Focus box)
1. Cell fractionation allows the researcher to isolate and individually study the organelles of a cell.
2. Differential centrifugation separates the cellular components by size and density.
3. Using these two techniques, researchers can obtain pure preparations of any cell component.
The Nucleus and Ribosomes
Nucleus
1. The nucleus has a diameter of about 5 μm.
2. Chromatin is a threadlike material that coils into chromosomes just before cell division occurs;
contains DNA, protein, and some RNA.
3. Nucleoplasm is the semifluid medium of the nucleus.
4. Chromosomes are rodlike structures formed during cell division; composed of coiled or folded
24
chromatin.
The nucleolus is a dark region of chromatin inside the nucleus; it is the site where ribosomal RNA
(rRNA) joins with proteins to form ribosomes.
6. The nucleus is separated from the cytoplasm by the nuclear envelope, which contains nuclear pores
to permit passage of substances (e.g., ribosomal subunits, messenger RNA, proteins, etc.) in and out of
the nucleus
Ribosomes
1. Ribosomes are the site of protein synthesis in the cell. In eukaryotic cells, ribosomes may occur freely
or in groups called polyribosomes.
2. Ribosomes receive messenger RNA (mRNA) from the nucleus, which instructs the ribosomes of the
correct sequence of amino acids in a protein to be synthesized.
The Endomembrane System
The endomembrane system is a series of intracellular membranes that compartmentalize the cell.
1. It consists of the nuclear envelope, the membranes of the endoplasmic reticulum, the Golgi apparatus,
and several types of vesicles.
Endoplasmic Reticulum
1. The endoplasmic reticulum (ER) is a system of membrane channels and saccules (flattened vesicles)
continuous with the outer membrane of the nuclear envelope.
2. Rough ER is studded with ribosomes on the cytoplasm side; it is the site where proteins are
synthesized and enter the ER interior for processing and modification.
3. Smooth ER is continuous with rough ER but lacks ribosomes; it is a site of various synthetic
processes, detoxification, and storage; smooth ER forms transport vesicles.
The Golgi Apparatus
1. It is named for Camillo Golgi, who discovered it in 1898.
2. The Golgi apparatus consists of a stack of slightly curved saccules.
3. The Golgi apparatus receives protein-filled vesicles that bud from the rough ER and lipid-filled
vesicles from the smooth ER.
4. Enzymes within the Golgi apparatus modify the carbohydrates that were placed on proteins in the ER;
proteins and lipids are sorted and packaged.
5. Vesicles formed from the membrane of the outer face of the Golgi apparatus move to different
locations in a cell; at the plasma membrane they discharge their contents as secretions, a process
called exocytosis because substances exit the cell.
Lysosomes
1. Lysosomes are membrane-bounded vesicles produced by the Golgi apparatus.
2. Lysosomes contain powerful digestive enzymes and are highly acidic.
3. Macromolecules enter a cell by vesicle formation; lysosomes fuse with vesicles and digest the contents
of the vesicle.
4. White blood cells that engulf bacteria use lysosomes to digest the bacteria.
5. Autodigestion occurs when lysosomes digest parts of cells.
6. Lysosomes participate in apoptosis, or programmed cell death, a normal part of development.
Endomembrane System Summary
1. Proteins produced in rough ER and lipids from smooth ER are carried in vesicles to the Golgi
apparatus.
2. The Golgi apparatus modifies these products and then sorts and packages them into vesicles that go to
various cell destinations.
3. Secretory vesicles carry products to the membrane where exocytosis produces secretions.
4. Lysosomes fuse with incoming vesicles and digest macromolecules.
Other Vesicles and Vacuoles
Peroxisomes are membrane-bounded vesicles that contain specific enzymes.
1. Peroxisome action results in production of hydrogen peroxide.
2. Hydrogen peroxide (H2O2) is broken down to water and oxygen by catalase.
3. Peroxisomes in the liver produce bile salts from cholesterol and also break down fats.
4. Peroxisomes also occur in germinating seeds where they convert oils into sugars used as nutrients by
the growing plant.
Vacuoles
1. Vacuoles are membranous sacs and are larger than vesicles.
2. Contractile vacuoles in some protists rid the cell of excess water.
5.
B.
4.5
A.
B.
C.
D.
E.
4.6
A.
B.
25
3.
4.
4.7
A.
B.
4.8
A.
B.
C.
D.
Digestive vacuoles digest nutrients.
Vacuoles generally store substances, e.g., plant vacuoles contain water, sugars, salts, pigments, and
toxic molecules.
5. The central vacuole of a plant cell maintains turgor pressure within the cell, stores nutrients and
wastes, and degrades organelles as the cell ages.
The Energy-Related Organelles
Chloroplasts are membranous organelles (a type of plastid) that serve as the site of photosynthesis.
1. Photosynthesis is represented by the equation:
solar energy + carbon dioxide + water → carbohydrate + oxygen
2. Only plants, algae, and certain bacteria are capable of conducting photosynthesis.
3. The chloroplast is bound by a double membrane organized into flattened disc-like sacs called
thylakoids formed from a third membrane; a stack of thylakoids is a granum.
4. Chlorophyll and other pigments capture solar energy, and the enzymes which synthesize carbohydrates
are located in the chloroplasts.
5. Chloroplasts have both their own DNA and ribosomes, supporting the endosymbiotic hypothesis.
6. Other types of plastids, which differ in color, form, and function from chloroplasts, include
chromoplasts and leucoplasts.
Mitochondria are surrounded by a double membrane: the inner membrane surrounds the matrix and is
convoluted to form cristae.
1. Mitochondria are smaller than chloroplasts, and often vary their shape.
2. Mitochondria also can be fixed in one location or form long, moving chains .
3. Mitochondria contain ribosomes and their own DNA.
4. The matrix of the mitochondria is concentrated with enzymes that break down carbohydrates.
5. ATP production occurs on the cristae.
6. More than forty different diseases involving mitochondria have been described.
The Cytoskeleton
The cytoskeleton is a network of connected filaments and tubules; it extends from the nucleus to the plasma
membrane in eukaryotes.
1. Electron microscopy reveals an organized cytosol.
2. Immunofluorescence microscopy identifies protein fibers.
3. Elements of the cytoskeleton include: actin filaments, intermediate filaments, and microtubules.
Actin Filaments
1. Actin filaments are long, thin fibers (about 7 nm in diameter) that occur in bundles or meshlike
networks.
2. The actin filament consists of two chains of globular actin monomers twisted to form a helix.
3. Actin filaments play a structural role, forming a dense complex web just under the plasma membrane;
this accounts for the formation of pseudopods in amoeboid movement.
4. Actin filaments in microvilli of intestinal cells likely shorten or extend cell into intestine.
5. In plant cells, they form tracks along which chloroplasts circulate.
6. Actin filaments move by interacting with myosin; myosin combines with and splits ATP, binding to
actin and changing configuration to pull actin filament forward.
7. Similar action accounts for pinching off cells during cell division.
Intermediate Filaments
1. Intermediate filaments are 8–11 nm in diameter, between actin filaments and microtubules in size.
2. They are rope-like assemblies of fibrous polypeptides.
3. Some support the nuclear envelope; others support plasma membrane and form cell-to-cell junctions.
Microtubules
1. Microtubules are small hollow cylinders (25 nm in diameter and from 0.2–25 μm in length).
2. Microtubules are composed of a globular protein tubulin that occurs as α tubulin and β tubulin.
3. Assembly brings these two together as dimers and the dimers arrange themselves in rows.
4. Regulation of microtubule assembly is under control of a microtubule organizing center (MTOC): the
main MTOC is called a centrosome.
5. Microtubules radiate from the MTOC, helping maintain the shape of cells and acting as tracks along
which organelles move.
6. Similar to actin-myosin, the motor molecules kinesin and dynein are associated with microtubules.
7. Different kinds of kinesin proteins specialize to move one kind of vesicle or cell organelle.
26
8. Cytoplasmic dynein is similar to the molecule dynein found in flagella.
E. Centrioles
1. Centrioles are short cylinders with a ring pattern (9 + 0) of microtubule triplets.
2. In animal cells and most protists, centrosome contains two centrioles lying at right angles to each
other.
3. Plant and fungal cells have the equivalent of a centrosome, but they do not contain centrioles.
4. Centrioles serve as basal bodies for cilia and flagella.
F. Cilia and Flagella
1. Cilia are short, usually numerous hairlike projections that can move in an undulating fashion (e.g.,
Paramecium; lining of human upper respiratory tract).
2. Flagella are longer, usually fewer, projections that move in whip-like fashion (e.g., sperm cells).
3. Both have similar construction, but differ from prokaryotic flagella.
a. Membrane-bounded cylinders enclose a matrix containing a cylinder of nine pairs of microtubules
encircling two single microtubules (9 + 2 pattern of microtubules).
b. Cilia and flagella move when the microtubules slide past one another.
c. Cilia and flagella have a basal body at base with the same arrangement of microtubule triples as
centrioles.
d. Cilia and flagella grow by the addition of tubulin dimers to their tips.
Lecture Enrichment Ideas
Experience Base: Many students will have had some experience working with a microscope, although the
movement toward using a microscope-mounted television camera to demonstrate slides may remove this critical
experience from a student’s experience base. Providing microscopic or photographic images during lecture and
discussions is critical for students to visualize most cell components.
1.
Describe the history of observing cells—from light microscopes to electron microscopes and other new
techniques. Discuss how research has to await new technology and different methods to allow smaller structures
to be examined.
2.
Describe the difference between magnification (the ability to make something appear larger) and resolution (the
ability to distinguish between two adjacent structures). Discuss why the electron microscope has greater
resolving power than the light microscope at the same magnification.
3.
Consider why bacteria are so numerous and how they compare to eukaryotic cells in size and complexity. For
instance, if a eukaryotic cell is the size of the normal classroom or lecture hall, the bacterium would be the size
of a desk.
4.
Describe the functions of the eukaryotic organelles and how the same cellular functions are conducted in a
bacterial cell.
5.
Follow the pathway a protein molecule might take through the endomembrane system from its production on
the rough endoplasmic reticulum through release from a vesicle at the cell’s surface. How would this pathway
be different if the protein were made on a free ribosome in the cytosol? Explain that the protein’s beginning was
in a particular gene in the DNA.
6.
Display slides or transparencies of eukaryotic organelles as seen through a light microscope or an electron
microscope. Point out the various organellar structures (e.g., cristae) and the functions that are associated with
those functions.
7.
Calculate how a cell (cubelike or spherical) increases its size by doubling its dimensions, and consider the
relationship of volume and surface area. Consider the increase in surface area relative to volume when the cell
is spherical or cuboidal versus elongated or with the surface drawn out into microvilli.
3.
Note that movement (beyond the natural flow of physical currents) is normally perceived by non-scientists as a
mysterious property of life, but is fully understood and explained here as rather simple chemistry, portions of
which can be demonstrated outside the living cell in a test tube.
27
Critical Thinking
Question 1. When living tissues are viewed through the microscope, the cells reveal complex internal movements
that amazed the early microscopists. “Vitalism” is the belief that some additional vital force is necessary to explain
life and this movement inside cells. Why are modern biologists, who see these complex cells and their movements
every day, not “vitalists?”
Answer: As complex as cell movement is, it can be understood along with muscle cell movement, as in chemical
reactions, such as in sliding actin and myosin filaments, and the release of energy from ATP molecules.
Question 2. Freezing and thawing of vegetables destroys their structure because small sharp ice crystals pierce the
cell walls and destroy the structure. Although meat lacks cell walls, repeated freezing and thawing produces the bad
taste of “freezer burn.” What is the main organelle involved in this partial digestion of the cells in the meat?
Answer: The lysosome contains enzymes for self-digestion but these are usually walled off from the cell contents.
Freezing and thawing forms ice crystals that pierce the lysosome wall, and the enzymes begin to pre-digest the cell
contents.
Question 3. How is cell biology linked to the development of scientific technology?
Answer: The recognition of cells and the larger cellular structures had to await the development of the light
microscope. Structural details of most cell organelles were not observed until the development of the electron
microscope. Techniques and equipment to separate cellular components were necessary to study isolated organelles.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
28
CHAPTER
5
MEMBRANE STRUCTURE AND FUNCTION
The complex structure and function of the plasma membrane are described, along with the
macromolecules that comprise the membrane. The mechanisms by which substances move in
and out of cells are discussed, as are the general chemical processes of diffusion and osmosis.
Important cell surface modifications and their significance (e.g., in cellular junctions) are also
detailed.
Chapter Outline
5.1 Plasma Membrane Structure and Function
1. The plasma membrane is a phospholipid bilayer with embedded proteins.
2. Phospholipids have both hydrophilic and hydrophobic regions; nonpolar tails (hydrophobic) are
directed inward, polar heads (hydrophilic) are directed outward to face both extracellular and
intracellular fluid.
3. The proteins form a mosaic pattern on the membrane.
4. Cholesterol is a lipid found in animal plasma membranes; it stiffens and strengthens the membrane.
5. The plasma membrane is asymmetrical; glycolipids and proteins occur only on outside and
cytoskeletal filaments attach to peripheral proteins only on the inside surface.
6. Integral proteins are usually found in the membrane and are held in place by the cytoskeleton and the
extracellular matrix (ECM).
7. ECM are only found in animals and their functions include supporting the plasma membrane and
communicating between cells.
A. Fluid-Mosaic Model
1. The fluid-mosaic model describes the plasma membrane.
2. The fluid component refers to the phospholipids bilayer of the plasma membrane.
3. Fluidity of the plasma membrane allows cells to be pliable.
4. Fluidity is affected by cholesterol molecules in the plasma membrane.
5. The mosaic component refers to the protein content in the plasma membrane.
6. Protins bond to the ECM and/or cytoskeleton to prevent movement in the fluid phospholipid bilayer.
B.
Carbohydrate Chains
1. Glycolipids have a structure similar to phospholipids except the hydrophilic head is a variety of sugar;
they are protective and assist in various functions.
2. Glycoproteins have an attached carbohydrate chain of sugar that projects externally.
3. In animal cells, the glycocalyx is a “sugar coat” of carbohydrate chains; it has several functions.
4. Cells are unique in that they have highly varied carbohydrate chains (a “fingerprint”).
5. The immune system recognizes foreign tissues that have inappropriate carbohydrate chains.
6. Carbohydrate chains are the basis for A, B, and O blood groups in humans.
C. The Functions of the Proteins
1. Channel proteins allow a particular molecule to cross membrane freely (e.g., Cl channels).
2. Carrier proteins selectively interact with a specific molecule so it can cross the plasma membrane
(e.g., Na+-K+ pump).
3. Cell recognition proteins are glycoproteins that allow the body’s immune system to distinguish
between foreign invaders and body cells.
4. Receptor proteins are shaped so a specific molecule (e.g., hormone) can bind to it.
5. Enzymatic proteins carry out specific metabolic reactions.
6. Junction proteins join animal cells so tissues can function.
D. How Do Cells Talk to One Another? (Science Focus box)
1. Cell Signaling
a. Molecules, or chemical messengers, “talk” to other cells and may change cells, tissues, or organs.
b. These cells do not respond to all molecules. They require binding to a receptor protein.
29
c.
Once the signaling molecule is bound to a receptor, the signal follows through a transduction
pathway.
d. The cell’s response to the transduction pathway can change the shape or movement of the cell,
alter the metabolism or function of the cell, or alter the gene expression and amount of a cell
protein.
E. Permeability of the Plasma Membrane
1. The plasma membrane is differentially (selectively) permeable; only certain molecules can pass
through.
a. Small non-charged lipid molecules (alcohol, oxygen) pass through the membrane freely.
b. Small polar molecules (carbon dioxide, water) move “down” a concentration gradient, i.e., from
high to low concentration.
c. Ions and charged molecules cannot readily pass through the hydrophobic component of the bilayer
and usually combine with carrier proteins.
2. Both passive and active mechanisms move molecules across membrane.
a. Passive transport moves molecules across membrane without expenditure of energy; includes
diffusion and facilitated transport.
b. Active transport requires a carrier protein and uses energy (ATP) to move molecules across a
plasma membrane; includes active transport, exocytosis, endocytosis, and pinocytosis.
3. The presence of a membrane channel protein called an aquaporin allows water to cross membranes
quickly.
4. Substances enter or exit a cell through bulk transport.
5.2 Passive Transport Across a Membrane
1. Diffusion is the movement of molecules from higher to lower concentration (i.e., “down” the
concentration gradient).
a. A solution contains a solute, usually a solid, and a solvent, usually a liquid.
b. In the case of a dye diffusing in water, the dye is a solute and water is the solvent.
c. Once a solute is evenly distributed, random movement continues but with no net change.
d. Membrane chemical and physical properties allow only a few types of molecules to cross by
diffusion.
e. Gases readily diffuse through the lipid bilayer; e.g., the movement of oxygen from air sacs
(alveoli) to the blood in lung capillaries depends on the concentration of oxygen in alveoli.
f. Temperature, pressure, electrical currents, and molecular size influence the rate of diffusion.
A. Osmosis
1. Osmosis is the diffusion of water across a differentially (selectively) permeable membrane.
a. Osmosis is illustrated by the thistle tube example:
1) A differentially permeable membrane separates two solutions.
2) The beaker has more water (lower percentage of solute) and the thistle tube has less water
(higher percentage of solute).
3) The membrane does not permit passage of the solute; water enters but the solute does not exit.
4) The membrane permits passage of water with a net movement of water from the beaker to the
inside of the thistle tube.
b. Osmotic pressure is the pressure that develops in such a system due to osmosis.
c. Osmotic pressure results in water being absorbed by the kidneys and water being taken up from
tissue fluid.
2. Tonicity is strength of a solution with respect to osmotic pressure.
a. Isotonic solutions occur where the relative solute concentrations of two solutions are equal; a
0.9% salt solution is used in injections because it is isotonic to red blood cells (RBCs).
b. A hypotonic solution has a solute concentration that is less than another solution; when a cell is
placed in a hypotonic solution, water enters the cell and it may undergo cytolysis (“cell bursting”).
c. Swelling of a plant cell in a hypotonic solution creates turgor pressure; this is how plants
maintain an erect position.
d. A hypertonic solution has a solute concentration that is higher than another solution; when a cell
is placed in a hypertonic solution, it shrivels (a condition called crenation).
e. Plasmolysis is shrinking of the cytoplasm due to osmosis in a hypertonic solution; as the central
vacuole loses water, the plasma membrane pulls away from the cell wall.
3. Facilitated Transport
a. Facilitated transport is the transport of a specific solute “down” or “with” its concentration
30
gradient (from high to low), facilitated by a carrier protein; glucose and amino acids move across
the membrane in this way.
5.3 Active Transport Across a Membrane
A. Active transport is transport of a specific solute across plasma membranes “up” or “against” (from low to
high) its concentration gradient through use of cellular energy (ATP).
1. Iodine is concentrated in cells of thyroid gland, glucose is completely absorbed into lining of digestive
tract, and sodium is mostly reabsorbed by kidney tubule lining.
2. Active transport requires both carrier proteins and ATP; therefore cells must have high number of
mitochondria near membranes where active transport occurs.
3. Proteins involved in active transport are often called “pumps”; the sodium-potassium pump is an
important carrier system in nerve and muscle cells.
4. Salt (NaCl) crosses a plasma membrane because sodium ions are pumped across, and the chloride ion
is attracted to the sodium ion and simply diffuses across specific channels in the membrane.
B. Bulk Transport
1. In exocytosis, a vesicle formed by the Golgi apparatus fuses with the plasma membrane as secretion
occurs; insulin leaves insulin-secreting cells by this method.
2. During endocytosis, cells take in substances by vesicle formation as plasma membrane pinches off by
either phagocytosis, pinocytosis, or receptor-mediated endocytosis.
3. In phagocytosis, cells engulf large particles (e.g., bacteria), forming an endocytic vesicle.
a. Phagocytosis is commonly performed by ameboid-type cells (e.g., amoebas and macrophages).
b. When the endocytic vesicle fuses with a lysosome, digestion of the internalized substance occurs.
4. Pinocytosis occurs when vesicles form around a liquid or very small particles; this is only visible with
electron microscopy.
5. Receptor-mediated endocytosis, a form of pinocytosis, occurs when specific macromolecules bind to
plasma membrane receptors.
a. The receptor proteins are shaped to fit with specific substances (vitamin, hormone, lipoprotein
molecule, etc.), and are found at one location in the plasma membrane.
b. This location is a coated pit with a layer of fibrous protein on the cytoplasmic side; when the
vesicle is uncoated, it may fuse with a lysosome.
c. Pits are associated with exchange of substances between cells (e.g., maternal and fetal blood).
d. This system is selective and more efficient than pinocytosis; it is important in moving substances
from maternal to fetal blood.
e. Cholesterol (transported in a molecule called a low-density lipoprotein, LDL) enters a cell from
the bloodstream via receptors in coated pits; in familial hypocholesterolemia, the LDL receptor
cannot bind to the coated pit and the excess cholesterol accumulates in the circulatory system.
5.4 Modification of Cell Surfaces
A. Cell Surfaces in Animals
1. The extracellular matrix is a meshwork of polysaccharides and proteins produced by animal cells.
a. Collagen gives the matrix strength and elastin gives it resilience.
b. Fibronectins and laminins bind to membrane receptors and permit communication between
matrix and cytoplasm; these proteins also form “highways” that direct the migration of cells
during development.
c. Proteoglycans are glycoproteins that provide a packing gel that joins the various proteins in
matrix and most likely regulate signaling proteins that bind to receptors in the plasma protein.
2. Junctions Between Cells are points of contact between cells that allow them to behave in a
coordinated manner.
a. Anchoring junctions mechanically attach adjacent cells.
b. In adhesion junctions, internal cytoplasmic plaques, firmly attached to cytoskeleton within each
cell, are joined by intercellular filaments; they hold cells together where tissues stretch (e.g., in
heart, stomach, bladder).
c. In desmosomes, a single point of attachment between adjacent cells connects the cytoskeletons of
adjacent cells.
d. In tight junctions, plasma membrane proteins attach in zipper-like fastenings; they hold cells
together so tightly that the tissues are barriers (e.g., epithelial lining of stomach, kidney tubules,
blood-brain barrier).
e. A gap junction allows cells to communicate; formed when two identical plasma membrane
channels join.
31
1) They provide strength to the cells involved and allow the movement of small molecules and
ions from the cytoplasm of one cell to the cytoplasm of the other cell.
2) Gap junctions permit flow of ions for heart muscle and smooth muscle cells to contract.
B. Plant Cell Walls
1. Plant cells are surrounded by a porous cell wall; it varies in thickness, depending on the function of the
cell.
2. Plant cells have a primary cell wall composed of cellulose polymers united into threadlike microfibrils
that form fibrils.
3. Cellulose fibrils form a framework whose spaces are filled by non-cellulose molecules.
4. Pectins allow the cell wall to stretch and are abundant in the middle lamella that holds cells together.
5. Non-cellulose polysaccharides harden the wall of mature cells.
6. Lignin adds strength and is a common ingredient of secondary cell walls in woody plants.
7. Plasmodesmata are narrow membrane-lined channels that pass through cell walls of neighboring cells
and connect their cytoplasms, allowing direct exchange of molecules and ions between neighboring
plant cells.
Lecture Enrichment Ideas
Experience Base: This chapter is heavily laden with terminology and chemical concepts that may
not be familiar to many students. Visuals and demonstrations are critical to understanding
membrane properties, especially the discussion of a “fluid” membrane and the concept of
“hypotonic” and “hypertonic” as pertains to solutions. Be aware that students will likely have a
negative concept of “cholesterol”; stress that it is necessary for animal life and is a vital raw
material for many important metabolic functions and is the basis of many hormones.
1.
2.
3.
4.
5.
6.
Describe how the sugar residues of glycoproteins and glycolipids are located only on the outside face of the
plasma membrane. Emphasize the importance of these molecules in cellular activity.
Examine scanning electron micrographs of freeze-fractured plasma membrane. Determine which face is the
cytoplasmic and which the external side of the membrane. (The proteins are more frequent in the cytoplasmic
face).
Describe the function of cholesterol in the plasma membranes of animal cells, and discuss why cholesterol is
missing in plant cells.
Describe and discuss the functions of the different kinds of proteins that are located in and attached to the
plasma membrane. Discuss why these membrane-associated proteins are also found in membrane-bound
organelles such as vesicles, vacuoles, mitochondria, and chloroplasts.
Prepare dialysis bags containing different molar solutions of saline, and place them in solutions with different
molarities. Determine which beakers represent a cell in a hypotonic, hypertonic, or isotonic solutions. An egg
with its shell “peeled away” or dissolved away with vinegar can also be used; it can swell to the size of an
orange as water continuously enters. (This experiment should be set up at the beginning of class and analyzed at
the end of class.)
Compare the functions of the different kinds of junctions that hold cells together and with what kinds of cells
they would likely be associated. Discuss the possible effects of various chemical agents that alter or eliminate
the function of each type of junction.
Critical Thinking
Question 1. If you do not water your houseplants, they will first wilt, then eventually die. Why do wilting and
dying not occur at the same time?
Answer: A plant maintains its rigid shape by holding water inside a large central vacuole; the water pushes against
32
the plasma membrane, which pushes against the rigid cell wall, and this turgor pressure holds the plant rigid. The
plant wilts when the water level drops to where there is not enough turgor pressure to maintain rigidity, but there is
plenty of water for life processes to continue. Loss of additional water will dehydrate the cell and eventually cause
death of plant tissue.
Question 2. You can “peel” a raw egg without breaking the membrane or “melt” away the shell in vinegar. If you
place it in a glass of distilled, deionized water, it will swell in size until it breaks. Why is the flow one way?
Answer: The egg white is albumin, a huge protein molecule that cannot pass through the membrane. Water is a
small molecule that can easily pass either way across the membrane. Since the water outside is 100% water and the
albumin cannot leave, the percentage of water inside is always lower than 100 percent; thus the water molecules will
continue to move from higher concentration outside the egg to a lower concentration inside.
Question 3. The DNA of a cell codes for sequences of amino acids in proteins but the main component of a plasma
membrane is phospholipids, a molecule that is not a protein. Where does the “new” plasma membrane come from
when cells reproduce?
Answer: As a lipid, the plasma membrane grows by adding lipid molecules from the cellular environment, a process
called accretion. The unique proteins that are embedded in the membrane would be encoded by DNA but all nonprotein elements of a cell must be developed by other cellular mechanisms.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
33
CHAPTER
6
METABOLISM: ENERGY AND ENZYMES
The nature of energy and the laws of thermodynamics are discussed, followed by a detailed
description of energy transformations that occur within the cell. The chemistry and functions of
ATP are described. The role of enzymes in metabolism, oxidation-reduction reactions, and the
cellular organelles in which these reactions take place are detailed.
Chapter Outline
6.1 Cells and the Flow of Energy
A. Forms of Energy
1. Energy is capacity to do work; cells continually use energy to develop, grow, repair, reproduce, etc.
2. Kinetic energy is energy of motion; all moving objects have kinetic energy.
3. Potential energy is stored energy.
4. Food is chemical energy; it contains potential energy.
5. Chemical energy can be converted into mechanical energy, e.g., muscle movement.
B. Two Laws of Thermodynamics
1. First law of thermodynamics (also called the law of conservation of energy)
a. Energy cannot be created or destroyed, but it can be changed from one form to another.
b. In an ecosystem, solar energy is converted to chemical energy by the process of photosynthesis;
some of the chemical energy in the plant is converted to chemical energy in an animal, which in
turn can become mechanical energy or heat loss.
c. Neither the plant nor the animal create energy, they convert it from one form to another.
d. Likewise, energy is not destroyed; some becomes heat that dissipates into the environment.
2. Second law of thermodynamics
a. Energy cannot be changed from one form into another without a loss of usable energy.
b. Heat is a form of energy that dissipates into the environment; heat can never be converted back to
another form of energy.
C. Cells and Entropy
1. Every energy transformation makes the universe less organized and more disordered; entropy is the
term used to indicate the relative amount of disorganization.
2. When ions distribute randomly across a membrane, entropy has increased.
3. Organized/usable forms of energy (as in the glucose molecule) have relatively low entropy;
unorganized/less stable forms have relatively high entropy.
4. Energy conversions result in heat; therefore, the entropy of the universe is always increasing.
5. Living things depend on a constant supply of energy from the sun, because the ultimate fate of all solar
energy in the biosphere is to become randomized in the universe as heat; the living cell is a temporary
repository of order purchased at the cost of a constant flow of energy.
6.2 Metabolic Reactions and Energy Transformations
1. Metabolism is the sum of all the biochemical reactions in a cell.
2. In the reaction A + B = C + D, A and B are reactants and C and D are products.
3. Free energy (G) is the amount of energy that is free to do work after a chemical reaction.
4. Change in free energy is noted as G; a negative G means that products have less free energy than
reactants; the reaction occurs spontaneously.
5. Exergonic reactions have a negative G and energy is released.
6. Endergonic reactions have a positive G; products have more energy than reactants; such reactions
can only occur with an input of energy.
A. ATP: Energy for Cells
1. Adenosine triphosphate (ATP) is the energy currency of cells; when cells need energy, they “spend”
ATP.
2. ATP is an energy carrier for many different types of reactions.
34
3.
B.
6.3
A.
B.
C.
When ATP is converted into ADP + P, the energy released is sufficient for biological reactions with
little wasted.
4. ATP breakdown is coupled to endergonic reactions in a way that minimizes energy loss.
5. ATP is a nucleotide composed of the base adenine and the 5-carbon sugar ribose and three phosphate
groups.
6. When one phosphate group is removed, about 7.3 kcal of energy is released per mole.
Coupled Reactions
1. Coupled reactions are reactions that occur in the same place, at the same time, and in a way that an
exergonic reaction is used to drive an endergonic reaction.
2. ATP breakdown is often coupled to cellular reactions that require energy.
3. ATP supply is maintained by breakdown of glucose during cellular respiration.
Metabolic Pathways and Enzymes
1. Enzymes are catalysts that speed chemical reactions without the enzyme being affected by the
reaction.
2. Every enzyme is specific in its action and catalyzes only one reaction or one type of reaction.
3. Ribozymes are made of RNA rather than proteins and also serve as catalysts.
4. A metabolic pathway is an orderly sequence of linked reactions; each step is catalyzed by a specific
enzyme.
5. Metabolic pathways begin with a particular reactant, end with a particular end product(s), and may
have many intermediate steps.
6. In many instances, one pathway leads to the next; since pathways often have one or more molecules in
common, one pathway can lead to several others.
7. Metabolic energy is captured more easily if it is released in small increments.
8. A reactant is the substance that is converted into a product by the reaction; often many intermediate
steps occur.
Energy of Activation
1. Molecules often do not react with each other unless activated in some way.
2. For metabolic reactions to occur in a cell, an enzyme must usually be present.
3. The energy of activation (Ea) is the energy that must be added to cause molecules to react; without an
enzyme (i.e., in a reaction vessel in the laboratory) this energy may be provided by heat, which causes
an increase in the number of molecular collisions.
Enzyme-Substrate Complex
1. A substrate is a reactant for an enzymatic reaction.
2. Enzymes speed chemical reactions by lowering the energy of activation (E a) by forming a complex
with their substrate(s) at the active site.
a. An active site is a small region on the surface of the enzyme where the substrate(s) bind.
b. When a substrate binds to an enzyme, the active site undergoes a slight change in shape that
facilitates the reaction. This is called the induced fit model of enzyme catalysis.
3. Only a small amount of enzyme is needed in a cell because enzymes are not consumed during
catalysis.
4. Some enzymes (e.g., trypsin) actually participate in the reaction.
5. A particular reactant(s) may produce more than one type of product(s).
a. Presence or absence of enzyme determines which reaction takes place.
b. If reactants can form more than one product, the enzymes present determine which product is
formed.
Factors Affecting Enzymatic Speed
1. Substrate concentration.
a. Because molecules must collide to react, enzyme activity increases as substrate concentration
increases; as more substrate molecules fill active sites, more product is produced per unit time.
2. Optimal pH
a. Every enzyme has optimal pH at which its rate of reaction is optimal.
b. A change in pH can alter the ionization of the R groups of the amino acids in the enzyme, thereby
disrupting the enzyme’s activity.
3. Temperature
a. As temperature rises, enzyme activity increases because there are more enzyme-substrate
collisions.
b. Enzyme activity declines rapidly when enzyme is denatured at a certain temperature, due to a
35
change in shape of the enzyme.
Enzyme cofactors
a. Many enzymes require an inorganic ion or non-protein cofactor to function.
b. Inorganic cofactors are ions of metals.
c. A coenzyme is an organic cofactor, which assists the enzyme (i.e., it may actually contribute atoms
to the reaction).
d. Vitamins are small organic molecules required in trace amounts for synthesis of coenzymes; they
become part of a coenzyme’s molecular structure; vitamin deficiency causes a lack of a specific
coenzyme and therefore a lack of its enzymatic action.
5. Enzyme inhibition
a. Enzyme inhibition occurs when a substance (called an inhibitor) binds to an enzyme and
decreases its activity; normally, enzyme inhibition is reversible.
b. In noncompetitive inhibition, the inhibitor binds to the enzyme at a location other than the active
site (the allosteric site), changing the shape of the enzyme and rendering it unable to bind to its
substrate.
c. In competitive inhibition, the substrate and the inhibitor are both able to bind to the enzyme’s
active site.
D. Enzyme Inhibitors Can Spell Death (Health Focus box)
1. Cyanide
a. Cyanide was used in human executions.
b. Cyanide binds to the mitochondrial enzyme that is necessary for ATP production.
2. MPTP
a. MPTP is an enzyme inhibitor that stops mitochondria from producing ATP.
b. MPTP’s toxic characteristics were discovered in the 1980s, after intravenous drug users developed
syndromes of Parkinson disease.
c. The drug users had injected a synthetic form of heroin contaminated with MPTP.
3. Sarin
a. Sarin inhibits enzymes at neuromuscular junctions.
b. The muscle contraction can’t turn off, muscles do not relax and become paralyzed.
c. In 1995, terrorists in Japan released sarin gas on a subway; 17 people died.
4. Warfarin
a. Warfarin is a chemical produced by the spoiling of the plant sweet clover.
b. Wafarin inhibits an enzyme for blood clotting.
c. Used as a rat poison, pets and small children also can consume the poison.
5. Coumadin
a. Coumadin is a medicine to prevent inappropriate blood clotting.
b. Coumadin contains a nonlethal dose of warfanin.
4.
6.4 Organelles and the Flow of Energy
A. Photosynthesis
1. Photosynthesis uses energy to combine carbon dioxide and water to produce glucose in the formula:
6 CO2 + 6 H2O + energy = C6H12O6 + 6 O2.
2. Oxidation is the loss of electrons.
3. Reduction is the gain of electrons.
4. When hydrogen atoms are transferred to carbon dioxide from water, water has been oxidized and
carbon dioxide has been reduced.
5. Input of energy is needed to produce the high-energy glucose molecule.
6. Chloroplasts capture solar energy and convert it by way of an electron transport system into the
chemical energy of ATP.
7. ATP is used along with hydrogen atoms to reduce glucose; when NADP+ (nicotinamide adenine
dinucleotide phosphate) donates hydrogen atoms (H+ + e) to a substrate during photosynthesis, the
substrate has accepted electrons and is therefore reduced.
8. The reaction that reduces NADP+ is:
NADP+ + 2e + H+ = NADPH.
B. Cellular Respiration
1. The overall equation for cellular respiration is opposite that of photosynthesis:
C6H12O6 + 6 O2 = 6 CO2 + 6 H2O + energy.
36
When NAD removes hydrogen atoms (H+ + e-) during cellular respiration, the substrate has lost
electrons and is therefore oxidized.
3. At the end of cellular respiration, glucose has been oxidized to carbon dioxide and water and ATP
molecules have been produced.
4. In metabolic pathways, most oxidations involve the coenzyme NAD+ (nicotinamide adenine
dinucleotide); the molecule accepts two electrons but only one hydrogen ion:
NAD+ + 2e + H+ = NADH.
C. Electron Transport Chain
1. Both photosynthesis and respiration use an electron transport chain consisting of membrane-bound
carriers that pass electrons from one carrier to another.
2. High-energy electrons are delivered to the system and low-energy electrons leave it.
3. The overall effect is a series of redox reactions; every time electrons transfer to a new carrier, energy is
released for the production of ATP.
D. ATP Production
1 ATP synthesis is coupled to the electron transport system.
2. Peter Mitchell received the 1978 Nobel Prize for his chemiosmotic theory of ATP production.
3. In both mitochondria and chloroplasts, carriers of electron transport systems are located within a
membrane.
4. H+ ions (protons) collect on one side of the membrane because they are pumped there by specific
proteins.
5. The electrochemical gradient thus established across the membrane is used to provide energy for ATP
production.
6. Enzymes and their carrier proteins, called ATP synthase complexes, span the membrane; each
complex contains a channel that allows H+ ions to flow down their electrochemical gradient.
7. In photosynthesis, energized electrons lead to the pumping of hydrogen ions across the thylakoid
membrane; as hydrogen ions flow through the ATP synthase complex, ATP is formed.
8. During cellular respiration, glucose breakdown provides energy for a hydrogen ion gradient on the
inner membrane of the mitochondria that also couples hydrogen ion flow with ATP formation.
2.
Lecture Enrichment Ideas
Experience Base: Generally, American students enter college with less physics and chemistry
coursework than overseas students, so the concepts of catalysts, redox reactions, etc., may be
new. In addition, the concept of entropy may not be well understood.
1.
Explain what happens to the heat that is released from energy transfers on earth. The “greenhouse effect” is
only a temporary storage of heat energy.
2. Describe how life can be organized when the level of entropy (disorganization) in the
universe continues to increase. The universe as a whole is “running down;” an individual
cell, organism, or ecosystem must be considered as an open system that requires a continuous
energy input from outside to maintain its organization. (This is a critical concept to counter
the possible misunderstanding that entropy prevents evolution.)
3.
Explain how an enzyme can lower the activation energy for a reaction and why there is an activation energy
even for a spontaneous reaction.
4. Demonstrate how changes in various conditions and inhibitors can affect the structure of an
enzyme and thus its ability to interact with a substrate to catalyze the enzymatic reaction.
5. Discuss why vitamins (in particular, the water-soluble vitamins) are required only in small
amounts and how they function in the cell.
37
Critical Thinking
Question 1. Why does the growth of a seven pound baby into a 100 pound adolescent not violate the second law of
thermodynamics?
Answer: To produce and support the extra 93 pounds of living tissue, a much larger amount, well over 1,000
pounds of food had to be digested. Thus, we are “islands of complexity” built up amid a sea of entropy. Such islands
of organized life can continue to be built up as long as there is an outside source of energy—the sun, providing that
input of energy. In physics, this would be described as an “open system” since energy is continually supplied from
the outside (the sun) to keep the systems on earth “running.”
Question 2. What are the key organelles that allow energy to flow through living systems?
Answer: The chloroplast and mitochondrion temporarily store energy as chemical energy so some of it flows
through living systems. When chloroplasts carry on photosynthesis, solar energy is used to produce carbohydrates,
and when mitochondria carry on cellular respiration, the energy stored in carbohydrates is converted to energy
stored in ATP. All organisms make use of energy stored in ATP before the energy is eventually lost as heat to the
universe.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
38
CHAPTER
7
PHOTOSYNTHESIS
Photosynthesis is the biochemical process by which organic molecules (i.e., sugars) are
synthesized for use by organisms throughout the food web. Photosynthesis takes place in the
chloroplasts of plant cells (and certain other types of organisms). The process includes the “light
reactions,” in which solar energy is captured, and the “Calvin Cycle reactions” (light
independent reactions, “dark reactions”), in which carbohydrates are synthesized. The two
pathways of light reactions are described, as are the Calvin Cycle reactions. The pigments
involved in photosynthesis are discussed, as are the C3, C4, and CAM photosynthetic
mechanisms.
Chapter Outline
7.1
Photosynthetic Organisms
1.
2.
Photosynthetic organisms (algae, plants, and cyanobacteria) transform solar energy into carbohydrates.
Photosynthetic organisms (plants, algae, cyanobacteria) are called autotrophs because they produce
their own food.
3. Organisms that must take in preformed organic molecules are called heterotrophs.
4. Both autotrophs and heterotrophs use organic molecules produced by photosynthesis as chemical
building blocks and as a source of energy.
A. Flowering Plants as Photosynthesizers
1. Raw materials for photosynthesis are carbon dioxide and water.
2. Roots absorb water that moves up vascular tissue in the stem until it reaches the leaf veins.
3. Carbon dioxide enters a leaf through small openings called stomata.
4. Carbon dioxide and water diffuse into the chloroplasts, the organelles that carry on photosynthesis.
5. In chloroplasts, a double membrane encloses a fluid-filled space called the stroma.
6. An internal membrane system within the stroma forms flattened sacs called thylakoids, which in some
cases are organized into stacks to form grana.
7. Spaces within all thylakoids are connected to form an inner compartment, the thylakoid space.
8. Chlorophyll and other pigments involved in absorption of solar energy reside within thylakoid
membranes; these pigments absorb solar energy, and energize electrons prior to reduction of CO 2 to a
carbohydrate.
7.2 The Process of Photosynthesis
1. The net equation of photosynthesis reads: 6CO2 + 6H2O = C6 H12O6 + 6O2.
2. Photosynthesis involves oxidation-reduction, where the carbon dioxide has been reduced by hydrogen
atoms and energy, and the water has been oxidized.
a. Solar energy is not used directly, but rather converted to ATP molecules.
b. Electrons required to reduce carbon dioxide are carried by coenzyme, NADP +.
3. In 1930, van Niel showed that O2 given off by photosynthesis comes from water and not from CO 2.
A. Two Sets of Reactions
1.
2.
3.
In 1905, Blackman proposed two sets of reactions for photosynthesis.
Light reactions take place only in the presence of light.
a. Light reactions are the energy-capturing reactions.
b. Chlorophyl within thylakoid membranes absorbs solar energy and energizes electrons.
c. When energized electrons move down an electron transport chain, energy is captured and used for
ATP production.
d. Energized electrons are also taken up by NADP +, converting it to NADPH.
Calvin cycle reactions
a. These reactions take place in the stroma; the reactions can occur in either the presence or the
39
b.
absence of light.
These are synthetic reactions that use NADPH and ATP to reduce CO2.
7.3 Plants as Solar Energy Converters
1. Higher energy wavelengths are screened out by the ozone layer in the upper atmosphere.
2. Lower energy wavelengths are screened out by water vapor and CO2.
3. Both the organic molecules within organisms and certain processes (e.g., vision, photosynthesis) are
adapted to visible light, the radiation that is most prevalent in the environment.
4. Photosynthetic pigments use primarily the visible light portion of the electromagnetic spectrum.
5. Pigments found in chlorophyll absorb various portions of visible light; this is called their absorption
spectrum.
6. Two major photosynthetic pigments are chlorophyll a and chlorophyll b.
7. Both chlorophylls absorb violet, blue, and red wavelengths best.
8. Very little green light is absorbed; most is reflected (this is why leaves appear green).
9. Carotenoids are yellow-orange pigments that absorb light in violet, blue, and green regions.
10. When chlorophyll breaks down in the fall, the yellow-orange pigments in leaves show through.
11. Absorption and action spectrum
a. A spectrophotometer measures the amount of light that passes through a sample.
1) As light is shone on a sample, some wavelengths are absorbed and others pass through the
sample.
2) A graph of percent of light absorbed at each wavelength is a compound’s absorption
spectrum.
A Light Reactions
1.
2.
This light reaction requires participation of two light-gathering units: photosystem I (PS I) and
photosystem II (PS II).
A photosystem is a photosynthetic unit comprised of a pigment complex and an electron acceptor;
solar energy is absorbed and high-energy electrons are generated.
B. Noncyclic Electron Pathway
1.
The noncyclic pathway begins with PSII; electrons move from H 2O through PS II to PS I and then on
to NADP+.
2. The PS II pigment complex absorbs solar energy; high-energy electrons (e) leave the reaction-center
chlorophyll a molecule.
3. PS II takes replacement electrons from H2O, which splits, releasing O2 and H+ ions:
H2O = 2 H+ + 2 e + ½ O2.
4. Oxygen is released as oxygen gas (O2).
5. The H+ ions temporarily stay within the thylakoid space and contribute to a H + ion gradient.
6. As H+ flow down electrochemical gradient through ATP synthase complexes, chemiosmosis occurs.
7. Low-energy electrons leaving the electron transport system enter PS I.
8. When the PS I pigment complex absorbs solar energy, high-energy electrons leave reaction-center
chlorophyll a and are captured by an electron acceptor.
9. The electron acceptor passes them on to NADP +.
10. NADP+ takes on an H+ to become NADPH: NADP+ + 2 e + H+ = NADPH.
11. NADPH and ATP (produced by noncyclic-flow electrons in the thylakoid membrane) are used by
enzymes in the stroma during the light-independent (dark) reactions.
C. The Organization of the Thylakoid Membrane
1. PS II consists of a pigment complex and electron-acceptor molecules; it oxidizes H2O and produces O2.
2. The electron transport system consists of cytochrome complexes and transports electrons and pumps H +
ions into the thylakoid space.
3. PS I has a pigment complex and electron-acceptor molecules; it is associated with an enzyme that
reduces NADP+ to NADPH.
4. ATP synthase complex has an H+ channel and ATP synthase; it produces ATP.
D. ATP Production
1. The thylakoid space acts as a reservoir for H+ ions; each time H2O is split, two H+ remain.
2. Electrons move carrier-to-carrier, giving up energy used to pump H + from the stroma into the thylakoid
space.
3. Flow of H+ from high to low concentration across thylakoid membrane provides energy to produce
40
ATP from ADP + P by using an ATP synthase enzyme.
4. This is called chemiosmosis because ATP production is tied to an electrochemical (H+) gradient.
E. Tropical Rain Forest Destruction and Global Warming (Ecology Focus box)
1. Global warming is an unexpected rise in the average global temperature during the 21 st century due to
the introduction of certain gases into the atmosphere.
2. For more than 1000 years before 1850, carbon dioxide levels remained fairly constant at 0.028%.
3. Following the 1850s (marked by the industrial revolution), the amount of carbon dioxide in the
atmosphere increased to 0.038%.
4. Role of Carbon Dioxide
a. Carbon dioxide, as well as other gases, traps radiant heat from the sun.
b. Factors adding carbon dioxide to the atmosphere include: burning of fossil fuels, and destructing
tropical rain forests.
5. Role of Tropical Rain Forests
a. Ten – 30 million hectares of rain forests are lost every year due to ranching, logging, and mining.
b. Each year, tropical rain forest deforestation accounts for 20–30% of all carbon dioxide in the
atmosphere.
c. Destruction of tropical rain forests is also troublesome because burning a forest adds carbon
dioxide to the atmosphere, and also removes trees that normally would absorb carbon dioxide.
6. The Argument for Preserving Forests
a. Tropical rain forests contribute to the uptake of carbon dioxide, and the productivity of
photosynthesis.
b. Tropical rain forests exist between the Tropic of Cancer and Tropic of Capricorn, temperatures are
about 26 C, and rainfall is 100-200 cm and regular.
c. Tropical rain forest tree characteristics include: large trees, buttressed trunks, broad, simple darkgreen leaves, and vines (lianas).
d. Researchers suggest that as temperatures rise, tropical rain forests may add to atmospheric carbon
dioxide accumulation and accelerate global warming rather than the reverse.
e. To combat deforestation, some countries, such as Costa Rica, have developed national park
systems and reserves to protect the forests from destruction.
7.4
Calvin Cycle Reactions
1.
A.
B.
C.
D.
The Calvin cycle is a series of reactions producing carbohydrates; these reactions follow the light
reactions.
2. The cycle is named for Melvin Calvin who used a radioactive isotope of carbon to trace the reactions.
3. The Calvin cycle includes carbon dioxide fixation, carbon dioxide reduction, and regeneration of
ribulose 1,5-bisphosphate (RuBP).
Fixation of Carbon Dioxide
1. CO2 fixation is the attachment of CO2 to an organic compound called RuBP.
2. RuBP (ribulose bisphosphate) is a five-carbon molecule that combines with carbon dioxide; the
resulting 6-carbon molecule then splits into two 3-carbon molecules.
3. The enzyme RuBP carboxylase (rubisco) speeds this reaction; this enzyme comprises 20–50% of the
protein content of chloroplasts—it is an unusually slow enzyme.
Reduction of Carbon Dioxide
1. With the reduction of carbon dioxide, a 3PG (3-phosphoglycerate) molecule forms.
2. Each of two 3PG molecules undergoes reduction to G3P (glyceraldehyde-3-phosphate) in two steps.
3. Light-dependent reactions provide NADPH (electrons) and ATP (energy) to reduce 3PG to G3P.
Regeneration of RuBP
1. For every three turns of the Calvin cycle, five molecules of G3P are used to re-form three molecules of
RuBP.
2. This reaction also uses ATP produced by the light reactions.
The Importance of the Calvin Cycle
1. G3P, the product of the Calvin Cycle can be converted into many other molecules.
2. Glucose phosphate is one result of G3P metabolism; it is a common energy molecule.
3. Glucose phosphate can bond with fructose to form sucrose.
4. Glucose phosphate is the starting point for synthesis of starch and cellulose.
5. The hydrocarbon skeleton of G3P is used to form fatty acids and glycerol; the addition of nitrogen
forms various amino acids.
41
7.5 Other Types of Photosynthesis
1. In C3 plants, the Calvin cycle fixes CO2 directly; the first molecule following CO2 fixation is 3PG.
2. In hot weather, stomata close to save water; CO2 concentration decreases in leaves; O2 increases.
3. This is called photorespiration since oxygen is taken up and CO2 is produced; this produces only one
3PG.
A. C4 Photosynthesis
1. In a C3 plant, mesophyll cells contain well-formed chloroplasts, arranged in parallel layers.
2. In C4 plants, bundle sheath cells as well as the mesophyll cells contain chloroplasts.
3. In C4 leaf, mesophyll cells are arranged concentrically around the bundle sheath cells.
4. C3 plants use RuBP carboxylase to fix CO2 to RuBP in mesophyll; the first detected molecule is G3P.
5. C4 plants use the enzyme PEP carboxylase (PEPCase) to fix CO 2 to PEP (phosphoenolpyruvate, a C3
molecule); the end product is oxaloacetate (a C4 molecule).
6. In C4 plants, CO2 is taken up in mesophyll cells and malate, a reduced form of oxaloacetate, is pumped
into the bundle-sheath cells; here CO2 enters Calvin cycle.
7. In hot, dry climates, net photosynthetic rate of C4 plants (e.g., corn) is 2–3 times that of C4 plants.
8. Photorespiration does not occur in C4 leaves because PEPCase does not combine with O2; even when
stomates are closed, CO2 is delivered to the Calvin cycle in bundle sheath cells.
9. C4 plants have advantage over C3 plants in hot and dry weather because photorespiration does not
occur; e.g., bluegrass (C3) dominates lawns in early summer, whereas crabgrass (C4) takes over in the
hot midsummer.
B. CAM Photosynthesis
1. CAM (crassulacean-acid metabolism) plants form a C4 molecule at night when stomates can open
without loss of water; found in many succulent desert plants including the family Crassulaceae.
2. At night, CAM plants use PEPCase to fix CO2 by forming C4 molecule stored in large vacuoles in
mesophyll.
3. C4 formed at night is broken down to CO2 during the day and enters the Calvin cycle within the same
cell, which now has NADPH and ATP available to it from the light-dependent reactions.
4. CAM plants open stomates only at night, allowing CO2 to enter photosynthesizing tissues; during the
day, stomates are closed to conserve water but CO2 cannot enter photosynthesizing tissues.
5. Photosynthesis in a CAM plant is minimal, due to limited amount of CO 2 fixed at night; but this does
allow CAM plants to live under stressful conditions.
C. Photosynthesis and Adaptation to the Environment
1. Each method of photosynthesis has its advantages, depending on the environment.
2. C4 plants are adapted to areas of high light intensities, high temperatures, and limited rainfall.
3. C3 plants do better in cooler climates.
4. CAM plants do well in an arid environment.
Lecture Enrichment Ideas
Experience Base: United States farm populations have dropped from nearly one-third of the
population after World War II to less than one percent today; as a result far fewer students have
any agricultural experience (crops, farms, etc.). Many students will have had no real experience
with simple gardening. As such, the use of visual media, lab experiments, and field work are
necessary in describing the concepts of photosynthesis.
1. Describe the relationship between the reactants and products of photosynthesis, paying
special attention to the carbohydrate ratio (CH2O). Note particularly that the water is split
into oxygen and the hydrogen that goes into the carbohydrate, and emphasize that the carbon
dioxide is involved in carbohydrate production.
2. Explain how electrons lose energy as they move “down” an electron transport system and
how some of that energy is used to move hydrogen ions across the thylakoid membrane to
form the electrochemical gradient used in making ATP.
42
3. Discuss how adding photosystem II to photosystem I makes photosynthesis more efficient,
and link the production of organic molecules (carbohydrates) to the production of ATP from
solar energy.
4.
Emphasize that the reaction as given for photosynthesis is actually a summary of numerous reactions that occur
in both light-dependent and light-independent reactions. Separate the parts that occur in each group of reactions.
5.
Stress that the various photosynthetic pathways are adaptations to various environments; none is superior under
all conditions. Discuss what would happen if one pathway was always superior in every environmental
condition.
Critical Thinking
Question 1. An astronaut is on a spaceship moving closer to the sun. The food supply is supplemented by an
onboard greenhouse. The light intensity is increasing and for visual comfort, the astronaut must block out some
sunlight. What color of glass-reflecting pane should be used to preserve the maximum photosynthesis, and why?
Answer: Given no other limitations, blocking out green light would do the least damage to the
plants since they use less light in the green wavelength and reflect green themselves.
Theoretically, if all green light was prevented from entering, and the leaves absorbed all other
visible wavelengths, the leaves in the space greenhouse would appear black!
Question 2. The Dutchman Van Helmont grew a tree in a large pot and found the soil did not
add to the weight of the tree. Unaware of the gases in the air, he therefore concluded a tree was
made of water, the only thing else that he was aware was added in his experiment. How could
you prove to Van Helmont that something in the air was also involved in making up the tree's
substance?
Answer: Many strategies are possible.
1) The air openings (stomata) on tree leaves are on the underside. Coating the upper surface of a
leaf with petroleum jelly does not affect its growth; coating its underside does.
2) Sealing a plant inside a vacuum will kill it; however, early researchers contended a vital force
had been shut off—the same force that kept a flame “alive.” This can be countered by sealing a
plant inside a jar with only carbon dioxide; this will often improve growth.
3) Modern radioisotope tracers can be used to trace the flow of molecules through organisms.
Students can invent other experimental procedures based upon plausible concepts.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
43
CHAPTER
8
CELLULAR RESPIRATION
This chapter undertakes a detailed study of the cellular process of respiration. The biochemical
reactions occurring in the cytoplasm (glycolysis, fermentation) and in the mitochondria (citric
acid cycle, electron transport, oxidative phosphorylation) are described, as are the molecules that
participate in the reactions. Oxidation-reduction chemistry is emphasized. The energy yield (i.e.,
ATP) production is calculated for the complete oxidation of a glucose molecule, and is compared
with the energy yield under anaerobic conditions. Other anabolic and catabolic reactions
occurring in the cell are discussed.
Chapter Outline
8.1 Cellular Respiration
1.
Cellular respiration involves various metabolic pathways that break down carbohydrates and other
metabolites with the concomitant buildup of ATP.
2. Cellular respiration consumes oxygen and produces CO2; because oxygen is required, cellular
respiration is aerobic.
3. Cellular respiration usually involves the complete breakdown of glucose into CO 2 and H2O.
4. The net equation for glucose breakdown is: C6H12O6 + 6 O2 = 6 CO2 + 6 H2O + energy.
5. Glucose is a high-energy molecule; CO2 and H2O are low-energy molecules; cellular respiration is thus
exergonic because it releases energy.
6. Electrons are removed from substrates and received by oxygen, which combines with H + to become
water.
7. Glucose is oxidized and O2 is reduced.
8. The reactions of cellular respiration allow energy in glucose to be released slowly; therefore ATP is
produced gradually.
9. In contrast, if glucose were broken down rapidly, most of its energy would be lost as non-usable heat.
10. The breakdown of glucose yields synthesis of 36 or 38 ATP (depending on certain conditions); this
preserves about 39% of the energy available in glucose.
11. This is relatively efficient compared to, for example, the 25% efficiency of a car burning gasoline.
A. NAD+ and FAD
1. Each metabolic reaction in cellular respiration is catalyzed by a specific enzyme.
2. As a metabolite is oxidized, NAD+ (nicotinamide adenine dinucleotide) accepts two electrons and a
hydrogen ion (H+); this results in NADH + H+.
3. Electrons received by NAD+ and FAD are high-energy electrons and are usually carried to the electron
transport chain.
4. NAD+ is a coenzyme of oxidation-reduction since it both accepts and gives up electrons; thus, NAD + is
sometimes called a redox coenzyme.
5. Only a small amount of NAD+ is needed in cells because each NAD+ molecule is used repeatedly.
6. FAD coenzyme of oxidation-reduction can replace NAD+; FAD accepts two electrons and two
hydrogen ions to become FADH2.
B. Phases of Cellular Respiration
1. Cellular respiration includes four phases:
a. Glycolysis is the breakdown of glucose in the cytoplasm into two molecules of pyruvate.
1) Enough energy is released for an immediate yield of two ATP.
2) Glycolysis takes place outside the mitochondria and does not utilize oxygen; it is therefore an
anaerobic process.
b. In the preparatory (prep) reaction, pyruvate enters a mitochondrion and is oxidized to a twocarbon acetyl group and CO2 is removed; this reaction occurs twice per glucose molecule.
c. The citric acid cycle:
1) occurs in the matrix of the mitochondrion and produces NADH and FADH2;
2) is a series of reactions that gives off CO2 and produces one ATP;
44
d.
3) turns twice because two acetyl-CoA molecules enter the cycle per glucose molecule;
4) produces two immediate ATP molecules per glucose molecule.
The electron transport chain:
1) is a series of carriers in the inner mitochondrial membrane that accept electrons from
glucose-electrons are passed from carrier to carrier until received by oxygen;
2) passes electrons from higher to lower energy states, allowing energy to be released and stored
for ATP production;
8.2 Outside the Mitochondria: Glycolysis
1.
2.
3.
A.
B.
8.3
A.
B.
C.
Glycolysis occurs in the cytoplasm outside the mitochondria.
Glycolysis is the breakdown of glucose into two pyruvate molecules.
Glycolysis is universally found in organisms; therefore, it likely evolved before the citric acid cycle
and electron transport chain.
4. Thycolosis can be divided into the energy-investment steps where ATP is used to “jump start”
glycolosis, and the energy-harvesting steps, where more ATP is made than used.
Energy-Investment Steps
1. Glycolysis begins with the activation of glucose with two ATP; the glucose splits into two C 3
molecules known as G3P, each of which carries a phosphate group.
Energy-Harvesting Steps
1. Oxidation of G3P occurs by removal of electrons and hydrogen ions.
2. Two electrons and one hydrogen ion are accepted by NAD +, resulting in two NADH; later, when the
NADH molecules pass two electrons to the electron transport chain, they become NAD + again.
3. The oxidation of G3P and subsequent substrates results in four high-energy phosphate groups, which
are used to synthesize four ATP molecules; this process is called substrate-level phosphorylation.
4. Two of four ATP molecules produced are required to replace two ATP molecules used in the initial
phosphorylation of glucose; therefore there is a net gain of two ATP from glycolysis.
5. Pyruvate enters a mitochondrion (if oxygen is available) and cellular respiration ensues.
6. If oxygen is not available, fermentation occurs and pyruvate undergoes reduction.
Fermentation
1. Fermentation is an anaerobic (i.e., occurs in the absence of oxygen) process which consists of
glycolysis plus reduction of pyruvate to either lactate or to alcohol and CO 2 (depending on the
organism).
2. Animal cell fermentation results in lactate.
3. Bacteria can produce an organic acid like lactate, or an alcohol and CO 2.
4. Yeasts produce ethyl alcohol and CO2.
5. NADH passes its electrons to pyruvate instead of to an electron transport chain; NAD + is then free to
return and pick up more electrons during earlier reactions of glycolysis.
Advantages and Disadvantages of Fermentation
1. Despite a low yield of two ATP molecules, fermentation provides a quick burst of ATP energy for
muscular activity.
2. Fermentation products are toxic to cells.
a. When blood cannot remove all lactate from muscles, lactate changes pH and causes muscles to
fatigue.
b. The individual is in oxygen debt because oxygen is needed to restore ATP levels and rid the body
of lactate.
c. Recovery occurs after lactate is sent to the liver where it is converted into pyruvate; some pyruvate
is then respired or converted back into glucose.
Efficiency of Fermentation
1. Two ATP produced per glucose molecule during fermentation is equivalent to 14.6 kcal.
2. Complete glucose breakdown to CO2 and H2O during cellular respiration represents a potential yield of
686 kcal per molecule.
3. Efficiency of fermentation is 14.6/686 or about 2.1%, far less efficient than complete breakdown of
glucose.
Fermentation Helps Produce Numerous Food Products (Science Focus box)
1. Yeast Fermentation
a. Baker’s yeast, Saccharomyces cerevisiae, is added to bread for leavening. The dough rises when
yeasts give off CO2.
45
b.
2.
3.
Yeasts ferment the carbohydrates of fruit to produce ethyl alcohol in wine, and ferment grains to
produce ethyl alcohol in beer.
c. The acetic acid bacteria, Acetobacter aceti, spoil wine, and convert the alcohol to acetic acide to
produce vinegar.
Bacterial Fermentation
a. Lactic acid bacteria cause milk to sour and produce yogurt, sour cream, and cheese.
b. Brine cucumber pickles, sauerkraut, and kimchi are pickled vegetables produced by acid
producing fermenting bacteria.
Soy Sauce Production
a. Yeast and fermenting bacteria are added to soy beans and wheat to produce soy sauce.
8.4 Inside the Mitochondria
1.
The next reactions of cellular respiration involve the preparatory reaction, the citric acid cycle, and
the electron transport chain.
2. These reactions occur in the mitochondria.
3. A mitochondrion has a double membrane with an intermembrane space (between the outer and inner
membrane).
4. Cristae are the inner folds of membrane that jut into the matrix, the innermost compartment of a
mitochondrion that is filled with a gel-like fluid.
5. The prep reaction and citric acid cycle enzymes are in the matrix; the electron transport chain is in the
cristae.
6. Most of the ATP produced in cellular respiration is produced in the mitochondria; therefore,
mitochondria are often called the “powerhouses” of the cell.
A. Preparatory Reaction
1. The preparatory reaction connects glycolysis to the citric acid cycle.
2. In this reaction, pyruvate is converted to a two-carbon acetyl group, and is attached to coenzyme A,
resulting in the compound acetyl-CoA.
3. This redox reaction removes electrons from pyruvate by a dehydrogenase enzyme, using NAD + as a
coenzyme.
4. This reaction occurs twice for each glucose molecule.
5. CoA carry the acetyl group to the citric acid cycle.
6. The two NADH carry electrons to the electron transport chain.
7. The CO2 diffuses out of animal cells into blood, transported to lungs, and exhaled.
B. Citric Acid Cycle
1. The citric acid cycle occurs in the matrix of mitochondria.
2. The cycle is sometimes called the Krebs cycle, named for Sir Hans Krebs, who described the
fundamentals of the reactions in the 1930s.
3. The cycle begins by the addition of a two-carbon acetyl group to a four-carbon molecule, forming a
six-carbon citrate (citric acid) molecule.
4. In the subsequent reactions, at three different times two electrons and one hydrogen ion are accepted
by NAD+, forming NADH.
5. At one time, two electrons and one hydrogen ion are accepted by FAD, forming FADH 2.
6. NADH and FADH2 carry these electrons to the electron transport chain.
7. Some energy is released and is used to synthesize ATP by substrate-level phosphorylation.
8. One high-energy metabolite accepts a phosphate group and transfers it to convert ADP to ATP.
9. The citric acid cycle turns twice for each original glucose molecule.
10. The products of the citric acid cycle (per glucose molecule) are 4 CO 2, 2 ATP, 6 NADH, and 2
FADH2.
11. Production of CO2
a. The six carbon atoms in the glucose molecule have now become the carbon atoms of six CO 2
molecules, two from the prep reaction and four from the citric acid cycle.
C. The Electron Transport Chain
1. The electron transport chain is located in the cristae of mitochondria and consists of carriers that pass
electrons successively from one to another.
2. NADH and FADH2 carry the electrons to the electron transport system.
3. Members of the Chain
a. NADH gives up its electrons and becomes NAD +; the next carrier then gains electrons and is
46
thereby reduced.
At each sequential redox reaction, energy is released to form ATP molecules.
Some of the protein carriers are cytochrome molecules, complex carbon rings with a heme (iron)
group in the center.
4. Cycling of Carriers
a. By the time electrons are received by O2, three ATP have been made.
b. When FADH2 delivers electrons to the electron transport system, two ATP are formed by the time
the electrons are received by O2.
c. Oxygen serves as the terminal electron acceptor and combines with hydrogen ions to form water.
5. The Cristae of a Mitochondrion and Chemiosmosis
a. The electron transport chain consists of three protein complexes and two protein mobile carriers
that transport electrons.
b. The three protein complexes include NADH-Q reductase complex, the cytochrome reductase
complex, and the cytochrome oxidase complex; the two protein mobile carriers are coenzyme Q
and cytochrome c.
c. Energy released from the flow of electrons down the electron transport chain is used to pump H +
ions, which are carried by NADH and FADH2, into intermembrane space.
d. Accumulation of H+ ions in this intermembrane space creates a strong electrochemical gradient.
e. ATP synthase complexes are channel proteins that serve as enzymes for ATP synthesis.
f. As H+ ions flow from high to low concentration, ATP synthase synthesizes ATP by the reaction:
ADP + P = ATP.
g. Chemiosmosis is the term used for ATP production tied to an electrochemical (H +) gradient
across a membrane.
h. Once formed, ATP molecules diffuse out of the mitochondrial matrix through channel proteins.
i. ATP is the energy currency for all living things; all organisms must continuously produce high
levels of ATP to survive.
D. Energy Yield from Glucose Metabolism
1. Substrate-Level Phosphorylation
a. Per glucose molecule, there is a net gain of two ATP from glycolysis in cytoplasm.
b. The citric acid cycle in the matrix of the mitochondria produces two ATP per glucose.
c. Thus, a total of four ATP are formed by substrate-level phosphorylation outside of the electron
transport chain.
2. ETC and Chemiosmosis
a. Most of the ATP is produced by the electron transport chain and chemiosmosis.
b. Per glucose, ten NADH and two FADH2 molecules provide electrons and H+ ions to the electron
transport chain.
c. For each NADH formed within the mitochondrion, three ATP are produced.
d. For each FADH2 formed by the citric acid cycle, two ATP are produced.
e. For each NADH formed outside mitochondria by glycolysis, two ATP are produced as electrons
are shuttled across the mitochondrial membrane by an organic molecule and delivered to FAD.
3. Efficiency of Cellular Respiration
a. The energy difference between total reactants (glucose and O 2) and products (CO2 and H2O) is
686 kcal.
b. An ATP phosphate bond has an energy of 7.3 kcal; 36 to 38 ATP are produced during glucose
breakdown for a total of at least 263 kcal.
c. This efficiency is 263/686, or 39% of the available energy in glucose is transferred to ATP; the
rest of the energy is lost as heat.
b.
c.
8.5
Metabolic Pool
1. In a metabolic pool, substrates serve as entry points for degeneration or synthesis of larger molecules.
2. Degradative reactions (catabolism) break down molecules; they tend to be exergonic.
3. Synthetic reactions (anabolism) build molecules; they tend to be endergonic.
A. Catabolism
1. Just as glucose is broken down in cellular respiration, other molecules in the cell undergo catabolism.
2. Fat breaks down into glycerol and three fatty acids.
a. Glycerol is converted to G3P, a metabolite in glycolysis.
b. An 18-carbon fatty acid is converted to nine acetyl-CoA molecules that enter the citric acid cycle.
47
c.
Respiration of fat products can produce 108 kcal in ATP molecules; fats are an efficient form of
stored energy.
3. Amino acids break down into carbon chains and amino groups.
a. Hydrolysis of proteins results in amino acids.
b. R-group size determines whether carbon chain is oxidized in glycolysis or the citric acid cycle.
c. A carbon skeleton is produced in the liver by removal of the amino group, by the process of
deamination.
d. The amino group becomes ammonia (NH3), which enters the urea cycle and ultimately becomes
part of excreted urea.
e. The size of the R-group determines the number of carbons left after deamination.
B. Anabolism
1. ATP produced during catabolism drives anabolism.
2. Substrates making up pathways can be used as starting materials for synthetic reactions.
3. The molecules used for biosynthesis constitute the cell’s metabolic pool.
4. Carbohydrates can result in fat synthesis: G3P converts to glycerol; acetyl groups join to form fatty
acids.
5. Some metabolites can be converted to amino acids by transamination, the transfer of an amino acid
group to an organic acid.
6. Plants synthesize all the amino acids they need; animals lack some enzymes needed to make some
amino acids.
7. Humans synthesize 11 of 20 amino acids; the remaining 9 essential amino acids must be provided by
the diet.
C. The Energy Organelles Revisited
1. Chloroplasts and mitochondria may be related based on their likeness, yet they carry out opposite
processes.
a. The inner membrane of the chloroplasts forms the thylakoids of the grana. The inner membrane of
the mitochondrion forms the convoluted cristae.
b. In chloroplasts the electrons passed down the ETC have been energized by the sun. In
mitochondria the electrons passed down the ETC have been removed from glucose products.
c. In chloroplasts the stroma contains the enzymes of the Calvin cycle. In the mitochondria the
matrix contains the enzymes of the citric acid cycle.
2. Flow of Energy
a. Energy flows through organisms. For example, the sun is the energy source for producing
carbohydrates in chloroplasts. In the mitochondria, the carbohydrate energy is converted into ATP
molecules during cellular respiration.
b. Chemicals cycle throughout cells. Mitochondria use carbohydrates and oxygen produced in
chloroplasts, and chloroplasts use carbon dioxide and water produced in the mitochondria.
Lecture Enrichment Ideas
Experience Base: Some students are confused when the term “respiration” is also used for the
ventilation or breathing process; early distinction will help avoid confusion.
1. Trace the flow of carbon atoms through glycolysis and cellular respiration, focusing on the
points at which they enter and leave each set of reactions.
2. Do the same with hydrogen, and be sure to emphasize that some of the hydrogens being
carried off by NADH and FADH2 are coming from water and are not the remnants of the
glucose. (Water can be placed on both sides of the general equation for cellular respiration to
account for this entry of water into the process.)
3. Point out the chemical relationships between photosynthesis and cellular respiration; note
that these are complementary processes that are part of the interrelatedness of all life and the
cyclic renewal of reactants and products.
4. Compare and contrast the production of ATP through chemiosmosis in the mitochondrion
48
and the chloroplast.
5. Discuss how the production and breakdown of ATP must be cyclic and extremely rapid to
allow the many chemical reactions to occur. Note the limits on the production of ATP (i.e.,
must have a pool of ADP and phosphate), and how recycling of these materials must take
place across the outer membrane of the mitochondria.
6.
Rotifers and tardigrades are unique in being able to completely shut down cellular respiration and begin it again
decades later. While biologists are uncomfortable with the concept of calling such inanimate stages “dead,”
these organisms are not carrying on active metabolism. Therefore the definition of life must for these organisms
be modified to the “potential to resume metabolism.” (A similar concept can be addressed when studying the
endospores of bacteria.) Even though most water is lost, a high concentration of trehalose sugar appears to help
the dried cells maintain structural integrity and the ability to resume life when water is restored.
Critical Thinking
Question 1. Cyanide interrupts the cytochrome system of electron transport. Why is cyanide a
universal poison effective in all organisms with mitochondria?
Answer: The electron transport system is the producer of ATP from ADP in aerobic respiration.
Stopping the electron transport system stops ATP production, which stops metabolic reactions,
and this is essentially a universal system.
Question 2. Breaking apart a molecule through combination with oxygen is burning. However, this is a “slow
burn” inside of cells. Today, we hear of “spontaneous human combustion” where people allegedly burn up from
runaway metabolism. Why is it impossible for rapid direct oxidation to originate in the cell environment?
Answer: Fire requires fuel, an abundant supply of gaseous oxygen, and a kindling temperature. The body does have
fats, sugars, and other good fuel molecules. However, cells are 70 –90 percent water, oxygen is sparse and dissolved
in this fluid, and the kindling temperature is far beyond the body temperatures that permit life or even boiling! In
addition, the cellular respiration process described in this chapter shows how the breakdown of the glucose molecule
is slowed by stages located in various parts of the cell and by metabolic pathways that take time, spreading out the
“burn” so the released energy can be harvested for ATP rather than lost as heat.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
49
PART
II
GENETIC BASIS OF LIFE
The science of Genetics is explained using a historical approach with genetic problems that stress practical
aspects. The cell cycle, human genetics, cancer concepts, and biotechnology have been updated.
9 The Cell Cycle and Cellular Reproduction
10 Meiosis and Sexual Reproduction
11
12
13
14
Mendelian Patterns of Inheritance
Molecular Biology of the Gene
Regulation of Gene Activity
Biotechnology and Genomics
CHAPTER
9 THE CELL CYCLE AND CELLULAR REPRODUCTION
The stages of the cell cycle are described in detail, as are currently-understood mechanisms by which the
cell cycle is controlled and regulated. The structures of the eukaryotic and prokaryotic chromosome are
described, as are the processes involved in chromosomal replication. Mitosis and cytokinesis are examined.
Current issues, such as stem cells, therapeutic cloning, and reproductive cloning are discussed. A detailed
study of cancer is presented.
Chapter Outline
9.1 The Cell Cycle
1. The cell cycle is an orderly set of stages from the first division to the time the
daughter cells divide.
2.
When a cell is preparing for division, it grows larger, the number of organelles doubles, and
the DNA replicates.
A. Interphase
1. Most of a cell’s life is spent in interphase, in which the cell performs its usual functions.
2. Time spent in interphase varies by cell type: nerve and muscle cells do not complete the cell
cycle and remain in the G0 stage while embryonic cells complete the cycle every few hours.
3. The G1 stage is just prior to DNA replication; a cell grows in size, organelles increase in
number, and material accumulates for DNA synthesis.
4. The S stage is the DNA synthesis (replication) period; proteins associated with DNA are also
synthesized; at the end of the S stage, each chromosome has two identical DNA double helix
molecules, called sister chromatids.
5. The G2 stage occurs just prior to cell division; the cell synthesizes proteins needed for cell
division, such as proteins in microtubules.
6. Interphase therefore consists of G1, S, and G2.
B. M (Mitotic) Stage
1. M stage (M = mitosis) is the entire cell division stage, including both mitosis and cytokinesis.
2. Mitosis (also known as karyokinesis) is nuclear division, cytokinesis is division of the
cytoplasm.
3. When division of the cytoplasm is complete, two daughter cells are produced.
C. Control of the Cell Cycle
1. A signal is an agent that influences the activities of a cell.
50
2.
3.
Growth factors are external signals received at the plasma membrane.
Cell Cycle Checkpoints
a. There appear to be three checkpoints where the cell cycle either stops or
continues onward, depending on the internal signals it receives.
b. Researchers have identified a family of proteins called cyclins, internal signals that
increase or decrease during the cell cycle.
c. Cyclin must be present for the cell to move from the G1 stage to the S
stage, and from the G2 stage to the M stage.
d. The primary checkpoint of the cell cycle is the G1 checkpoint.
e. DNA damage stops the cycle at the G1 checkpoint by the protein p53; if
the DNA is not repaired, p53 triggers apoptosis.
f. Another protein, RB (retinoblastoma), is responsible for interpreting
growth and nutrient availability signals.
g. The cell cycle stops at the G2 stage if DNA has not finished replicating;
stopping the cell cycle at this stage allows time for repair of possible
damaged DNA.
h. Also, the cycle stops if chromosomes are not properly attached to the
mitotic spindle.
D. Apoptosis
1. Apoptosis is programmed cell death and involves a sequence of cellular events involving:
a. fragmenting of the nucleus,
b. blistering of the plasma membrane, and
c. engulfing of cell fragments by macrophages and/or neighboring cells.
2. Apoptosis is caused by enzymes called caspases.
3. Cells normally hold caspases in check with inhibitors.
4. Caspases are released by internal or external signals.
5. Apoptosis and cell division are balancing processes that maintain the normal level of somatic
(body) cells.
6. Cell death is a normal and necessary part of development: frogs, for example, must destroy
tail tissue they used as tadpoles, and the human embryo must eliminate webbing found
between fingers and toes.
7. Death by apoptosis prevents a tumor from developing.
E. The G1 Checkpoint (Science Focus box)
1. During cell division, only certain cells in an adult body are actively dividing.
2. Once cell division occurs, cells enter G1 stage.
3. Following the G1 stage, cells enter the G1 checkpoint to make sure that the correct conditions
occur to continue cell division.
4. Evaluating Growth Signals
a. Signal molecules are sent to encourage or discourage cells from entering the cell cycle.
Cells may enter a G0 stage, complete G1 and enter S stage.
b. Cell division promoting signals can cause a cyclin-dependent-kinase (CDK) to add a
phosphate group to RB, which is a regulator of the G 1 checkpoint.
c. When RB is phosphorylated, the shape of RB changes and it releases the protein E2F,
rather than binding to E2F.
d. E2F then binds to DNA and activates genes to complete the cell cycle.
e. If growth signals are sufficient, a cell will pass through the G 1 checkpoint and cell
division will occur.
5. Determining Nutrient Availability
a. Cells require adequate nutrient levels prior to cell division.
b. When nutrients are available, CDKs bring about phosphorylation of RB, which releases
E2F. E2F binds to DNA to produce proteins.
c. When nutrients are not available, the cell enters the G0 stage and does not progress to the
G1 stage.
6. Assessing DNA Integrity
51
a.
b.
DNA must be free of errors and damage in order for cell division to occur.
If DNA damage is detected, CDK phosphorylates p53. Rather than being broken down,
p53 levels in the nucleus rise.
c. The phosphorylated p53 binds to DNA to produce DNA repair proteins.
d. If DNA damage cannot be repaired, p53 levels continue to rise to trigger apoptosis.
However, if DNA damage is repaired, p53 levels fall, and cell completes G 1 stage as long
as other requirements are met.
9.2 Mitosis and Cytokinesis
A. Eukaryotic Chromosomes
1. DNA in chromosomes of eukaryotic cells is associated with proteins; histones organize
chromosomes.
2. When a cell is not undergoing division, DNA in the nucleus is a tangled mass of threads
called chromatin.
3. At cell division, chromatin becomes highly coiled and condensed and is now visible as
individual chromosomes.
4. Each species has a characteristic number of chromosomes.
a. The diploid (2n) number includes two sets of chromosomes of each type.
1) The diploid number is found in all the non-sex cells of an organism’s
body (with a few exceptions).
b. The haploid (n) number contains one of each kind of chromosome.
1) In the life cycle of many animals, only sperm and egg cells have the
haploid number.
B. Preparations for Mitosis
1. Cell division in eukaryotes involves nuclear division and cytokinesis.
a. Somatic cells undergo mitosis for development, growth, and repair.
1) This nuclear division leaves the chromosome number constant.
2) A 2n nucleus replicates and divides to provide daughter nuclei that are
also 2n.
b. A chromosome begins cell division with two sister chromatids.
1) Sister chromatids are two strands of genetically identical
chromosomes.
2) At the beginning of cell division, they are attached at a centromere, a
region of constriction on a chromosome.
2. The centrosome, the main microtubule organizing center of the cell, divides
before mitosis begins.
3. Each centrosome contains a pair of barrel-shaped organelles called centrioles.
4. The mitotic spindle contains many fibers, each composed of a bundle of
microtubules.
5.
Microtubules are made of the protein tubulin.
a. Microtubules assemble when tubulin subunits join, disassemble when tubulin subunits
become free, and form interconnected filaments of cytoskeleton.
b. Microtubules disassemble as spindle fibers form.
C. Phases of Mitosis
1. Mitosis is divided into five phases: prophase, prometaphase, metaphase, anaphase, and
telophase.
2. Prophase
a. Nuclear division is about to occur: chromatin condenses and chromosomes become
visible.
b. The nucleolus disappears and the nuclear envelope fragments.
c. Duplicated chromosomes are composed of two sister chromatids held together by a
centromere; chromosomes have no particular orientation in the cell at this time.
d. The spindle begins to assemble as pairs of centrosomes migrate away from each other.
52
e. An array of microtubules called asters radiates toward the plasma membrane from the
centrosomes.
3. Prometaphase (Late Prophase)
a. Specialized protein complexes (kinetochores) develop on each side of the centromere for
future chromosome orientation.
b. An important event during prometaphase is attachment of the chromosomes to the spindle
and their movement as they align at the metaphase plate (equator) of the spindle.
c. The kinetochores of sister chromatids capture kinetochore spindle fibers.
d. Chromosomes move back and forth toward alignment at the metaphase plate.
4. Metaphase
a. Chromosomes, attached to kinetochore fibers, are now aligned at the metaphase plate.
b. Non-attached spindle fibers, called polar spindle fibers, can reach beyond the metaphase
plate and overlap.
c. A cell checkpoint called the M checkpoint delays the start of anaphase until kinetochores
are properly attached to the spindle fibers, and chromosomes are properly aligned along
metaphase plate.
5. Anaphase
a. The two sister chromatids of each duplicated chromosome separate at the centromere.
b. Daughter chromosomes, each with a centromere and single chromatid, move to opposite
poles.
1) Polar spindle fibers lengthen as they slide past each other.
2) Kinetochore spindle fibers disassemble at the kinetochores; this pulls
daughter chromosomes to poles.
3) The motor molecules kinesin and dynein are involved in this sliding
process.
4) Anaphase is the shortest stage of mitosis.
6.
Telophase
a. Spindle disappears in this stage.
b. The nuclear envelope reforms around the daughter chromosomes.
c. The daughter chromosomes diffuse, again forming chromatin.
d. The nucleolus reappears in each daughter nucleus.
D. Cytokinesis in Animal and Plant Cells
1. Cytokinesis in Animal Cells
a. A cleavage furrow indents the plasma membrane between the two
daughter nuclei at a midpoint; this deepens to divide the cytoplasm during
cell division.
b. Cytoplasmic cleavage begins as anaphase draws to a close and organelles
are distributed.
c. The cleavage furrow deepens as a band of actin filaments, called the
contractile ring, constricts between the two daughter cells.
d. A narrow bridge exists between daughter cells during telophase until
constriction completely separates the cytoplasm.
2.
Cytokinesis in Plant Cells
a. The rigid cell wall that surrounds plant cells does not permit cytokinesis
by furrowing.
b. The Golgi apparatus produces vesicles, which move along the
microtubules to a small flattened disc that has formed.
c. Vesicles fuse forming a cell plate; their membranes complete the plasma
membranes of the daughter cells.
d. The new membrane also releases molecules from the new plant cell walls;
the cell walls are strengthened by the addition of cellulose fibrils.
E. The Functions of Mitosis
53
1. Mitosis permits growth and repair.
2. In flowering plants, the meristematic tissue retains the ability to divide
throughout the life of the plant; this accounts for the continued growth, both in
height and laterally, of a plant.
3.
In mammals, mitosis is necessary as a fertilized egg becomes an embryo and as the embryo
becomes a fetus; throughout life, mitosis allows a cut to heal or a broken bone to mend.
F. Stem Cells
1. Many mammalian organs contain stem cells (or adult stem cells), which retain
the ability to divide.
2. Red bone marrow stem cells repeatedly divide to produce the various types of
blood cells.
3. Therapeutic cloning to produce human tissues can begin with either adult
stem cells or embryonic stem cells.
4. Embryonic stem cells can be used for reproductive cloning, the production of
a new individual.
G. Reproductive and Therapeutic Cloning (Science Focus box)
1. There are two types of cloning: reproductive cloning and therapeutic cloning.
2. Reproductive (somatic cell) cloning
a. In reproductive cloning, the donor cells are first starved, then the nucleus
is taken out of cell and transplanted into enucleated egg.
b. The donor cell stops dividing and goes into G0 stage.
c. Embryonic stem cells are formed and implanted into the embryo of a
surrogate mother.
d. When embryo is fully developed, a clone is born.
3. Therapeutic cloning
a. One way to conduct therapeutic cloning is by the same procedure as
reproductive cloning, embryonic stem cells are separated and subjected to
treatment that causes it to develop into different types of cells: red blood
cells, muscle cells, nerve cells.
b. The other way to conduct therapeutic cloning is to use adult stem cells.
c. One drawback to using adult stem cells is they are limited in the number
of cell types they may become. However, researchers are trying to
overcome this obstacle.
9.3 The Cell Cycle and Cancer
1. Cancer is a cellular growth disorder that occurs when cells divide uncontrollably; i.e., cancer
results from the loss of control and a disruption of the cell cycle.
2. Most cancers begin as abnormal cell growth that is benign, or not cancerous
3. When additional mutations occur, the growth becomes malignant, or cancerous, and
possesses the ability to spread.
A. Characteristics of Cancer Cells
1. Cancer cells lack differentiation.
a. Unlike normal cells that differentiate into muscle or nerves cells, cancer cells have an
abnormal form and are nonspecialized.
b. Normal cells enter the cell cycle only about 50 times; cancer cells are immortal in that
they can enter the cell cycle repeatedly.
2. Cancer cells have abnormal nuclei.
a. The nuclei may be enlarged and may have an abnormal number of chromosomes.
b. The chromosomes have mutated; some chromosomes may be duplicated or deleted.
3. Cancer cells do not undergo apoptosis
a. Whereas ordinary cells with DNA damage undergo apoptosis, cancer cells do not.
54
4.
5.
Cancer cells form tumors.
a. Normal cells are anchored and stop dividing when in contact with other cells; i.e., they
exhibit contact inhibition.
b. Cancer cells invade and destroy normal tissue and their growth is not inhibited.
c. Cancer cells pile on top of each other to form a tumor.
Cancer cells undergo metastasis and angiogenesis.
a. A benign tumor is encapsulated and does not invade adjacent tissue.
b. Many types of cancer can undergo metastasis, in which new tumors form which are
distant from the primary tumor.
c. Angiogenesis, the formation of new blood vessels, is required to bring nutrients and
oxygen to the tumor.
B. Origin of Cancer
1.
2.
3.
4.
Proto-oncogenes code for proteins that stimulate the cell cycle and prevent apoptosis.
Tumor-suppressor genes code for proteins that inhibit the cell cycle and promote apoptosis.
Mutations of either of these genes can cause cancer.
Proto-oncogenes are at the end of a stimulatory pathway from the plasma membrane to the
nucleus; a growth factor binding at the plasma membrane can result in turning on an
oncogene.
5. Proto-oncogenes can undergo mutation to become oncogenes (cancer-causing genes).
6. An oncogene may code for a faulty receptor in the stimulatory pathway.
7. Or an oncogene can specify an abnormal protein product or abnormally high levels of a
normal product that stimulates the cell cycle.
8. Tumor-suppressor genes are at the end of an inhibitory pathway; a growth-inhibitory factor
can result in turning on a tumor suppressor gene that inhibits the cell cycle.
9. The balance between stimulatory and inhibitory signals determines whether proto-oncogenes
or
tumor-suppressor genes are active, and therefore whether or not cell division occurs.
10. Researchers have identified about a half dozen tumor-suppressor genes.
11. The RB tumor-suppressor gene prevents retinoblastoma, a cancer of the retina, and has been
found to malfunction in cancers of the breast, prostate, bladder, and small-cell lung
carcinoma.
12. The p53 tumor-suppressor gene is more frequently mutated in human cancers than any other
known gene; it normally functions to trigger cell cycle inhibitors and stimulate apoptosis.
13. In some cancer cells, mutation of an enzyme that regulates the length of telomeres causes the
telomeres to remain at a constant length, which allows the cancer cells to continue dividing.
9.4 Prokaryotic Cell Division
1. Unicellular organisms reproduce via asexual reproduction, in which the offspring are
genetically identical to the parent.
A. The Prokaryotic Chromosome
1. Prokaryotic cells (bacteria and archaea) lack a nucleus and other membranous organelles.
2. The prokaryotic chromosome is composed of DNA and associated proteins,
but much less protein than eukaryotic chromosomes.
3. The chromosome appears as a nucleoid, an irregular-shaped region that is not
enclosed by a membrane.
4. The chromosome is a circular loop attached to the inside of the plasma
membrane; it is about 1,000 times the length of the cell.
B. Binary Fission
1. Binary fission of prokaryotic cells produces two genetically identical
daughter cells.
2. Before cell division, DNA is replicated—both chromosomes are attached to a
special site inside the plasma membrane.
3. The two chromosomes separate as a cell lengthens and pulls them apart.
4. When the cell is approximately twice its original length, the plasma membrane
grows inward, a septum (consisting of new cell wall and plasma membrane)
55
forms, dividing the cell into two daughter cells.
5. The generation time of bacteria depends on the species and environmental
conditions; Escherichia coli’s generation time is about 20 minutes.
C. Comparing Prokaryotes and Eukaryotes
1. Both binary fission and mitosis ensure that each daughter cell is genetically
identical to the parent.
2. Bacteria and protists use asexual reproduction to produce identical offspring.
3. In multicellular fungi, plants, and animals, cell division is part of the growth
process that produces and repairs the organism.
4. Prokaryotes have a single chromosome with mostly DNA and some associated
protein; there is no spindle apparatus.
5. Eukaryotic cells have chromosomes with DNA and many associated proteins;
histone proteins organize the chromosome.
6. The spindle is involved in distributing the daughter chromosomes to the
daughter nuclei.
Lecture Enrichment Ideas
Experience Base: Without laboratory experience with cell division, most of the cell
cycle will be abstract and visuals will be critical for any meaningful understanding.
Public concepts of cancer are riddled with misconceptions, especially since
understanding based on convergent research has become clear recently. However, most
students are eager to learn the meaning of terms they have heard in unclear context, such
as metastasis, malignant, neoplasm, etc.
1. Be sure to clarify terms that closely resemble each other:
chromosome:chromatin:chromatid, centriole:centromere, etc.
2. Contrast the differences in cell division between binary fission and mitosis with
respect to: the amount of DNA, the number of chromosomes needed to govern
smaller and larger cells, and the increased needs of a multicellular organism’s
genome. Note that DNA replication must occur before mitosis begins and that each
chromatid represents one copy of the DNA molecule.
3. Emphasize why it is important that each daughter cell receives exactly one copy of
each gene in the genome of the organism. Discuss how mitosis is more effective in
separating the many strands of DNA in a human (46 chromosomes) than binary
fission.
4. Discuss the parts of the cell cycle and what might provide the direct and indirect
control of the cycle. Why would it be important to have internal control when a cell
replicates its DNA or starts to divide? Consider why external signals would be
important in controlling the division of cells in a multicellular organism.
5.
Explain how a modification of cell cycle controls and control mechanisms for cell division could
contribute to cancer development.
6.
Most students will have some experience (relative, friend, etc.) with cancer and know some of the
symptoms, diagnosis, treatment, etc. If they wish to volunteer personal observations, it is possible to
point out the “filter points” for cancer in lungs, liver, etc., and the reasons that many treatment regimes
56
make sense in the light of this.
7. Describe the differences between and discuss the uses of reproductive cloning and
therapeutic cloning. These are state-of-the-art research and medical techniques of
which students need to have a working knowledge.
Critical Thinking
Question 1. Human red blood cells develop in the bone marrow from stem cells, and
lose their nucleus before being released into the bloodstream. While this gives a cell that
can be densely packed with hemoglobin molecules, what are the consequences as far as
the longevity of the cell and its ability to replicate?
Answer: Lacking DNA coding, the red blood cells will not themselves be able to
replicate, and we must rely on the stem cells in the bone marrow to continue red blood
cell production. In addition, the red blood cells will be limited in life span and unable to
make DNA-encoded repairs.
Question 2. In some birds and true bugs, the number of chromosomes is hard to determine since
chromosomes get smaller and smaller until they are too small to see. Yet, in animals it is rare to find
chromosomes numbering over a hundred pairs. What is the probable reason for keeping chromosome
numbers low?
Answer: A spindle forms to provide an orderly distribution of chromosomes to the
daughter cells. It is not practical for spindle fibers to accurately engineer the separation of
thousands of chromosomal bits in a minuscule cell with limited space.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
57
CHAPTER
10
MEIOSIS AND SEXUAL REPRODUCTION
Meiosis occurs at different times in the life cycle of plants, animals, and fungi, but its phases are the same. In
humans, meiosis is a part of spermatogenesis and oogenesis. This chapter presents a detailed description of the
stages of meiosis, and various meiotically-related mechanisms involved in genetic variation in the offspring. Meiosis
and mitosis are compared and contrasted. The contribution of meiosis to the evolutionary process is discussed. The
abnormal situations of chromosome number change (e.g., monosomy, trisomy, etc.) are described, as are the
situations of chromosome structural change (e.g., deletion, translocation, etc.). Many human diseases involving
chromosomal abnormalities are described.
Chapter Outline
10.1 Halving the Chromosome Number
1. Meiosis is nuclear division, reducing the chromosome number from the diploid (2n)
to the haploid (n) number.
2. The haploid (n) number is half of the diploid number of chromosomes.
3. Sexual reproduction requires gamete (reproductive cell, often sperm and egg)
formation and then fusion of gametes to form a zygote.
4. A zygote always has the full or diploid (2n) number of chromosomes.
5. If gametes contained same number of chromosomes as body cells, doubling would
soon fill cells.
A. Homologous Pairs of Chromosomes
1. In diploid body cells, chromosomes occur as pairs.
a. Each set of chromosomes is a homologous pair; each member is a homologous
chromosome or homologue.
b. Homologues look alike, have the same length and centromere position, and have a
similar banding pattern when stained.
c. A location on one homologue contains gene for the same trait that occurs at this
locus on the other homologue, although the genes may code for different
variations of that trait; alternate forms of a gene are called alleles.
2. Chromosomes duplicate immediately prior to nuclear division.
a. Duplication produces two identical parts called sister chromatids; they are held
together at the centromere.
3. One member of each homologous pair is inherited from the male parent, the other
member from the female parent.
4. One member of each homologous pair will be placed in each sperm or egg.
B. Overview of Meiosis
1. Meiosis involves two nuclear divisions and produces four haploid daughter cells.
2. Each daughter cell has half the number of chromosomes found in the diploid parent
nucleus.
3. Meiosis I is the nuclear division at the first meiotic division.
a. Prior to meiosis I, DNA replication occurs, each chromosome thus has two sister
chromatids.
b. During meiosis I, homologous chromosomes pair; this is called synapsis.
58
c. During synapsis, the two sets of paired chromosomes lay alongside each other as
a bivalent (sometimes called a tetrad).
4. In meiosis II, the centromeres divide and daughter chromosomes (derived as sister
chromatids) separate.
a. No replication of DNA is needed between meiosis I and II because chromosomes
are already doubled (DNA replication occurred prior to meiosis I).
b. Chromosomes in the four daughter cells have only one chromatid.
c. Counting the number of centromeres verifies that parent cells were diploid; each
daughter cell is haploid.
d. In the animal life cycle, daughter cells become gametes that fuse during
fertilization.
e. Fertilization restores the diploid number in cells.
C. Fate of Daughter Cells
1. In plant life cycle, daughter cells become haploid spores that germinate to become a haploid
generation.
2. In the animal life cycle, the daughter cells become the gametes, either sperm or egg.
10.2 Genetic Variation
A. Genetic Recombination
1. Due to genetic recombination, offspring have a different combination of genes than
their parents.
2. Without recombination, asexual organisms must rely on mutations to generate
variation among offspring; this is sufficient because they have great numbers of
offspring.
3. Meiosis brings about genetic recombination in two ways: crossing-over and
independent assortment.
4. Crossing-over of non-sister chromatids results in exchange of genetic material
between non-sister chromatids of a bivalent; this introduces variation.
5. At synapsis, homologous chromosomes are held in position by a nucleoprotein lattice
(the synaptonemal complex).
6. As the lattice of the synaptonemal complex breaks down at the beginning of anaphase
I, homologues are temporarily held together by chiasmata, regions where the nonsister chromatids are attached due to crossing-over.
7. The homologues separate and are distributed to daughter cells.
8. Due to this genetic recombination, daughter chromosomes derived from sister
chromatids are no longer identical.
B. Independent Assortment of Homologous Chromosomes
1. During independent assortment, the homologous chromosomes separate
independently or in a random manner.
2. Independent assortment in a cell with only three pairs of chromosomes is 23 or eight
combinations of maternal and paternal chromosomes.
C. Significance of Genetic Variation
1. In humans with 23 pairs of chromosomes, the combinations possible are 223 or
8,388,608, and this does not consider the variation from crossing-over.
2. When gametes fuse at fertilization, chromosomes donated by parents combine.
3. The chromosomally different zygotes from same parents have (223)2 or
70,368,744,000,000 combinations possible without crossing-over.
4. If crossing-over occurs once, then (423)2 or 4,951,760,200,000,000,000,000,000,000
genetically different zygotes are possible for one couple.
59
5. A successful parent in a particular environment can reproduce asexually and produce
offspring adapted to that environment.
6. If the environment changes, differences among offspring provide the sexual parents
with much improved chances of survival.
10.3 The Phases of Meiosis
A.
B.
C.
D.
E.
F.
1. Both meiosis I and meiosis II have four phases: prophase, metaphase, anaphase, and
telophase.
Prophase I
1. Nuclear division is about to occur: nucleolus disappears; nuclear envelope fragments;
centrosomes migrate away from each other; and spindle fibers assemble.
2. Homologous chromosomes undergo synapsis to form bivalents; crossing-over may
occur at this time in which case sister chromatids are no longer identical.
3. Chromatin condenses and chromosomes become microscopically visible.
Metaphase I
1. Bivalents held together by chiasmata have moved toward the metaphase plate at the
equator of the spindle.
2. In metaphase I, there is a fully formed spindle and alignment of the bivalents at the
metaphase plate.
3. Kinetochores, protein complexes just outside the centromeres attach to spindle fibers
called kinetochore spindle fibers.
4. Bivalents independently align themselves at the metaphase plate of the spindle.
5. Maternal and paternal homologues of each bivalent may be oriented toward either
pole.
Anaphase I
1. The homologues of each bivalent separate and move toward opposite poles.
2. Each chromosome still has two chromatids.
Telophase I
1. In animals, this stage occurs at the end of meiosis I.
2. When it occurs, the nuclear envelope reforms and nucleoli reappear.
3. This phase may or may not be accompanied by cytokinesis.
Interkinesis
1. This period between meiosis I and II is similar to the interphase between mitotic
divisions; however, no DNA replication occurs (the chromosomes are already
duplicated).
Meiosis II and Gamete Formation
1. During metaphase II, the haploid number of chromosomes align at the metaphase
plate.
2. During anaphase II, the sister chromatids separate at the centromeres; the two
daughter chromosomes move toward the poles.
3. Due to crossing-over, each gamete can contain chromosomes with different types of
genes.
4. At the end of telophase II and cytokinesis, there are four haploid cells.
5. In animals, the haploid cells mature and develop into gametes.
6. In plants, the daughter cells become spores and divide to produce a haploid
generation; these haploid cells fuse to become a zygote that develops into a diploid
generation.
7. The type of life cycle of alternating haploid and diploid generations is called
60
alternation of generations.
10.4 Meiosis Compared to Mitosis
1. Meiosis requires two nuclear divisions; mitosis requires only one nuclear division.
2. Meiosis produces four daughter nuclei and four daughter cells; mitosis produces only
two.
3. The daughter cells produced by meiosis are haploid; the daughter cells produced by
mitosis are diploid.
4. The daughter cells produced by meiosis are not genetically identical; the daughter
cells produced by mitosis are genetically identical to each other and to the parental
cell.
A Occurrence
1. In humans, meiosis occurs only in reproductive organs to produce gametes.
2. Mitosis occurs in all tissues for growth and repair.
B. Meiosis I Compared to Mitosis
1. DNA is replicated only once before both mitosis and meiosis; in mitosis there is only
one nuclear division; in meiosis there are two nuclear divisions.
2. During prophase I of meiosis, homologous chromosomes pair and undergo crossingover; this does not occur during mitosis.
3. During metaphase I of meiosis, bivalents align at the metaphase plate; in mitosis
individual chromosomes align.
4. During anaphase I in meiosis, homologous chromosomes (with centromeres intact)
separate and move to opposite poles; in mitosis at this stage, sister chromatids
separate and move to opposite poles.
C. Meiosis II Compared to Mitosis
1. Events of meiosis II are the same stages as in mitosis.
2. However, in meiosis II, the nuclei contain the haploid number of chromosomes.
10.5 The Human Life Cycle
1. Life cycle refers to all reproductive events between one generation and next.
2. In animals, the adult is always diploid [Instructors note: some bees, etc., have haploid
male adults].
3. In plants, there are two adult stages: one is diploid (called the sporophyte) and one is
haploid (called the gametophyte).
4. Mosses are haploid most of their cycle; the majority of higher plants are diploid most
of their cycle.
5. In fungi and some algae, only the zygote is diploid, and it undergoes meiosis.
6. In human males, meiosis is part of spermatogenesis (the production of sperm), and
occurs in the testes.
7. In human females, meiosis is part of oogenesis (the production of eggs), and occurs in
the ovaries.
8. After birth, mitotic cell division is involved in growth and tissue regeneration of
somatic tissue.
A. Spermatogenesis and Oogenesis in Humans
1. Spermatogenesis
a. In the testes of males, primary spermatocytes with 46 chromosomes undergo
meiosis I to form two secondary spermatocytes, each with 23 duplicated
chromosomes.
b. Secondary spermatocytes divide (meiosis II) to produce four spermatids, also
61
with 23 daughter chromosomes.
c. Spermatids then differentiate into sperm (spermatozoa).
d. Meiotic cell division in males always results in four cells that become sperm.
2. Oogenesis
a. In the ovaries of human females, primary oocytes with 46 chromosomes undergo
meiosis I to form two cells, each with 23 duplicated chromosomes.
b. One of the cells, a secondary oocyte, receives almost all the cytoplasm; the other
cell, a polar body, disintegrates or divides again.
c. The secondary oocyte begins meiosis II and then stops at metaphase II.
d. At ovulation, the secondary oocyte leaves the ovary and enters an oviduct where it
may meet a sperm.
e. If a sperm enters secondary oocyte, the oocyte is activated to continue meiosis II
to completion; the result is a mature egg and another polar body, each with 23
daughter chromosomes.
f. Meiosis produces one egg and three polar bodies; polar bodies serve to discard
unnecessary chromosomes and retain most of the cytoplasm in the egg.
g. The cytoplasm serves as a source of nutrients for the developing embryo.
10.6 Change in Chromosome Number and Structure
A. Aneuploidy
1. Chromosomal mutations are changes in chromosome number or structure.
2. Mutations, along with crossing-over, recombination of chromosomes during meiosis,
and gamete fusion during fertilization, increase the amount of variation among
offspring.
3. The correct number of chromosomes in a species is called euploidy; changes in
chromosome number resulting from nondisjunction during meiosis is called
aneuploidy.
4. Monosomy (2n – 1) occurs when an individual has only one of a particular type of
chromosome.
5. Trisomy (2n + 1) occurs when an individual has three of a particular type of
chromosome.
6. Nondisjunction is the failure of chromosomes to separate at meiosis—both members
of the homologous pair go into the same gamete.
a. Primary nondisjunction occurs during meiosis I when both members of a
homologous pair go into the same daughter cell
b. Secondary nondisjunction occurs during meiosis II when the sister chromatids
fail to separate and both daughter chromosomes go into the same gamete.
7. Monosomy and trisomy occur in plants and animals; in autosomes of animals, it is
generally lethal.
8. Trisomy 21 is the most common autosomal trisomy.
a. Trisomy 21 (also called Down syndrome) occurs when three copies of
chromosome 21 are present.
b. Usually two copies of chromosome 21 are contributed by the egg; in 23% of the
cases, the sperm had the extra chromosome 21.
c. A Down syndrome child has many characteristic signs and symptoms, including a
tendency for leukemia, cataracts, faster aging, mental retardation, and an
increased chance of developing Alzheimer disease later in life.
d. Chances of a woman having a Down syndrome child increase with age.
62
e. A karyotype, a visual display of the chromosomes arranged by shape, size, and
banding pattern, may be performed to identify babies with Down syndrome and
other aneuploid conditions.
9. Nondisjunction during oogenesis can result in too few or too many X chromosomes;
nondisjunction during spermatogenesis can result in missing or too many Y
chromosomes.
10. Turner syndrome females have only one sex chromosome, an X; thus, they are XO,
with O signifying the absence of a second sex chromosome.
a. Turner females are short, have a broad chest and folds of skin on back of neck.
b. Ovaries of Turner females never become functional; therefore, females do not
undergo puberty.
c. They usually have normal intelligence and can lead fairly normal lives with
hormone supplements.
11. Klinefelter syndrome males have one Y chromosome and two or more X
chromosomes (e.g., XXY).
a. Affected individuals are sterile males; the testes and prostate are underdeveloped.
b. Individuals have large hands and feet, long arms and legs, and lack facial hair.
c. Presence of the Y chromosome drives male formation but more than two X
chromosomes may result in mental retardation.
d. A Barr body, usually only seen in the nuclei of a female’s cells, is seen in this
syndrome due to the two X chromosomes.
12. Poly-X females (or superfemale) have three or more X chromosomes and therefore
extra Barr bodies in the nucleus.
a. There is no increased femininity; most lack any physical abnormalities.
b. XXX individuals are not mentally retarded but may have delayed motor and
language development; XXXX females are usually tall and severely mentally
retarded.
c. Some experience menstrual irregularities but many menstruate regularly and are
fertile.
13. Jacobs syndrome (XYY) are males with two Y chromosomes instead of one.
a. This results from nondisjunction during spermatogenesis.
b. Males are usually taller than average, suffer from persistent acne, and tend to have
speech and reading problems.
c. Earlier claims that XYY individuals were likely to be aggressive were not correct.
B. Living with Klinefelter Syndrome (Health Focus box)
1. Stefan Schwartz is a man who has Klinefelter syndrome.
2. As a child, some of his symptoms included being shy, trouble making friends, and
emotional instability.
3. Teacher and doctors thought he had “learning disabilities,” and was lazy.
4. Eventually a doctor tested his blood and discovered he had Klinefelter sydrome, a
genetic disorder having sex chromosomes XXY.
5. Physical treatment can include testosterone injection. Emotional treatment can
include support groups.
C.
Changes in Chromosome Structure
1. Environmental factors including radiation, chemicals, and viruses, can cause
chromosomes to break; if the broken ends do not rejoin in the same pattern, this
causes a change in chromosomal structure.
63
2. Deletion: a type of mutation in which an end of a chromosome breaks off or when
two simultaneous breaks lead to the loss of a segment.
3. Translocation: a chromosomal segment is removed from one chromosome and
inserted into another nonhomologous chromosome; in Down syndrome, 5% of cases
are due to a translocation between chromosome 21 and 14, a situation that runs in the
family of the father or mother.
4. Duplication: the presence of a chromosomal segment more than once on the same
chromosome.
a. A broken segment from one chromosome can simply attach to its homologue or
unequal crossing-over may occur.
b. A duplication may also involve an inversion where a segment that has become
separated from the chromosome is reinserted at the same place but in reverse; the
position and sequence of genes are altered.
D. Human Syndromes
1. Deletion Syndromes
a. Williams syndrome occurs when chromosome 7 loses an end piece: children look
like pixies, have poor academic skills but good verbal and musical skills; lack of
elastin causes cardiovascular problems and skin aging.
b. Cri du chat syndrome (“cry of the cat”) is a deletion in which an individual has a
small head, is mentally retarded, has facial abnormalities, and an abnormal glottis
and larynx resulting in a cry resembling that of a cat.
2. Translocation Syndromes
a. If a translocation results in the normal amount of genetic material, the person will
remain healthy; if a person inherits only one of the translocated chromosomes,
that person may have only one allele or three alleles rather than the normal two.
b. In Alagille syndrome, chromosomes 2 and 20 exchange segments, causing a small
deletion on chromosome 20 that may produce some abnormalities.
Lecture Enrichment Ideas
Experience Base: Many students will likely have a rudimentary understanding of the processes of meiosis and
mitosis. Nonetheless, visuals (illustrations, microscopic slides, etc.) are critical for developing full understanding of
these mechanisms, especially in keeping track of the chromosome/chromatids and in contrasting the processes of
meiosis and mitosis.
1.
Discuss the differences in sexual and asexual reproduction, and explain why some organisms that usually
reproduce asexually may go through sexual reproduction occasionally to provide genetic recombination.
2.
Consider the advantages of sexual reproduction in evolution and discuss whether evolution can occur in
asexually reproducing organisms.
3.
Discuss the advantages of a mainly diploid life cycle (e.g., animals) and of a mainly haploid life cycle (e.g.,
fungi and some algae).
4.
Consider why oogenesis and spermatogenesis would have such different kinds of cell products. Explain why
there would be only one functional gamete from the female and four from the male.
5.
Consider the possible effects of an X-linked disease on a carrier female; given the random inactivation of one X
in each cell, half of her cells would be expected to have the abnormal X active. Lead students through examples
of color blindness or Duchenne muscular dystrophy.
64
Critical Thinking
Question 1. Meiosis, or duplication-division-division, is not the only way to reduce
chromosome numbers by half. It is theoretically possible to simply divide the original diploid
number of chromosomes to produce two haploid cells, and there is reportedly a primitive
organism that does this. What would be a drawback?
Answer: The absence of a tetrad and loss of crossing-over would decrease potential diversity of
gametes.
Question 2. Bees and ants have a haploid-diploid system for determining the sex of offspring.
The queen can withhold sperm in her seminal receptacle and the unfertilized egg develops into a
female. One species of ant has just two chromosomes in the diploid male and one in the haploid
female. What effect would such a low chromosome number have on the standard “advantages”
of sexual reproduction?
Answer: The diversity that can come from independent assortment is no longer possible in the
haploid cell. Another source of variation, the possibility for crossing-over, is lost on the male
side.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
65
CHAPTER
11 MENDELIAN PATTERNS OF INHERITANCE
This chapter presents a study of the science of genetics, focusing on its history and the laws
governing inheritance (Mendelian genetics). Genetic crosses are presented and analyzed: onetrait, two-trait, etc. The concepts of dominant and recessive traits, polygenic inheritance,
incomplete dominance, etc. are explained. Statistical interpretations of genetic crosses are
presented. Many human genetic diseases are discussed.
Chapter Outline
11.1 Gregor Mendel
A. The Blending Concept of Inheritance
1. This theory stated that offspring would have traits intermediate between those of the
parents.
2. Red and white flowers produce pink flowers; any return to red or white offspring was
considered instability in the genetic material.
3. Charles Darwin wanted to develop a theory of evolution based on hereditary
principles; blending theory was of no help.
a. A blending theory did not account for variation (differences) and could not
explain species diversity.
B. Mendel’s Particulate Theory of Inheritance
1. Mendel was an Austrian monk.
2. Mendel formulated two fundamental laws of heredity in the early 1860s.
3. He had previously studied science and mathematics at the University of Vienna.
4. At time of his research, he was a substitute science teacher at a local technical high
school.
5. Because Mendel had a mathematical background, he used a statistical basis for his
breeding experiments.
6. Mendel’s particulate theory is based on the existence of minute particles—now called
genes.
C. Mendel Worked with the Garden Pea
1. Mendel prepared his experiments carefully and conducted preliminary studies.
a. He chose the garden pea, Pisum sativum, because peas were easy to cultivate, had
a short generation time, and could be cross-pollinated by hand.
b. From many varieties, Mendel chose 22 true-breeding varieties for his
experiments.
c. True-breeding varieties had all offspring like the parents and like each other.
d. Mendel studied simple traits (e.g., seed shape and color, flower color, etc.).
2. He used his understanding of mathematical principles of probability to interpret
results.
11.2 Mendel’s Law
A. Law of Segregation
1. Mendel confirmed that his tall plants always had tall offspring, i.e., were true66
breeding, before crossing two different strains that differed in only one trait—this is
called a monohybrid cross.
2. A monohybrid cross is between two parent organisms true-breeding for two distinct
forms of one trait.
3. Mendel tracked each trait through two generations.
a. P generation is the parental generation in a breeding experiment.
b. F1 generation is the first-generation offspring in a breeding experiment.
c. F2 generation is the second-generation offspring in a breeding experiment.
4. He performed reciprocal crosses, i.e., pollen of tall plant to stigma of short plant and
vice versa.
5. His results were contrary to those predicted by a blending theory of inheritance.
6. He found that the F1 plants resembled only one of the parents.
7. Characteristics of other parent reappeared in about 1/4 of F2 plants; 3/4 of offspring
resembled the F1 plants.
8. Mendel saw that these 3:1 results were possible if:
a. F1 hybrids contained two factors for each trait, one being dominant and the other
recessive;
b. factors separated when gametes were formed; a gamete carried one copy of each
factor;
c. and random fusion of all possible gametes occurred upon fertilization.
9. Results of his experiments led Mendel to develop his first law of inheritance—the
law of segregation:
a. Each organism contains two factors for each trait.
b. Factors segregate in the formation of gametes.
c. Each gamete contains one factor for each trait.
d. Fertilization gives each new individual two factors for each trait.
B. Mendel’s Cross as Viewed by Classical Genetics
1. The gene locus is the specific location of alleles on homologous chromosomes.
2. Alternate versions of a genes are called alleles.
3. A dominant allele masks or hides expression of a recessive allele; it is represented
by an uppercase letter.
4. A recessive allele is an allele that exerts its effect only in the homozygous state; its
expression is masked by a dominant allele; it is represented by a lowercase letter.
5. The process of meiosis explains Mendel’s law of segregation.
6. In Mendel’s cross, the parents were true-breeding; each parent had two identical
alleles for a trait–they were homozygous, indicating they possess two identical alleles
for a trait.
7. Homozygous dominant genotypes possess two dominant alleles for a trait.
8. Homozygous recessive genotypes possess two recessive alleles for a trait.
9. After cross-pollination, all individuals of the F1 generation had one of each type of
allele.
10. Heterozygous genotypes possess one of each allele for a particular trait.
11. The allele not expressed in a heterozygote is a recessive allele.
C. Genotype Versus Phenotype
1. Two organisms with different allele combinations can have the same outward
appearance (e.g., TT and Tt pea plants are both tall; therefore, it is necessary to
distinguish between alleles present and the appearance of the organism).
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2. Genotype refers to the alleles an individual receives at fertilization (dominant,
recessive).
3. Phenotype refers to the physical appearance of the individual (tall, short, etc.).
D. Mendel’s Law of Independent Assortment
1. This two-trait (dihybrid) cross is between two parent organisms that are truebreeding for different forms of two traits; it produces offspring heterozygous for both
traits.
2. Mendel observed that the F1 individuals were dominant in both traits.
3. He further noted four phenotypes among F2 offspring; he deduced second law of
heredity.
4. Mendel’s law of independent assortment states that members of one pair of factors
assort independently of members of another pair, and that all combinations of factors
occur in gametes.
5. The law of independent assortment only applies to alleles on different chromosomes.
6. A phenotypic ratio of 9:3:3:1 is expected when heterozygotes for two traits are
crossed and simple dominance is present for both genes.
7. Independent assortment during meiosis explains these results.
E. Mendel’s Laws and Meiosis (Science Focus box)
1. Scientists now know that Mendel’s laws hold true because of meiosis.
2. For example, a parent cell will have two pairs of homologous chromosomes.
3. During metaphase I, all alignments of homologous chromosomes can occur,
following the lines of Mendel’s law of independent assortment.
4. During metaphase II, there is only one member of each homologous pair, following
the lines of Mendel’s law of segregation.
5. All possible gametes result since one daughter cell has both dominant alleles (AB),
one daughter cell has both recessive alleles (ab), and two daughter cells have one
dominant and one recessive allele (Ab and aB), following Mendel’s two laws.
F. Mendel’s Law of Probability
1. A Punnett square is used for two-trait crosses.
2. Probability is the likely outcome a given event will occur from random chance.
a. For example, with every coin flip there is a 50% chance of heads and 50% chance
of tails.
3. The product rule of probability states that the chance of two or more independent
events occurring together is the product of the probability of the events occurring
separately.
a. The chance of inheriting a specific allele from one parent and a specific allele
from another is ½ x ½ or 1/4.
b. Possible combinations for the alleles Ee of heterozygous parents are the
following:
EE = ½ x ½ = 1/4
eE = ½ x ½ = 1/4
Ee = ½ x ½ = 1/4
ee = ½ x ½ =
¼
4. The sum law of probability calculates the probability of an event that occurs in two or
more independent ways; it is the sum of individual probabilities of each way an event
can occur; in the above example where unattached earlobes are dominant (EE, Ee,
and eE), the chance for unattached earlobes is 1/4 + 1/4 + 1/4 = 3/4.
G. Testcrosses
1. A testcross is used to determine if an individual with the dominant phenotype is
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homozygous dominant or heterozygous for a particular trait.
2. By Mendel performing a testcross, the law of segregation was supported.
3. A one-trait testcross is used between an individual with dominant phenotype and an
individual with a recessive phenotype to see if the individual with dominant
phenotype is homozygous or heterozygous.
4. A two-trait testcross tests if individuals showing two dominant characteristics are
homozygous for both or for one trait only, or heterozygous for both.
a. If an organism heterozygous for two traits is crossed with another recessive for
both traits, the expected phenotypic ratio is 1:1:1:1.
b. In dihybrid genetics problems, the individual has four alleles, two for each trait.
H. Mendel’s Laws and Human Genetic Disorders
1. Genetic disorders are medical conditions caused by alleles inherited from parents.
2. An autosome is any chromosome other than a sex (X or Y) chromosome.
3. In a pedigree chart, males are designated by squares, females by circles; shaded
circles and squares are affected individuals; line between square and circle represents
a union; vertical line leads to offspring.
4. A carrier is a heterozygous individual with no apparent abnormality but able to pass
on an allele for a recessively-inherited genetic disorder.
5. Autosomal dominant and autosomal recessive alleles have different patterns of
inheritance.
a. Characteristics of autosomal dominant disorders
1)
Affected children usually have an affected parent.
2)
Heterozygotes are affected.: two affected parents can
produce unaffected child; two unaffected parents will not have affected children.
b. Characteristics of autosomal recessive disorders
1)
Most affected children have normal parents since
heterozygotes have a normal phenotype.
2)
Two affected parents always produce an affected child.
3)
Close relatives who reproduce together are more likely to
have affected children.
I. Autosomal Recessive Disorders
1. Methemoglobinemia
a. Relatively harmless disorder resulting from an accumulation of methemoglobin in
the blood.
b. Cause and genetic link still remain a mystery.
c. Symptoms include bluish-purple skin due to inability to clear abmornal blue
protein from blood.
d. People with methemoglobinemia lack the enzyme diaphorase, which is coded for
by a gene on chromosome 22.
2. Cystic Fibrosis
a. This is the most common lethal genetic disease in Caucasians in the U.S.
b. About 1 in 20 Caucasians is a carrier, and about 1 in 3,000 newborns has this
disorder.
c. An increased production of a viscous form of mucus in the lungs and pancreatic
ducts is seen.
1) The resultant accumulation of mucus in the respiratory tract interferes with
gas exchange.
69
2) Digestive enzymes must be mixed with food to supplant the pancreatic juices.
d. New treatments have raised the average life expectancy to up to 35 years.
e. Chloride ions (Cl–) fail to pass plasma membrane proteins.
f. Since water normally follows Cl–, lack of water in the lungs causes thick mucus.
g. The cause is a gene on chromosome 7; attempts to insert the gene into nasal
epithelium has had little success.
h. Genetic testing for adult carriers and fetuses is possible.
3. Niemann-Pick Disease
a. Infant symptoms include jaundice, difficulty feeding, enlarged abdomen, and
pronounced mental retardation.
b. Type A and B forms of Niemann-Pick disease are caused by defective versions of
the same gene located on chromosome 11.
c. This disease is marked by the inability to break down lipids. Lipid droplets
accumulate in liver, lymph nodes, and spleen, and in severe cases, the brain.
J. Autosomal Dominant Disorders
1. Osteogenesis Imperfecta
a. This is an autosomal dominant disorder that affects one in 5,000 newborns and is
distributed equally around the world.
b. Affected individuals have weakened, brittle bones. Additional symptoms include
unusual blue tine in the sclerea of the eye, reduced skin elasticity, weakened teeth,
and sometimes heart valve abnormalities.
c. The disease may be treated by long-term medicine.
2. Hereditary Spherocytosis
a. This genetic blood disorder results from a defective copy of a gene found on
chromosome 8.
b. Symptoms include: spherical shape of red blood cells, and enlarged spleen.
c. Hereditary spherocytosis affects 1 in 5,000 people and is one of the most common
hereditary blood disorders.
H. Testing for Genetic Disorders (Science Focus box)
1. Two genetic disorders resulting from faulty genes are Huntington disease and cystic
fibrosis.
2. Researchers are tests that can detect particular DNA base sequencing that may be able to
identify individuals who may either have a genetic disease or if they are carriers to a
particular genetic disease.
a. A carrier is a person who does not exhibit traits of the disease, but who has the
potential of passing the recessive allele of a genetic disorder.
3. In order to develop a test for a particular genetic disorder, scientists must first obtain
family pedigrees.
a. Family pedigrees trace particular genes through many family generations.
b. In the example of Huntington disease, the family pedigree illustrated that the
offspring of an affected individual has a 50% of having the disease.
c. When blood testing can be conducted, DNA base sequencing is determined and
compared to see if there are similarities in base sequencing with people who have the
disease.
d. However, this gene is only linked to the disease and not the disease itself.
e. More than one allele can occur on the same chromosome, meaning the alleles are
linked.
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f. Linked alleles are found together on the same gamete. However, even though they
are considered to be linked, crossing over and unlinking can occur.
4. Assocation studies are another method to discover potential base sequencing to identify
if an individual has a genetic disorder.
a. DNA of the general population is tested to identify similar base sequences.
b. The exploration of the human genome project has made it possible to identify genes
that may be linked to particular genetic disorders.
5. Base sequencing identification can be used for prenatal testing and carrier testing.
11.3
Extending the Range of Mendelian Genetics
A. Multiple Allelic Traits
1. This occurs when a gene has many allelic forms or alternative expressions.
2. ABO Blood Types
a. The ABO system of human blood types is a multiple allele system.
b. Two dominant alleles (IA and IB) code for presence of A and B glycoproteins on
red blood cells.
c. This also includes a recessive allele (iO) coding for no A or B glycoproteins on red
blood cells.
d. As a result, there are four possible phenotypes (blood types): A, B, AB, and O.
e. This is a case of codominance, where both alleles are fully expressed.
3. The Rh factor is inherited independently from the ABO system; the Rh+ allele is
dominant.
B. Incomplete Dominance and Incomplete Penetrance
1. Incomplete dominance: offspring show traits intermediate between two parental
phenotypes.
a. True-breeding red and white-flowered four-o’clocks produce pink-flowered
offspring.
b. Incomplete dominance has a biochemical basis; the level of gene-directed protein
production may be between that of the two homozygotes.
c. One allele of a heterozygous pair only partially dominates expression of its
partner.
d. This does not support a blending theory; parental phenotypes reappear in F2
generation.
2. Human Examples of Incomplete Dominance
a. Curly versus Straight Hair
1) A curly-haired Caucasian and a straight-haired Caucasian will have wavyhaired offspring.
2) Two wavy-haired parents will produce a 1:2:1 ratio of curly-wavy-straight
hair children.
b. Cystic fibrosis is considered an example of incomplete dominance.
3. Incomplete penetrance: offspring does not always show dominant allele.
a. Polydactyly is an example of incomplete penetrance.
C. Pleioptropic Effects
1. Pleiotropy describesva gene that affects more than one characteristic of an
individual.
2. Examples include Marfan syndrome, porphyria, and sickle-cell anemia.
3. Marfan syndrome
a. The gene is on chromosome 15.
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b. Symptoms include disproportionately long arms, legs, hands, and feet; weakened
aorta; poor eyesight.
4. Porphyria
a. This disease is caused by a chemical insufficiency in the production of
hemoglobin.
b. Symptoms include photosensitivity, strong abdominal pain, port-wine-colored
urine, paralysis in arms and legs.
5. Sickle-cell anemia
a. This disease is the most common inherited disorder in blacks, affecting about 1 in
500 African Americans.
b. The gene is on chromosome 11.
c. In affected individuals, the red blood cells are shaped like sickles—an abnormal
hemoglobin molecule, Hbs, causes the defect.
1) Normal hemoglobin, HbA, differs from Hbs by one amino acid in the protein
globin.
d. Sickling of the red blood cells occurs when the oxygen content of the person’s
blood is low, thereby slowing down blood flow and clogging small vessels.
e. Signs and symptoms include anemia, weakness, fever, pain, rheumatism, low
resistance to disease, kidney and heart failure.
f. Treatment includes pain management, blood transfusions, and bone marrow
transplants.
g. The disease can be diagnosed prenatally.
h. Individuals with the sickle cell trait (carriers), who normally do not have any
sickle-shaped cells unless they experience dehydration or mild oxygen
deprivation, are resistant to the disease malaria.
D. Polygenic Inheritance
1. Polygenic inheritance occurs when one trait is governed by two or more sets of
alleles.
2. Dominant alleles have a quantitative effect on the phenotype: each adds to the effect.
3. The more genes involved, the more continuous is the variation in phenotypes,
resulting in a bell-shaped curve.
4. Multifactorial traits are controlled by polygenes subject to environmental
influences.
a. The coats of Siamese cats and Himalayan rabbits have darker tipped ears, nose,
paws, etc., due to the enzyme encoded by an allele which is only active at the
extremities at low temperatures.
5. Human Examples of Polygenic Inheritance
a. A hybrid cross for skin color provides a range of intermediates.
b. Parents with intermediate skin color can produce children with the full range of
skin colors.
c. Other examples include cleft lip, clubfoot, congenital dislocations of the hip,
hypertension, diabetes, schizophrenia, allergies, and cancers.
d. Behavioral traits including suicide, phobias, alcoholism, and homosexuality may
be associated with particular genes but are not likely completely predetermined.
e. All behavioral traits are partly heritable, and genes work together and are
susceptible to environmental influences.
E. X-Linked Inheritance
72
1.
Sex chromosomes in the human female are XX; those of the male are XY.
2. Males produce X-containing and Y-containing gametes; therefore males determine the
sex of offspring.
3. Besides genes that determine sex, sex chromosomes carry many genes for traits unrelated
to sex.
4. An X-linked gene is any gene located on X chromosome; used to describe genes on X
chromosome that are missing on the Y chromosome.
5. Work with fruit flies (Drosophila) by Thomas Hunt Morgan (early 1900s) confirmed
genes were on chromosomes.
a. Fruit flies are easily and inexpensively raised in common laboratory glassware.
b. Females only mate once and lay hundreds of eggs.
c. The fruit fly generation time is short, allowing rapid experiments.
6. Fruit flies have an XY sex chromosome system similar to the human system; experiments
can be correlated to the human situation.
a. Newly discovered mutant male fruit flies had white eyes.
b. Cross of the hybrids from the white-eyed male crossed with a dominant red-eyed
female yielded the expected 3:1 red-to-white ratio; however, all of the white-eyed
flies were males.
c. An allele for eye color on the X but not on the Y chromosome supports the results
of this cross.
d. Behavior of this allele corresponds to the behavior of the chromosome; this
confirmed the chromosomal theory of inheritance.
7. Solving X-Linked Genetic Problems
a. X-linked alleles are designated as superscripts to the X chromosome.
b. Heterozygous females are carriers; they do not show the trait but can transmit it.
c. Males are never carriers but express the one allele on the X chromosome; the
allele could be dominant or recessive.
d. One form of color-blindness is X-linked recessive.
F. Human X-Linked Disorders
1. More males have X-linked traits because recessive alleles on the X chromosome in
males are expressed in males.
2. Color Blindness
a. Color blindness can be an X-linked recessive disorder involving mutations of
genes coding for green or red sensitive cone cells, resulting in the inability to
perceive green or red, respectively; the pigment for blue-sensitive protein is
autosomal.
b. About 8% of Caucasian men have red-green color blindness.
3. Menkes Syndrome (kinky hair syndrome)
a. Caused by a defective allele on the X chromosome.
b. Symptoms include: poor muscle tone, seizures, low body temperature, skeletal
abnormalities, and brittle, steely hair.
c. Treatments include copper injections, but prognosis is poor and many individuals
die within the first few years of life.
4. Muscular Dystrophy
a. Duchenne muscular dystrophy is the most common form and is characterized by
wasting away of muscles, eventually leading to death; it affects one out of every
3,600 male births.
73
b. This X-linked recessive disease involves a mutant gene that fails to produce the
protein dystrophin.
c. Signs and symptoms (e.g., waddling gait, toe walking, frequent falls, difficulty in
rising) soon appear.
d. Muscles weaken until the individual is confined to a wheelchair; death usually
occurs by age 20.
e. Affected males are rarely fathers; the gene passes from carrier mother to carrier
daughter.
f. Lack of dystrophin protein causes calcium ions to leak into muscle cells; this
promotes action of an enzyme that dissolves muscle fibers.
g. As the body attempts to repair tissue, fibrous tissue forms and cuts off blood
supply to the affected muscles.
h. A test now detects carriers of Duchenne muscular dystrophy; treatments are being
attempted.
5. Adrenoleukodystrophy (ADL)
a. This disease is an X-linked recessive disorder.
b. Fatty acids are not broken down and severe nervous system damage occurs.
c. Symptoms include failure to properly develop after the age of five, loss of adrenal
gland function, exhibit poor coordination, and show progressive loss of hearing,
speech, vision.
6. Hemophilia
a. About one in 10,000 males is a hemophiliac with impaired ability of blood to clot.
b. The two common types: Hemophilia A, due to the absence of clotting factor IX;
Hemophilia B, due to the absence of clotting factor VIII.
c. Hemophiliacs bleed externally after an injury and also suffer internal bleeding
around joints.
d. Hemorrhages stop with transfusions of blood (or plasma) or concentrates of
clotting protein.
e. Factor VIII is now available as a genetically-engineered product.
f. Of Queen Victoria’s 26 offspring, five grandsons had hemophilia and four
granddaughters were carriers.
Lecture Enrichment Ideas
Experience Base: While the pea plant traits selected by Mendel matched a clear and simple
mathematical model, students are more interested in their own genetic traits. However, recent
work has shown most traits to be polygenic or complicated by other complex genetic factors. If
you use tongue-rolling, eye-color, etc., be sure to qualify that these are oversimplifications used
for teaching purposes. The common genetic diseases discussed in this text reveal that it is highly
likely that there will be students in classes who have personal experience with some of these
disorders (family, friend); they may ask questions and provide information vis-a-vis the lecture
material. Visuals will likewise put a face on the tragedy of many of these conditions. Students’
math level relative to both statistics and algebra will be a critical factor in understanding both
probability and related upcoming concepts. (The mathematics involved does assume some
working ability of statistics usually achieved by college level students, but this assumption
should be checked by questioning students during mathematical examples.)
74
1. Describe the “loss” of Mendel’s work for thirty-five years and the rediscovery of the same
laws of heredity by three scientists in 1900. Consider the timeliness of research and how
some findings can be “ahead of their time” and therefore unappreciated.
2. If Mendel’s pea plant traits had been cases of epistasis, polygenic inheritance, etc., ask how
this would have confounded Mendel’s work and perhaps have misled him.
3. Consider polygenic inheritance and how it affects the expression of such traits as height, skin
color, and intelligence. Cover the idea of a normal distribution, and consider what kind of
offspring two very tall individuals would have. You might talk about the regression to the
mean and why two very tall individuals would be likely to have children closer to the average
height.
4. Discuss how interaction between the environment and the genotype produces the phenotype.
Students often don’t realize that the phenotype can change over time, as when hair changes
from blonde in childhood, to dark in the adult, to white in the older person, even though the
genes remain basically the same. Also consider that there are some changes in the genotype
(e.g., producing cancer) as a person ages.
5. Speculate what an awareness of Mendel’s findings might have done for Darwin’s
presentation of his theory of evolution.
6. Describe the methods of prenatal diagnosis and the means of recognizing carriers so that
couples at risk for having a homozygous recessive child will know to test for a particular
disorder.
Critical Thinking
Question 1. Consider the proposal that handedness is inherited with right-handedness
dominant (RR, Rr) and left handedness recessive (rr). Of all the possible combinations of parents
and offspring by phenotype, what combination of parents and offspring would cast doubt on this
simple hypothesis?
Answer: Two left-handed parents (rr) having a right-handed child (Rr or RR) would contradict
this proposal since there would be no way for the offspring to inherit the dominant gene.
However, two right-handed parents (could include carriers) or one left-handed and one righthanded parent combination (known from phenotype only) could result in any combination of
handedness in offspring.
Question 2. Corn has 10 pairs or 20 total chromosomes. If Mendel chose corn instead of pea
plants to conduct his first experiments, what is the maximum number of traits he could have
documented in breeding experiments in order to establish his law of segregation (each trait is
determined by two factors) and independent assortment (the factors assort independent of other
factors)?
Answer: Only ten traits will segregate or sort independently in corn. An eleventh trait would
have to be on the same chromosome as a previously tallied trait and would be linked in
occurrence to the other trait. Indeed, the fact that Mendel used the maximum number of traits
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allowed by the pea plant chromosome number has led some scientists to suspect that Mendel
experimented with more traits and did not get the mathematical results he expected.
Question 3. Malaria has not been a serious health problem in the southern United States this
century, but it remains a serious disease in Africa today. If we examined just the African
American population descended from Africans brought to the United States from the malaria
regions of Africa, how would the proportion of sickle-cell genes in these gene pools compare?
What would occur to these gene frequencies over time if there was a resurgence of malaria in the
United States?
Answer: Because the heterozygous carriers of the sickle-cell trait are somewhat protected from
malaria, the gene would be more prevalent in Africa than in the United States where this conveys
no advantage and is moderately disadvantageous. If, however, malaria returned to the southern
United States at a level that eliminated individuals in spite of the hygiene, housing and the health
care system, the frequency of the sickle-cell gene would increase in the carriers who are more
likely to survive than the normal individuals.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
76
CHAPTER
12 MOLECULAR BIOLGOY OF THE GENE
This chapter presents a comprehensive study of the chemistry, the structure, and the cellular
functions of DNA, with considerable attention given to the history of DNA research. The
mechanism of semiconservative replication in both prokaryotes and eukaryotes is discussed in
depth; detailed illustrations accompany the textual material. A brief outline of the repair of
replication errors is presented. The history of the research recognizing and elaborating gene
action is discussed. The processes of transcription and translation are described in detail, and are
accompanied with detailed graphics. The chemical nature of the RNAs and their roles in gene
expression are outlined.
Chapter Outline
12.1 The Genetic Material
1.
Early researchers knew that the genetic material must be:
a. able to store information used to control both the development and the metabolic activities of
cells;
b. stable so it can be replicated accurately during cell division and be transmitted for generations;
and,
c. able to undergo mutations providing the genetic variability required for evolution.
A. Transformation of Bacteria
1. Bacteriologist Frederick Griffith (1931) experimented with Streptococcus pneumoniae (a
pneumococcus that causes pneumonia in mammals).
2. Mice were injected with two strains of pneumococcus: an encapsulated (S) strain and a nonencapsulated (R) strain.
a. The S strain is virulent (the mice died); it has a mucous capsule and forms “shiny” colonies.
b. The R strain is not virulent (the mice lived); it has no capsule and forms “dull” colonies.
3. In an effort to determine if the capsule alone was responsible for the virulence of the S strain, he
injected mice with heat-killed S strain bacteria; the mice lived.
4. Finally, he injected mice with a mixture of heat-killed S strain and live R strain bacteria.
a. The mice died; living S strain pneumococcus were recovered from their bodies.
b. Griffith concluded that some substance necessary for synthesis of the capsule —and therefore for
virulence—must pass from dead S strain bacteria to living R strain bacteria so the R strain were
transformed.
c. This change in phenotype of the R strain must be due to a change in the bacterial genotype,
suggesting that the transforming substance passed from S strain to R strain.
B. DNA: The Transforming Substance
1. Oswald Avery et al. (1944) reported that the transforming substance was DNA.
2.
In the early twentieth century, it was shown that nucleic acids contain four types of nucleotides.
a. DNA was composed of repeating units, each of which always had just one of each of four different
nucleotides (a nitrogenous base, a phosphate, and a pentose).
b. In this model, DNA could not vary between species and therefore could not be the genetic
material; therefore some other protein component was expected to be the genetic material.
3. Purified DNA is capable of bringing about the transformation. Evidence:
a. DNA from S strain pneumococcus causes R strain bacteria to be transformed.
b. Digestion of the transforming substance with enzyme that digests DNA prevents transformation.
c. The molecular weight of the transforming substance is great enough for some genetic variability.
d. Enzymes that degrade proteins cannot prevent transformation, nor can enzymes that digest RNA.
4. Avery’s experimental results demonstrated DNA is genetic material and DNA controls biosynthetic
properties of a cell.
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C. Transformation of Organisms Today
1. Transformation experiments today are important especially in biotechnology labs.
2. Transformation of organisms are being used in commercial products.
3. In order to illustrate that transferring genes was possible from one organism to another, scientists used
a green fluorescent protein from jellyfish and transferred it to other organisms. The result was that
these organisms glowed in the dark.
4. Mammalian genes have the ability to function in other species: bacteria, invertebrates, plants.
D.
The Structure of DNA
1. Erwin Chargaff (1940s) analyzed the base content of DNA.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
It was known DNA contained four different nucleotides:
a. two with purine bases, adenine (A) and guanine (G); a purine is a type of nitrogen-containing base
having a double-ring structure.
b. two with pyrimidine bases, thymine (T) and cytosine (C); a pyrimidine is a type of nitrogencontaining base having a single-ring structure.
Results: DNA does have the variability necessary for the genetic material.
For a species, DNA has the constancy required of genetic material.
This constancy is given in Chargaff’s rules:
a. The amount of A, T, G, and C in DNA varies from species to species.
b. In each species, the amount of A = T and the amount of G = C (A +G = T +C).
The tetranucleotide hypothesis (proposing DNA was repeating units of one of four bases) was disproved:
each species has its own constant base composition.
The variability is in base sequences is staggering; a human chromosome contains about 140 million base
pairs.
Since any of the four possible nucleotides can be present at each nucleotide position, the total number of
6
possible nucleotide sequences is 4140 x 10 = 4140,000,000.
Rosalind Franklin produced X-ray diffraction photographs.
Franklin’s work provided evidence that DNA had the following features:
a. DNA is a helix.
b. Some portion of the helix is repeated.
American James Watson joined with Francis H. C. Crick in England to work on the structure of DNA.
Watson and Crick received the Nobel Prize in 1962 for their model of DNA.
Using information generated by Chargaff and Franklin, Watson and Crick constructed a model of DNA as a
double helix with sugar-phosphate groups on the outside, and paired bases on the inside.
Their model was consistent with both Chargaff’s rules and Franklin’s X-ray diffraction studies.
Complementary base pairing is the paired relationship between purines and pyrimidines in DNA: A is
hydrogen-bonded to T and G is hydrogen-bonded to C.
12.2 Replication of DNA
1.
DNA replication is the process of copying a DNA molecule. Replication is semiconservative, with each
strand of the original double helix (parental molecule) serving as a template (mold or model) for a new
strand in a daughter molecule. This process consists of:
a. Unwinding: old strands of the parent DNA molecule are unwound as weak hydrogen bonds between
the paired bases are “unzipped” and broken by the enzyme helicase.
b. Complementary base pairing: free nucleotides present in the nucleus bind with complementary bases
on unzipped portions of the two strands of DNA; this process is catalyzed by DNA polymerase.
c. Joining: complementary nucleotides bond to each other to form new strands; each daughter DNA
molecule contains an old strand and a new strand; this process is also catalyzed by DNA polymerase.
A. Aspects of DNA Replication (Science Focus box)
1. For complementary base pairing to occur, the DNA strands need to be antiparallel, as discovered by
Watson and Crick.
2. One strand of DNA is 5’ at the top and the other strand is 3’ at the top of the strand.
3. During replication the DNA polymerase can only join to the free 3’ end of the previous nucleotide.
4. DNA polymerase cannot start the synthesis of a DNA chain, so an RNA polymerase lays out an RNA
primer that is complementary to the replicated strand.
5. Now the DNA polymerase can join the DNA nucleotides to the 3’ end of the new strand.
6. The helicase enzyme unwinds the DNA and one strand (called the leading new strand) can be copied in the
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direction of the replication fork.
The other strand of DNA is copied in the direction away from the fork, and replication begins again.
a. This new lagging strand is discontinuous and each segment is called an Okazaki fragment, after the
scientist who discovered them.
8. Replication is only complete when RNA primers are removed.
9. During replication, DNA molecules gets smaller and smaller.
10. The end of eukaryotic DNA molecules have nucleotide sequences called teleromers.
a. Teleromeres don’t code for proteins. They are repeats of short nucleotide sequences (i.e., TTAGGG).
11. Normal mammalian cells divide approximately 50 times and then stop. However in cancer cells the
telomerase can be turned on and cancer cells then divide without limit.
B. Prokaryotic Versus Eukaryotic Replication
1. Prokaryotic DNA Replication
a. Bacteria have a single loop of DNA that must replicate before the cell divides.
b. Replication in prokaryotes may be bidirectional from one point of origin or in only one direction.
c. Replication only proceeds in one direction, from 5' to 3'.
d. Replication begins at a special site on a bacterial chromosome, called the origin of replication.
e. Bacterial cells can complete DNA replication in 40 minutes; eukaryotes take hours.
2. Eukaryotic DNA Replication
a. Replication in eukaryotes starts at many points of origin and spreads with many replication
bubbles—places where the DNA strands are separating and replication is occurring.
b. Replication forks are the V-shape ends of the replication bubbles; the sites of DNA replication.
c. Eukaryotes replicate their DNA at a slower rate—500 to 5,000 base pairs per minute.
d. Eukaryotes take hours to complete DNA replication.
C. Accuracy of Replication
1. A mismatched nucleotide may occur once per 100,000 base pairs, causing a pause in replication.
2. Proofreading is the removal of a mismatched nucleotide; DNA repair enzymes perform this
proofreading function and reduce the error rate to one per billion base pairs.
12.3 The Genetic Code of Life
1. Sir Archibald Garrod (early 1900s) introduced the phrase inborn error of metabolism.
a. Garrod proposed that inherited defects could be caused by the lack of a particular enzyme.
b. Knowing that enzymes are proteins, Garrod suggested a link between genes and proteins.
2. Linus Pauling and Harvey Itano (1949) compared hemoglobin in red blood cells of persons with
sickle-cell disease and normal individuals.
a. They discovered that the chemical properties of a protein chain of sickle-cell hemoglobin differed
from that of normal hemoglobin.
b. Pauling and Itano formulated the one gene–one polypeptide hypothesis: each gene specifies one
polypeptide of a protein, a molecule that may contain one or more different polypeptides.
A. RNA Carries the Information
1. Like DNA, RNA is a polymer of nucleotides.
2. Unlike DNA, RNA is single-stranded, contains the sugar ribose, and the base uracil instead of thymine
(in addition to cytosine, guanine, and adenine).
3. There are three major classes of RNA.
a. Messenger RNA (mRNA) takes a message from DNA in the nucleus to ribosomes in the
cytoplasm.
b. Ribosomal RNA (rRNA) and proteins make up ribosomes where proteins are synthesized.
c. Transfer RNA (tRNA) transfers a particular amino acid to a ribosome.
B. The Genetic Code
1. DNA undergoes transcription to mRNA, which is translated to a protein.
2. DNA is a template for RNA formation during transcription.
3. Transcription is the first step in gene expression; it is the process whereby a DNA strand serves as a
template for the formation of mRNA.
4. During translation, an mRNA transcript directs the sequence of amino acids in a polypeptide.
5. The genetic code is a triplet code, comprised of three-base code words (e.g., AUG).
6. A codon consists of 3 nucleotide bases of DNA.
7. Four nucleotides based on 3-unit codons allows up to 64 different amino acids to the specified.
8. Finding the Genetic Code
a. Marshall Nirenberg and J. Heinrich Matthei (1961) found that an enzyme that could be used to
7.
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construct synthetic RNA in a cell-free system; they showed the codon UUU coded for
phenylalanine.
b. By translating just three nucleotides at a time, they assigned an amino acid to each of the RNA
codons, and discovered important properties of the genetic code.
c. The code is degenerate: there are 64 triplets to code for 20 naturally occurring amino acids; this
protects against potentially harmful mutations.
d. The genetic code is unambiguous; each triplet codon specifies one and only one amino acid.
e. The code has start and stop signals: there are one start codon and three stop codons.
9. The Code Is Universal
a. The few exceptions to universality of the genetic code suggest the code dates back to the very first
organisms and that all organisms are related.
b. Once the code was established, changes would be disruptive.
12.4 First Step: Transcription
A. Messenger RNA Is Formed
1. A segment of the DNA helix unwinds and unzips.
2. Transcription begins when RNA polymerase attaches to a promoter on DNA. A promoter is a region
of DNA which defines the start of the gene, the direction of transcription, and the strand to be
transcribed.
3. As RNA polymerase (an enzyme that speeds formation of RNA from a DNA template) moves along
the template strand of the DNA, complementary RNA nucleotides are paired with DNA nucleotides of
the coding strand. The strand of DNA not being transcribed is called the noncoding strand.
4. RNA polymerase adds nucleotides to the 3'-end of the polymer under construction. Thus, RNA
synthesis is in the 5’-to-3’ direction.
5. The RNA/DNA association is not as stable as the DNA double helix; therefore, only the newest
portion of the RNA molecule associated with RNA polymerase is bound to DNA; the rest dangles off
to the side.
6. Elongation of mRNA continues until RNA polymerase comes to a stop sequence.
7. The stop sequence causes RNA polymerase to stop transcribing DNA and to release the mRNA
transcript.
8. Many RNA polymerase molecules work to produce mRNA from the same DNA region at the same
time.
9. Cells produce thousands of copies of the same mRNA molecule and many copies of the same protein
in a shorter period of time than if a single copy of RNA were used to direct protein synthesis.
B. RNA Molecules Are Processed
1. Newly formed pre-mRNA transcript is processed before leaving the nucleus.
2. Pre-mRNA transcript is the immediate product of transcription; it contains exons and introns.
3. The ends of the mRNA molecule are altered: a cap is put on the 5' end and a poly-A tail is put on the 3'
end.
a. The “cap” is a modified guanine (G) where a ribosome attaches to begin translation.
b. The “poly-A tail” consists of a 150–200 adenine (A) nucleotide chain that facilitates transport of
mRNA out of the nucleus and inhibits enzymatic degradation of mRNA.
4. Portions of the primary mRNA transcript, called introns, are removed.
a. An exon is a portion of the DNA code in the primary mRNA transcript eventually expressed in the
final polypeptide product.
b. An intron is a non-coding segment of DNA removed by spliceosomes before the mRNA leaves
the nucleus.
5. Ribozymes are RNAs with an enzymatic function restricted to removing introns from themselves.
a. RNA could have served as both genetic material and as the first enzymes in early life forms.
6. Spliceosomes are complexes that contain several kinds of ribonucleoproteins.
a. Spliceosomes cut the primary mRNA transcript and then rejoin adjacent exons.
7. Smaller nucleolar RNA (snoRNA) is present in the nucleolus, to assist in the processing of rRNA and
tRNA molecules.
C. Function of Introns
1. Introns give a cell the ability to decide which exons will go in a particular mRNA.
2. mRNA do not have all of the possible exons available from a DNA sequence. In one mRNA what is an
exon could be an intron in another mRNA. This process is termed alternative mRNA splicing.
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3.
Some introns give rise to microRNAs (miRNA). miRNA regulate mRNA translation by bonding with
mRNA through complementary base pairing and preventing translation from occurring.
4. Exon shuffling occurs when introns encourage crossing over during meiosis.
12.5 Second Step: Translation
1. Translation takes place in the cytoplasm of eukaryotic cells.
2. Translation is the second step by which gene expression leads to protein synthesis.
3. One language (nucleic acids) is translated into another language (protein).
A. The Role of Transfer RNA
1. Transfer RNA (tRNA) molecules transfer amino acids to the ribosomes.
2. The tRNA is a single-stranded ribonucleic acid that doubles back on itself to create regions where
complementary bases are hydrogen-bonded to one another.
3. The amino acid binds to the 3’ end; the opposite end of the molecule contains an anticodon that binds
to the mRNA codon in a complementary fashion.
4. There is at least one tRNA molecule for each of the 20 amino acids found in proteins.
5. There are fewer tRNAs than codons because some tRNAs pair with more than one codon; if an
anticodon contains a U in the third position, it will pair with either an A or G—this is called the
wobble hypothesis.
6. The tRNA synthetases are amino acid-activating enzymes that recognize which amino acid should
join which tRNA molecule, and covalently joins them. This requires ATP.
7. An amino acid–tRNA complex forms, which then travels to a ribosome to “transfer” its amino acid
during protein synthesis.
B. The Role of Ribosomal RNA
1. Ribosomal RNA (rRNA) is produced from a DNA template in the nucleolus of the nucleus.
2. The rRNA is packaged with a variety of proteins into ribosomal subunits, one larger than the other.
3. Subunits move separately through nuclear envelope pores into the cytoplasm where they combine
when translation begins.
4. Ribosomes can float free in cytosol or attach to endoplasmic reticulum.
5. Prokaryotic cells contain about 10,000 ribosomes; eukaryotic cells contain many times more.
6. Ribosomes have a binding site for mRNA and binding sites for two tRNA molecules.
7. They facilitate complementary base pairing between tRNA anticodons and mRNA codons; rRNA acts
as an enzyme (ribozyme) that joins amino acids together by means of a peptide bond.
8. A ribosome moves down the mRNA molecule, new tRNAs arrive, the amino acids join, and a
polypeptide forms.
9. Translation terminates once the polypeptide is formed; the ribosome then dissociates into its two
subunits.
10. Polyribosomes are clusters of several ribosomes synthesizing the same protein.
11. To get from a polypeptide to a function protein requires correct bending and twisting; chaperone
molecules assure that the final protein develops the correct shape.
12. Some proteins contain more than one polypeptide; they must be joined to achieve the final threedimensional shape.
C. Translation Requires Three Steps
1. During translation, mRNA codons base-pair with tRNA anticodons carrying specific amino acids.
2. Codon order determines the order of tRNA molecules and the sequence of amino acids in polypeptides.
3. Protein synthesis involves initiation, elongation, and termination.
4. Enzymes are required for all three steps; energy (ATP) is needed for the first two steps.
5. Chain Initiation
a. A small ribosomal subunit attaches to mRNA in the vicinity of the start codon (AUG).
b. First or initiator tRNA pairs with this codon; then the large ribosomal subunit joins to the small
subunit.
c. Each ribosome contains three binding sites: the P (for peptide) site, the A (for amino acid) site,
and the E (for exit) site.
d. The initiator tRNA binds to the P site although it carries one amino acid, methionine.
e. The A site is for the next tRNA carrying the next amino acid.
f. The E site is to discharge tRNAs from the ribosome.
g. Initiation factor proteins are required to bring together the necessary translation components: the
small ribosomal subunit, mRNA, initiator tRNA, and the large ribosomal subunit.
6. Chain Elongation
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a.
The tRNA with attached polypeptide is at the P site; a tRNA-amino acid complex arrives at the A
site.
b. Proteins called elongation factors facilitate complementary base pairing between the tRNA
anticodon and the mRNA codon.
c. The polypeptide is transferred and attached by a peptide bond to the newly arrived amino acid in
the A site.
d. This reaction is catalyzed by a ribozyme, which is part of the larger subunit.
e. The tRNA molecule in the P site is now empty.
f. Translocation occurs with mRNA, along with peptide-bearing tRNA, moving to the P site and the
spent tRNA moves from the P site to the E site and exits the ribosome.
g. As the ribosome moves forward three nucleotides, there is a new codon now located at the empty
A site.
h. The complete cycle is rapidly repeated, about 15 times per second in Escherichia coli.
i. The ribosomes will reach a stop codon, termination will occur, and the peptide will be released.
7. Chain Termination
a. Termination of polypeptide synthesis occurs at a stop codon that does not code for amino acid.
b. The polypeptide is enzymatically cleaved from the last tRNA by a release factor.
c. The tRNA and polypeptide leave the ribosome, which dissociates into its two subunits.
d. Proteomics is a new field ob biology that aims to understand protein structures, and the functions
of metabolic pathways.
D. Protein Synthesis and the Eukaryotic Cell
1. The first few amino acids of a polypeptide act as a signal peptide that indicates where the polypeptide
belongs in the cell or if it is to be secreted by the cell.
2. After the polypeptide enters the lumen of the ER, it is folded and further processed by addition of
sugars, phosphates, or lipids.
3. Transport vesicles carry the proteins between organelles and to the plasma membrane.
12.6 Structure of Eukaryotic Chromosomes and Genes
1. The DNA is wound around a core of eight protein molecules (“beads on a string”); the proteins are
called histones and each “bead” is called a nucleosome.
a. Histones play a structural role in chromosome structure and package the large DNA into the small
nucleus.
b. The nucleosomes also contribute to the shortening of DNA by folding it into a “zigzag” structure.
2. During interphase, some chromatin is highly compact, darkly stained, and genetically inactive
heterochromatin.
3. The rest is diffuse, lightly colored euchromatin thought to be genetically active.
a. Euchromatin activity is related to the extent nucleosomes are coiled and condensed.
b. A nucleosome is a bead-like unit made of a segment of DNA wound around a
complex of histone proteins.
Lecture Enrichment Ideas
Experience Base: The names “Watson and Crick” may be part of our cultural recognition
vocabulary but many students will be unaware of the revolution they began in biology research.
Visuals of stringy strands of DNA may help students visualize the substance. Understanding of
the three-dimensional molecular structure of purines and pyrimidines will be beyond most
students without use of physical models.
1. Read passages from biographies on the discovery of the structure of DNA. There are
excellent biographies out on Rosalind Franklin, Maurice Wilkins, et al., in addition to
Watson and Crick. Linus Pauling was also working on a model with three strands. Readings
from this history may help students learn of the rivalry and arbitrary nature of many
decisions made in the process of doing science.
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2. Pose this paradox: the correct replication of DNA is of such importance that there are
different means of assuring it, yet it is important that the copying process not always be exact
so there are mutations and evolution.
3. Emphasize how the structure of the DNA molecule is related to its different functions of
storing information, replicating itself, and undergoing mutation. Focus on the bases in the
center, the hydrogen bonding between them, and on the complementary nature of the base
pairing.
4. Emphasize how the study of genetics has expanded to include the science of biochemistry:
genetic processes, the mechanism of DNA replication, the transfer of information to a usable
form in building proteins, and the use of proteins (generally as enzymes) to produce the
phenotype of the individual.
5. Explain how the link between the DNA nucleotide sequence and the amino acid sequence of
proteins has led to the current knowledge of what happens in a cell and how to “fix it”
through genetic engineering. This prepares students for the material in the next few chapters.
6. Follow the progression of scientific understanding of this material from the one gene-one
enzyme hypothesis to the one gene–one polypeptide hypothesis, and take the next step to the
one gene-one gene product hypothesis that would include tRNA and rRNA as gene products.
These, of course, do not fit the one gene-one polypeptide hypothesis, since they do not
involve polypeptides but yet are cellular structures defined by DNA gene sequences.
7. The similarity of the words “transcription” and “translation” often confuse students—much
like centriole-centrosome, chromosome-chromatid, etc. It obviously is critical that students
can differentiate these processes before moving on in their studies.
8. Recent work with prion diseases (mad cow disease, scrapie, kuru, etc.) shows the
transmission of an infectious agent that does not contain DNA or RNA (as do viruses,
eukaryotic parasites, or bacteria). This strange situation met with early criticism from the
biology community because it avoids the central dogma of DNA → RNA → protein for the
promulgation of infectious agents. However, subsequent work appears to confirm that one
protein (prion) can alter the tertiary shape of an adjacent protein, this eventually cascading
into massive protein damage, without any nucleic acid involved. Note that this does not
overthrow the central dogma as much as simply add exceptions that can only be understood
when operating from the central dogma, and that nature has tremendous complexity.
9. The discovery that the human genome is substantially smaller than predicted means that there
may be an apparent shortage of DNA to account for all the diversity in proteins. This may
indicate that the role of spliceosomes is far greater than previously known. If one sequence of
DNA can be read in different ways to produce many varied products, the definition of a gene
may also become far more complicated.
Critical Thinking
Question 1.
components?
In what human cells would you find the highest concentration of DNA compared with other cell
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Answer: Sperm cells are mostly DNA. Although they are haploid, they are reduced down to the DNA plus an
acrosome tip plus mitochondria and tail. Other cells have substantial cytoplasmic portions. Some early research with
DNA was conducted with fish sperm.
Question 2. As quartz crystals “grow,” new silicon dioxide molecules are aligned alongside
the previous surfaces. Likewise, as aluminum silicates and other components of clay are
dissolved and precipitate out, they too align with previous molecules, even perpetuating cracks
and faults. Technically, these are self-replicating molecular assemblages and yet the quartz or
clay has not risen to become “life.” What feature(s) of DNA makes it different from quartz and
clay?
Answer: DNA serves as a template for the production of more DNA and stores information that specifies the
sequence of amino acids in proteins. While the correct replication of DNA is of such importance that there are
different means of assuring it, it is also important that the copying process not always be exact so there are mutations
and evolution. Quartz and clay cannot store information for other molecules, nor are they robust enough to mutate
and evolve.
Question 3.
How can a cell produce the largest amount of coded protein product in the shortest time?
Answer: Cells can produce thousands of copies from the same mRNA molecule and many copies of coded protein
in a shorter period of time than if a single copy of DNA were used for protein synthesis.
Question 4.
If bacteria do not have introns and we have many, what is the likely status of simpler eukaryotes?
Answer: Investigators have found that the simpler the eukaryote, the less likely that introns will be present. While
the role of introns is still being investigated (they may allow crossing over during meiosis, or introns may divide a
gene into regions that can be joined in different combinations for different products), the advantage apparently
accrues to advanced organisms.
Question 5.
Why would evolution not be able to select for “twins” for codons (instead of “triplets”); that is,
only two base pairs would code for an amino acid?
Answer: With any of four base pairs in the first and second positions, there are only 4 x 4 or 16 possible codons and
this is not enough to code for 20 amino acids plus stop and start codons.
Question 6.
The final product of gene activity is a protein. But cells are made of many other molecules also,
including the phospholipid bilayer membrane, carbohydrate-derived structures such as cellulose, etc. Also the recent
elaboration of the human genome has revealed far fewer genes than the diversity of proteins in the human body
cells. How are these a result of gene activity?
Answer: In some cases, such as the phospholipid membrane, the cell structure grows by accretion. Other nonprotein structures may be products of metabolic synthesis provided by mainly protein pathways. And one segment of
DNA can be read in different ways by spliceosomes—a concept extended beyond what is covered here—thus
producing more proteins than genes. And some proteins can be mis-folded, as in prions in mad cow disease, and
result in different metabolic functions although they possess the same amino acid sequence. These new paths of
study do not overthrow the central dogma, but do make the process far more complex and now provide for
excetions.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
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CHAPTER
13 REGULATION OF GENE ACTIVITY
A detailed history and current understanding of operon research is followed by a description of
the various levels of control of gene expression in eukaryotic cells. Various forms of genetic
mutation—their causes, effects, and their consequences—are described. A study of cancer as a
failure in genetic control is presented. A Science Focus box describes alternative mRNA splicing
in disease.
Chapter Outline
13.1 Prokaryotic Regulation
1.
Bacteria do not require the same enzymes all the time; they produce just those needed at the moment.
2. Francois Jacob and Jacques Monod (1961) proposed the operon model to explain
regulation of gene expression in prokaryotes.
a. In the operon model, several genes code for an enzyme in the same metabolic
pathway and are located in a sequence on a chromosome; expression of structural
genes is controlled by the same regulatory genes.
b. An operon is a group of structural and regulatory genes that function as a single
unit; it includes the following:
1) A regulator gene, located outside the operon, codes for a repressor protein
molecule that controls whether the operon is active or not.
2) A promotor is the sequence of DNA where RNA polymerase attaches when a
gene is to be transcribed.
3) An operator is a short sequence of DNA where an active repressor binds,
preventing RNA polymerase from attaching to the promotor—transcription
therefore does not occur.
4) Structural genes are one to several genes coding for enzymes of a metabolic
pathway that are transcribed as a unit.
A. The trp Operon
1. Some operons in E. coli usually exist in the “on” rather than the “off” condition.
2. E. coli produces five enzymes as part of the anabolic pathway to synthesize the amino
acid tryptophan.
3. If tryptophan is already present in medium, these enzymes are not needed and the
operon is turned off.
a. The regulator codes for a repressor that usually is unable to attach to the operator.
b. The repressor has a binding site for tryptophan (if tryptophan is present, it binds
to the repressor).
c. This changes the shape of the repressor that now binds to the operator.
4. The entire unit is called a repressible operon; tryptophan is the corepressor.
5. Repressible operons are involved in anabolic pathways that synthesize substances
needed by cells.
B. The lac Operon
1. If E. coli is denied glucose and given lactose instead, it makes three enzymes to
metabolize the lactose.
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2. These three enzymes are encoded by three genes.
a. One gene codes for beta-galactosidase that breaks lactose to glucose and
galactose.
b. A second gene codes for a permease that facilitates entry of lactose into the cell.
c. A third gene codes for enzyme transacetylase, which is an accessory in lactose
metabolism.
3. The three genes are adjacent on a chromosome and under control of one promoter and
one operator.
4. The regulator gene codes for a lac operon repressor protein that binds to the operator
and prevents transcription of the three genes.
5. When E. coli is switched to medium containing an allolactose, this lactose binds to
the repressor and the repressor undergoes a change in shape that prevents it from
binding to the operator.
6. Because the repressor is unable to bind to the operator, the promoter is able to bind to
RNA polymerase, which carries out transcription and produces the three enzymes.
7. An inducer is any substance (lactose in the case of the lac operon) that can bind to a
particular repressor protein, preventing the repressor from binding to a particular
operator; consequently, RNA polymerase can bind to the promoter and transcribe the
structural genes.
C. Further Control of the lac Operon
1. Since E. coli prefers to break down glucose, how does E. coli know how to turn on
when glucose is absent?
2. When glucose is absent, cyclic AMP (cAMP) accumulates; cAMP has only one
phosphate group and attaches to ribose at two locations.
a. CAP is a catabolite activator protein (CAP) in the cytoplasm.
b. When cAMP binds to CAP, the complex attaches to a CAP binding site next to
the lac promoter.
c. When CAP binds to DNA, DNA bends, exposing the promoter to RNA
polymerase.
d. Only then does RNA polymerase bind to the promoter; this allows expression of
the lac operon structural genes.
3. When glucose is present, there is little cAMP in the cell.
a. CAP is inactive and the lactose operon does not function maximally.
b. CAP affects other operons when glucose is absent.
c. This encourages metabolism of lactose and provides a backup system for when
glucose is absent.
4. Active repressors shut down the activity of an operon—this is negative control.
5. CAP is an example of positive control; when the molecule is active, it promotes the
activity of the operon.
6. Use of both positive and negative controls allows the cell to fine-tune control of its
metabolism.
7. If both glucose and lactose are present, the cell preferentially metabolizes glucose.
13.2 Eukaryotic Regulation
1. Different cells in the human body turn on different genes that code for different
protein products.
2. Eukaryotes have four levels of regulatory mechanisms to control gene expression;
two in the nucleus and two in the cytoplasm.
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3. There are several levels of control that can modify the amount of gene product.
a. Chromatin structure: if genes are not accessible to RNA polymerase, they
cannot be transcribed.
1) Chromatin structure is part of epigenetic inheritance, the transmission of
genetic information outside the coding sequences of a gene.
b. Transcriptional control in the nucleus determines which structural genes are
transcribed and the rate of transcription; it includes transcription factors initiating
transcription and transposons (DNA sequences that move between chromosomes
and shut down genes).
c. Posttranscriptional control occurs in the nucleus after DNA is transcribed and
preliminary mRNA forms.
1) This may involve differential processing of mRNA before it leaves the
nucleus.
2) The speed that mature mRNA leaves nucleus affects the ultimate amount of
gene product.
d. Translational control occurs in cytoplasm after mRNA leaves the nucleus but
before there is a protein product.
1) The life expectancy of mRNA molecules can vary, as well as their ability to
bind ribosomes.
2) Some mRNAs may need additional changes before they are translated at all.
e. Posttranslational control occurs in the cytoplasm after protein synthesis.
1) Polypeptide products may undergo additional changes before they are
biologically functional.
2) A functional enzyme is subject to feedback control; binding of an end product
can change the shape of an enzyme so it no longer carries out its reaction.
A. Chromatin Structure
4. Eukaryotic DNA is in the form of chromatin, a stringy material associated with
proteins.
5. During interphase, some chromatin is highly compact, darkly stained, and genetically
inactive heterochromatin.
6. Barr bodies are an example of heterochromatin.
a. Since human males have only one X chromosome, it might be supposed that they
produce half the gene product of a female with two X chromosomes.
b. However, females have in each nucleated cell a darkly staining Barr body, a
condensed, inactive X chromosome.
c. Which X chromosome is condensed in each cell is determined by chance.
d. Thus, the body of heterozygous females is “mosaic”; half her cells express alleles
on one X chromosome and half of her cells express the alleles on the other X
chromosome.
e. Female gonads do not show Barr bodies; both X chromosomes are needed in
development.
f. Only one active X chromosome in the female zygote means that lower X-coded
gene products are normal.
g. Other examples of this mosaic effect include: ocular albinism, Duchenne
muscular dystrophy, and female calico cat coat color.
7. Euchromatin activity is related to the extent nucleosomes are coiled and condensed.
c. A nucleosome is a bead-like unit made of a segment of DNA wound around a
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complex of histone proteins.
d. When DNA is transcribed, activators called remodeling proteins are able to push
aside the histone proteins so transcription can begin.
8. Epigenetic inheritance is the term used to describe inheritance patterns that do not
depend on the genes themselves.
a. Histones proteins have different chemical modifications in heterochromatin and
euchromatin.
b. Methylation of DNA accounts for a phenomenon called genomic imprinting: gene
expression is dependent on whether the chromosome carrying the gene is
inherited from the mother or the father.
c. Epigenetic inheritance explains unusual inheritance patterns, and may have
implications in growth, aging, and cancer.
B. Transcriptional Control
1. Transcriptional control is the most critical level of genetic control.
a. Transcription is controlled by DNA-binding proteins called transcription
factors.
b. Each cell contains many different types of transcription factors.
c. A group of transcription factors binds to a promoter adjacent to a gene; the
complex attracts and binds RNA polymerase, but transcription may still not begin.
d. Transcription activators are often involved in controlling transcription in
eukaryotes.
1) Different combinations may regulate different genes.
2) Transcription activators bind to DNA regions called enhancers.
3) Enhancers can be quite a distance from the promoter, but a hairpin loop in the
DNA brings the activator attached to an enhancer into contact with the
promotor.
4) Mediator proteins act as a bridge between transcription factors and
transcription factors at the promotor.
e. Transcription factors are always present in the cell and most likely they have to be
activated in some way (e.g., regulatory pathways involving kinases or
phosphatases) before they bind to DNA.
C. Posttranscriptional Control
1. Posttranscriptional control includes mRNA processing and the speed at which
mRNA leaves the nucleus.
2. Messenger RNA molecules are processed before they leave the nucleus and enter the
cytoplasm.
3. Differential excision of introns and splicing of mRNA can vary the type of mRNA
that leaves nucleus.
a. The hypothalamus and thyroid glands produce calcitonin but the mRNA that
leaves the nucleus is not the same in both types of cells.
b. Evidence of different patterns of mRNA splicing is found in cells that produce
neurotransmitters, muscle regulatory proteins, and antibodies.
4. Speed of transport of mRNA from nucleus into cytoplasm affects the amount of gene
product realized per unit time following transcription; there is a difference in the
length of time it takes various mRNA molecules to pass through nuclear pores.
D. Translational Control
1. Translational control begins when the processed mRNA molecule reaches the
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cytoplasm and before there is a protein product.
a. The longer an active mRNA molecule remains in the cytoplasm, the more product
is produced.
b. Mature red blood cells eject their nucleus but synthesize hemoglobin for several
months; the mRNAs must persist during this time.
c. Ribonucleases are enzymes associated with ribosomes that degrade mRNA.
d. Mature mRNA has non-coding segments at 3' cap and 5' poly-A tail ends;
differences in these segments influence how long the mRNA avoids being
degraded.
e. MicroRNAs are small, processed pieces of intron; after microRNAs are
degraded, they combine with protein and the complex binds to mRNAs,
destroying them.
E. Posttranslational Control
1. Posttranslational control begins once a protein has been synthesized and has
become active.
a. Some proteins are not active after synthesis; the polypeptide product has to
undergo additional changes before it is biologically functional.
b. Bovine proinsulin, for example, is inactive when first produced; a single long
polypeptide folds into a three-dimensional structure, a sequence of 30 amino acids
is removed from the middle, and the two polypeptide chains are bonded together
by disulfide bonds resulting in an active protein.
c. Many proteins are short-lived in cells and degraded or destroyed so they are no
longer active.
d. Giant protein complexes called proteasomes carry out this task.
e. One example is the cyclins that control the cell cycle; they are only temporarily
present.
F. Alternative mRNA Splicing in Disease (Science Focus box)
1. Gorlin syndrome is an autosomal dominant disease that causes an aggressive type of
skin cancer.
a. It is linked to the tumor suppressor genet, patched, which is found on chromosome 9.
b. It was previously thought that patched mutations occurred within the gene’s exons,
but current research demonstrated these mutations occur within the gene’s introns.
c. The mutations caused mRNA to be incorrectly spliced and then making the proteins
nonfunctional.
2. Spinal muscular dystrophy (SMD) is also caused by defective pre-mRNA splicing.
a. Exon 7 of the SMN2 gene is omitted from the mature mRNA in SMA patients,
making the protein nonfunctional.
b. SMA results in loss of spinal cord motor neurons, paralysis and skeletal muscle
atrophy.
c. Scientists are working with a new technique, antisense oligonucleotide technology, to
reverse the effects of the disorder.
d. The results support that targeting pre-mRNA splicing may be a tangible treatment for
many of these disorders.
3. Scientist are using pre-mRNA splicing to design more powerful medicine with less side
effects, as well as new therapy that may be more effective in battling many disorders.
13.3 Regulation Through Gene Mutations
1. A gene mutation is a permanent change in the sequence of bases in DNA; mutations
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range from having no effect to total inactivity.
A. Causes of Mutations
1. Spontaneous mutations result in abnormalities in normal biological processes.
a. Mutations due to replication errors are very rare.
b. DNA polymerase constantly proofreads new DNA against the old, and repairs any
irregularities, thereby reducing mistakes to one out of every one billion nucleotide
pairs replicated.
2. Induced mutations are changes in the DNA base sequencing from exposure to toxic
chemicals or radiation.
a. Carcinogens are mutagens that increase the chances of cancer.
1) The Ames test is commonly used to determine if a chemical is carcinogenic.
2) A histidine-requiring strain of bacteria is exposed to a chemical.
3) If the chemical is mutagenic, the bacteria regain the ability to grow without
histidine.
c. Tobacco smoke contains a number of known carcinogenic chemicals.
d. X rays and gamma rays are ionizing radiation that creates free radicals, ionized
atoms with unpaired electrons.
e. Ultraviolet (UV) radiation is easily absorbed by pyrimidines in DNA.
1) Where two thymine molecules are near each other, UV may bond them
together as thymine dimers.
2) Usually dimers are removed from damaged DNA by special enzymes called
DNA repair enzymes.
B. Effect of Mutations on Protein Activity
1. Point mutations change a single nucleotide and therefore change a single specific
codon.
a. The effect of the point mutation depends on the specific base change in the codon.
b. Changes to codons that code for the same amino acid have no effect; e.g., UAU to
UAC both code for tyrosine.
c. A change from UAC to UAG (a stop codon) results in a shorter protein, and a
change from UAC to CAC incorporates histidine instead of tyrosine.
d. Sickle cell disease results from a single base change in DNA where the beta-chain
of hemoglobin contains valine instead of glutamate at one location and the
resulting distorted hemoglobin causes red blood cells to clog vessels and die off
sooner.
2. Frameshift Mutations
a. The reading frame depends on the sequence of codons from the starting point:
THE CAT ATE THE RAT.
b. If, for example, C is deleted, the reading frame is shifted: THE ATA TET HER
AT.
c. Frameshift mutations occur when one or more nucleotides are inserted or deleted
from DNA.
d. The result of a frameshift mutation is a new sequence of codons and
nonfunctional proteins.
C. Nonfunctional Proteins
1. A single nonfunctioning protein can cause dramatic effects.
2. The human transposon Alu is responsible for hemophilia when it places a premature
stop codon in the gene for clotting factor IX.
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3. PKU results when a person cannot convert phenylalanine to tyrosine; phenylalanine
builds up in the system, leading to mental retardation.
4. A faulty code for an enzyme in the same pathway results in an albino individual.
5. Cystic fibrosis is due to inheriting a faulty code for a chloride transport protein in the
plasma membrane.
6. Androgen insensitivity is due to a faulty receptor for male sex hormones; body cells
cannot respond to testosterone and the individual develops as a female (even though
all of the body cells are XY).
D. Mutations Can Cause Cancer
1. The development of cancer involves a series of various types of mutations.
2. Tumor-suppressor genes normally act as brakes on cell division when it begins to
occur abnormally.
3. When proto-oncogenes mutate, they become oncogenes.
4. Tumor-suppressor genes and proto-oncogenes often code for transcription factors or
proteins that control transcription factors.
5. P53, a major tumor-suppressor gene, is more frequently mutated in human cancers
than any other known gene.
a. The p53 protein acts as a transcription factor to turn on the expression of genes
whose products are cell cycle inhibitors.
b. The p53 can also stimulate apoptosis (programmed cell death).
6. Other proto-oncogenes code for Ras proteins, which are needed for normal cell
growth and DNA synthesis.
Lecture Enrichment Ideas
Experience Base: This material is quite complex; media visuals are critical and a DNA and
RNA model are recommended. Most students have a general concept of mutations and cancer
that will be highly refined by the information in the chapter. The material lends itself nicely to
extensive class discussion.
1. Describe why it would be important for a cell to have multiple levels of control of gene
expression for fine-tuning the amount of product synthesis, and how that importance might
be different for a bacterial cell as opposed to a cell in a multicellular eukaryote.
2. Discuss why there do not seem to be any operons in eukaryotic cells, while in bacterial cells
there are numerous complete systems of structural proteins regulated by the same operator
gene and repressor protein. Is it more biologically strengthening to not have operons? Ask
students to consider why the lac operon is inducible and the trp operon is repressible, with
emphasis on the kind of product and its use in each case.
3. Discuss the presence of genetic diseases in which DNA repair is damaged and the individual
has skin damage and possibly cancer—as a result of exposure to the sun’s ultraviolet rays.
Consider where the damage of the disease would be located, and why cancer might result.
4. Propose a scenario in which 70 essential units, equivalent to the 20 essential amino acids,
were required for living cells. Walk students through the mathematics to solve the question
of whether our codon system with three base pairs would provide the information necessary.
Would it take four or five?
5. Frameshift and point mutations require visual illustrations to illustrate what is happening to
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the codon sequences.
6. Compared to infectious diseases and structural disorders, cancer has remained a complex and
elusive medical problem with early clues pointing to viruses, heredity, and other agents.
Students understanding this content should understand why cancer has been difficult to
conquer, and why, compared to infectious diseases caused by one agent, are far more
complex.
7. DNA polymerase “proofreading” of new strands may lead some students to again attribute
vitalistic features to this process. While the chemistry is more complex, they should be
assured this is a complex but understandable chemical process well within natural
explanation.
Critical Thinking
Question 1. Once human red blood cells have left the bone marrow, they have lost their
nucleus and with it the genetic DNA coding for hemoglobin. How can they continue to
synthesize hemoglobin for several months without a nucleus?
Answer: The mRNA molecules, that were produced before the nuclear DNA was ejected, persist
for several months in the cytoplasm and synthesize hemoglobin. However, this is one of many
factors that contribute to the life of the red blood cell being so short. As the ragged cells wear out
from lack of repair, they are filtered from the bloodstream by the liver and spleen.
Question 2. Work with retroviruses indicates that many types of animal cells have a
substantial load of DNA that is due not to their own genes, but to DNA that is inserted into their
genome by retroviruses in the environment. Would the retroviral DNA in these body cells end up
being inherited by the host’s offspring?
Answer: Only if the infected cells are germ line cells producing eggs or sperm would it be
inherited. Infected somatic cells only give rise to their specialized type, such as skin cells.
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CHAPTER
14 BIOTECHNOLOGY AND GENOMICS
This chapter presents a detailed study of the new areas of biological research: biotechnology and genomics. A
description of the process of DNA cloning (including the polymerase chain reaction) is followed by societal
applications of this technology (genetically modified organisms, genetic profiling, etc.). The chapter concludes with
a discussion of gene therapy. The implications of the technology are presented vis a vis its commercial application
and its use in the treatment of human disease.
Chapter Outline
14.1 DNA Cloning
1.
Cloning is the production of identical copies of DNA through some asexual means.
a. An underground stem or root sends up new shoots that are clones of the parent plant.
b. Members of a bacterial colony on a petri dish are clones because they all came from division of
the same cell.
c. Human identical twins are clones; the original single embryo separate to become two individuals.
2. Gene cloning is production of many identical copies of the same gene.
a. If the inserted gene is replicated and expressed, we can recover the cloned gene or protein product.
b. Cloned genes have many research purposes: determining the base sequence between normal and
mutated genes, altering the phenotype, obtaining the protein coded by a specific gene, etc.
c. Humans can be treated with gene therapy: alteration of the phenotype in a beneficial way.
d. Otherwise transgenetic organisms are used to produce products desired by humans.
e. To clone DNA, scientists use recombinant DNA (rDNA) and polymerase chain reaction
(PCR).
A. Recombinant DNA Technology
1. Recombinant DNA (rDNA) contains DNA from two or more different sources.
2. To make rDNA, a technician selects a vector.
3. A vector is a plasmid or a virus used to transfer foreign genetic material into a cell.
4. A plasmid is a small accessory ring of DNA in the cytoplasm of some bacteria.
5. Plasmids were discovered in research on reproduction of intestinal bacteria Escherichia coli.
6. Introduction of foreign DNA into vector DNA to produce rDNA requires two enzymes.
a. Restriction enzyme is a bacterial enzyme that stops viral reproduction by cleaving viral DNA.
1) The restriction enzyme is used to cut DNA at specific points during production of rDNA.
2) It is called a restriction enzyme because it restricts growth of viruses but it acts as a molecular
scissors to cleave any piece of DNA at a specific site.
3) Restriction enzymes cleave vector (plasmid) and foreign (human) DNA.
4) Cleaving DNA makes DNA fragments ending in short single-stranded segments with “sticky
ends.”
5) The “sticky ends” allow insertion of foreign DNA into vector DNA.
b. DNA ligase seals the foreign gene into the vector DNA.
1) Treated cells take up plasmids, and then bacteria and plasmids reproduce.
2) Eventually, there are many copies of the plasmid and many copies of the foreign gene.
3) When DNA splicing is complete, an rDNA (recombinant DNA) molecule is formed.
7. If the human gene is to express itself in a bacterium, the gene must be accompanied by the regulatory
regions unique to bacteria and meet other requirements.
a. The gene cannot contain introns because bacteria do not have introns.
b. An enzyme called reverse transcriptase can be used to make a DNA copy of mRNA.
c. This DNA molecule is called complementary DNA (cDNA) and does not contain introns.
d. A laboratory DNA synthesizer can produce small pieces of DNA without introns.
B. The Polymerase Chain Reaction (PCR)
1. PCR can create millions of copies of a single gene or a specific piece of DNA in a test tube.
2. PCR is very specific—the targeted DNA sequence can be less than one part in a million of the total
dxcDNA sample; therefore a single gene can be amplified using PCR.
3. PCR uses the enzyme DNA polymerase to carry out multiple replications (a chain reaction) of target
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4.
5.
6.
DNA.
PCR automation is possible because heat-resistant DNA polymerase from Thermus aquaticus, which
grows in hot springs, is an enzyme that withstands the temperature necessary to separate doublestranded DNA.
Analyzing DNA
a. Mitochondria DNA sequences in modern living populations can decipher the evolutionary history
of human populations.
b. Short tandem repeat (STR) profiling is a technique used to analyze DNA.
1) Advantages include not being limited to using restriction enzymes.
2) Since the chromosomal location of STRs are known, these are the only locations that are
subjected to PCR and gel electrophoresis.
3) The band patterns are different in each person because each individual has their own number
of STR repeats at different locations.
c. DNA fingerprinting is a technique of using DNA fragment lengths, resulting from restriction
enzyme cleavage and amplified by PCR, to identify particular individuals.
d. DNA is treated with restriction enzymes to cut it into different sized fragments.
e. During gel electrophoresis, fragments separate according to length, resulting in a pattern of bands.
f. DNA fingerprinting can identify deceased individuals from skeletal remains, perpetrators of
crimes from blood or semen samples, and genetic makeup of long-dead individuals or extinct
organisms.
PCR amplification and DNA analysis is used to:
a. detect viral infections, genetic disorders, and cancer;
b. determine the nucleotide sequence of human genes, and, because it is inherited,
c. associate samples with DNA of parents.
14.2 Biotechnology Products
1. Genetically engineered organisms can produce biotechnology products.
2. Organisms that have had a foreign gene inserted into them are transgenic.
A. Genetically Modified Bacteria
1. Bacteria are grown in large vats called bioreactors.
a. Foreign genes are inserted and the product is harvested.
b. Products on the market include insulin, hepatitis B vaccine, t-PA, and human growth hormone.
2. Transgenic bacteria have been produced to protect and improve the health of plants.
a. Frost-minus bacteria protect the vegetative parts of plants from frost damage.
b. Root-colonizing bacteria receive genes from bacteria for insect toxin, protecting the roots.
c. Bacteria that colonize corn roots can be endowed with genes for insect toxin.
3. Transgenic bacteria can degrade substances.
a. Bacteria selected for ability to degrade oil can be improved by bioengineering.
b. Bacteria can be bio-filters to prevent airborne chemical pollutants from being vented into the air.
c. Bacteria can also remove sulfur from coal before it is burned and help clean up toxic dumps.
d. Bacteria can also be given “suicide genes” that cause them to die after they have done their job.
4. Transgenic bacteria can produce chemical products.
a. Genes coding for enzymes can be manipulated to catalyze synthesis of valuable chemicals.
b. Phenylalanine used in artificial sweetener can be grown by engineered bacteria.
5. Transgenic bacteria process minerals.
a. Many major mining companies already use bacteria to obtain various metals.
b. Genetically engineered “bio-leaching” bacteria extract copper, uranium, and gold from low-grade
ore.
B. Genetically Modified Plants
1. Plant cells that have had the cell wall removed are called protoplasts.
2. Electric current makes tiny holes in the plasma membrane through which genetic material enters.
3. The protoplasts then develop into mature plants.
4. Foreign genes now give cotton, corn, and potato strains the ability to produce an insect toxin and
soybeans are now resistant to a common herbicide.
5. Plants are being engineered to produce human proteins including hormones, clotting factors, and
antibodies in their seeds; antibodies made by corn deliver radioisotopes to tumor cells and a soybean
engineered antibody can treat genital herpes.
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C. Genetically Modified Animals
1. Animal use requires methods to insert genes into eggs of animals.
a. It is possible to microinject foreign genes into eggs by hand.
b. Vortex mixing places eggs in an agitator with DNA and silicon-carbide needles that make tiny
holes through which the DNA can enter.
c. Using this technique, many types of animal eggs have been injected with bovine growth hormone
(bGH) to produce larger fishes, cows, pigs, rabbits, and sheep.
2. Gene pharming is the use of transgenic farm animals to produce pharmaceuticals; the product is
obtainable from the milk of females.
a. Genes for therapeutic proteins are inserted into animal’s DNA; animal’s milk produces proteins.
b. Drugs obtained through gene pharming are planned for the treatment of cystic fibrosis, cancer,
blood diseases, and other disorders.
D. Cloning Transgenic Animals
1. For many years, it was believed that adult vertebrate animals could not be cloned; the cloning of Dolly
in 1997 demonstrated this can be done.
2. Cloning of an adult vertebrate would require that all genes of an adult cell be turned on again.
3. Cloning of mammals involves injecting a 2n nucleus adult cell into an enucleated egg.
4. The cloned eggs begin development in vitro and are then returned to host mothers until the clones are
born.
E. Applications of Transgenic Animals
1. Scientists are using transgenic animals to illustrate that maleness is due to a section of DNA called SRY
(the sex determining region of the Y chromosome).
2. Transgenic animals are also being used to investigate various treatments for diseases by eliminating
genes from these animals.
3. Organ transplants from transgenic animal donors to human recipients is called xenltransplantation.
14.3 Gene Therapy
1. Gene therapy involves procedures to give patients healthy genes to make up for a faulty gene.
2. Gene therapy also includes the use of genes to treat genetic disorders and various human illnesses.
3. There are ex vivo (outside body) and in vivo (inside body) methods of gene therapy.
A. Ex Vivo Gene Therapy
1. Children with severe combined immunodeficiency (SCID) underwent ex vivo gene therapy.
a. Lacking the enzyme ADA involved in maturation of T and B cells, they faced life-threatening
infections.
b. Bone marrow stem cells are removed, infected with a retrovirus that carries a normal gene for the
enzyme ADA, and returned.
c. Use of bone marrow stem cells allows them to divide and produce more cells with the same genes.
d. Patients who undergo this procedure show significant improvement.
2. Gene therapy include treatment of familial hypercholesterolemia where liver cells lack a receptor for
removing cholesterol from blood.
a. High levels of blood cholesterol make the patient subject to fatal heart attacks when young.
b. A small portion of the liver is surgically removed and infected with retrovirus with normal gene
for receptor.
c. This has lowered cholesterol levels following the procedure.
B. In Vivo Gene Therapy
1. Cystic fibrosis patients lack a gene for trans-membrane chloride ion carriers; patients die from
respiratory tract infections.
a. Liposomes, microscopic vesicles that form when lipoproteins are in solution, are coated with
healthy cystic fibrosis genes and sprayed into a patient’s nostrils.
b. Various methods of delivery are being tested for effectiveness.
2. A gene for vascular endothelial growth factor (VEGF) can be injected alone or within a virus into the
heart to stimulate branching of coronary blood vessels.
3. Another strategy is to make cancer cells more vulnerable, and normal cells more resistant, to
chemotherapy.
4. Injecting a retrovirus containing a normal p53 gene—that promotes apoptosis—into tumors may stop
the growth of tumors.
14.4 Genomics
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A.
B.
C.
D.
E.
F.
G.
1. Genetics in the 21st century concerns genomics: the study of genomes of humans and other organisms.
Sequencing the Genome
1. The Human Genome Project has produced a record of all the base pairs in all our chromosomes.
2. The task took 13 years to learn the sequence of the three billion base pairs along the length of our
chromosomes.
3. Most of the DNA regions that differed among individuals are called single nucleotide polymorphisms
(SNPs)
a. Many SNPs have no effect and others could contribute to enzymatic differences affecting the
phenotype.
4. Following functional genomics, structural genomics investigates base sequences and the number of
genes in humans.
Genome Comparisons
1. There is little difference between the sequence of our bases and other organisms whose DNA
sequences are known.
2. We share a large number of genes with simpler organisms (e.g., bacteria, yeast, mice); perhaps our
uniqueness is due to regulation of these genes.
3. Researchers found that certain genes on chromosome 22 differed in humans and chimpanzees: those
for speech development, hearing, and smell.
4. Many genes found were responsible for human diseases.
Eukaryotic Gene Structure
1. Eukaryotic chromosome structure is much more complex than prokaryotic chromosome structure.
2. Generally speaking, more complex organisms have more complex genes with more and larger introns.
3. Introns are currently regarded as gene expression regulators and determine which genes are expressed
and how they are spliced.
Intergenic Sequences
1. Intergenic sequences are DNA sequences that occur between genes.
2. Generally speaking, as the complexity of an organism increases, so does the proportion of its nonprotein-coding DNA sequences.
3. Most of the human chromosomes are comprised of intergeneic sequences, genes represent 1.5% of
human’s total DNA, and the remainder represents “junk DNA,” which scientists believe serve many
important functions.
4. Three types of intergenic sequences found in the human genome are: repetitive elements, tranposons,
and unknown sequences.
Repetative Elements
1. Repetative DNA elements occur when the same sequence of two or more nucleotides are repeated
many times along the length of one or more chromosomes.
2. They make up nearly half of the human genome and scientists believe that their true significance have
yet to be discovered.
3. Repetative DNA elements occur as tandem repeats—meaning the repeated sequences are next to each
other on the chromosome.
4. One type of tandem repeat sequence, short tandem repeats (STRs), are a standard method in forensic
science for identifying one individual from another and also determining family relationships.
5. Another type of repetitive DNA element is called an interspersed repeat—meaning the repetitions
may be placed intermittently along a single chromosome, or across multiple chromosomes.
6. Interspersed repeat are common and thought to play a role in the evolution of new genes.
Transposons
2. Transposons are specific DNA sequences that move with and between chromosomes.
a. Their movement may increase or decrease the expression of neighboring genes.
b. They are among the 40% of the human genome consisting of the same short sequence of DNA
continuously repeated.
c. They are noncoding sequences that play regulatory functions, and could thus be cansidered part of
epigenetic inheritance.
Unknown Sequences
1. Unknown DNA sequences were once dismissed as junk DNA since scientists could not identify any
function they have.
2. Recently scientists believe observed that many of the unknown sequences is transcribed into RNA.
3. This RNA may carry out regulatory functions more easily than proteins at times.
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H. What Is a Gene?
1. A modern definition of a gene, suggested by Mark Gerstein and associates, “…is a genomic sequence
(either DNA or RNA) directly encoding functional products, either RNA or protein.”
2. This definition recognizes that the genetic material involved does not have to be DNA, and that coding
does not only mean DNA sequencing, but the gene product could be RNA or protein as well.
I. Functional and Comparative Genomics
1. The emphasis on genome structure is both on functional and comparative genomics.
2. Functional genomics aims to understand the exact role of the genome in cells or organisms.
a. To meet the aim of functional genomics, DNA microarrays can be used (see Science Focus box).
b. DNA microarrays rapidly identify a person’s complete genotype; this is called the genetic profile.
3. Comparative genomics aims to compare the human genome to the genome of other organisms.
a. This type of genomics has revealed little difference between DNA sequence of our bases and those
of many other organisms.
J. Proteomics
1. Proteomics is the study of the structure, function, and interaction of cellular proteins.
2. The information obtained from proteomic studies can be used in designing better drugs, and to
correlate drug treatment to the particular genome of the individual.
K. Bioinformatics
1. Bioinformatics is the application of computer technologics to the study of the genome.
2. Information obtained from computer analysis of the genome can show relationships between genetic
profiles and genetic disorders.
L. DNA Microarray Technology (Science Focus box)
1. DNA microarray technology is being used to identify genes associated with gene tissues, and identify links
between disease and chromosomal variation.
2. DNA microarray technology techniques begin with labeling the test mRNA with a fluorescent dye added to
the chip.
3. The tagged mRNA can then be followed by identifying the genes that exhibit the fluorescent dye.
M. Copy Number Variations (Science Focus box)
1. Scientists have recently discovered small duplications and deletions in chromosomes, referred to as copy
number variations (CNVs).
2. The change in genes may arise from fork stalling and template switching.
3. Some CNVs may be linked to diseases such as HIV, Lupus, and autism.
4. Evolutionarily speaking, it may be advantageous for species to have multiple copies of genes because if one
copy of an allele does not function properly, the duplicate allele may be able to restore normal
function.
5. In addition, having two normal alleles may free the extra gene copy to accumulate mutations, which could
ultimately lead to the formation of a new gene.
Lecture Enrichment Ideas
Experience Base: Nearly all concepts in this section are somewhat abstract and build upon the
previous chapters. Visuals are critical for elaborating the recombinant DNA processes, vector
mechanisms, and genomic library. However, new developments are reported regularly in the
media and provide leads into discussion of the human genome project, cloning, and transgenic
bacterial, plant and animal products.
1. Use current news reports on the production of a giant mouse through use of human or rat
growth hormone, the production of pigs with leaner meat from use of bovine or human
growth hormone, or some of the recent clinical trials on gene transfer in cancer and other
disease treatment. Genetically-modified (GM) organisms are a source of tension and trade
disagreement among many countries.
2. Explain why it is necessary to remove the introns from genes added to bacteria. Explain why
it would work to use yeast to transcribe genes containing introns that could not be transcribed
97
and translated into usable proteins in bacteria.
3. The distinction between ex vivo and in vivo methods of gene therapy is important relative to
the ability to confirm that the inoculum is transgenic; in vivo procedures are sometimes
difficult to confirm.
4. The polymerase chain reaction (PCR) is central to much modern molecular biology work;
note the action of the DNA polymerase and the use of heat to denature the DNA and release
the newly synthesized strands. Include the source of the heat-resistant DNA polymerase from
Thermus aquaticus, which grows in hot springs and speculate on whether this process could
have been invented without it.
5. A decade ago, fabulous claims were being made for mapping the human genome, as if that
would ultimately reveal a full understanding of the human organism. Note that knowing the
full sequence of DNA base pairs does not automatically provide an understanding of how that
cell physiology works or provide us with instant cures, but shifts our research to
understanding developmental biology.
6. Many students will be surprised at the new lower estimate of the number of genes in the
human genome (~25,000), especially since textbooks often continue to publish much higher
numbers. The evidence presented in the previous chapters on multiple products from
common pathways, and recent research in developmental biology both indicate far fewer
“genes” than previously calculated.
Critical Thinking
Question 1.
Whenever anything dies, DNA and RNA are released into the environment. Ocean water is full of
fragments of these molecules. Why are DNA and RNA not constantly seeping into all living organisms?
Answer: From the text discussion of the need for packages for DNA and RNA, it is obvious that both molecules are
easily denatured in the environment and must be protected to be delivered in plasmids, etc.
Question 2.
For a mammalian gene to be expressed in a bacterium, what two provisions must be met?
Answer: The mammalian gene must include all proper regulatory regions for it to function, and the introns must be
removed because bacteria lack the enzymes to process intron mRNA.
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98
PART
III EVOLUTION
Evolution provides the unifying concepts for the following chapters that consider the diversity of
organisms and the progressive development of derived anatomy and physiology. This section
discusses the history and mechanisms of evolution, the history of life, and the most recent
developments in classifying organisms.
15
16
17
18
19
Darwin and Evolution
How Populations Evolve
Speciation and Macroevolution
Origin and History of Life
Systematics and Phylogeny
CHAPTER
15 Darwin and Evolution
Chapter Outline
15.1 History of Evolutionary Thought
1. In 1831, Charles Darwin, a 22-year-old naturalist, accepted a position aboard the ship
HMS Beagle that began a voyage around the world; it provided Darwin with many
observations.
2. The pre-Darwinian world-view was different from the post-Darwinian.
a. Pre-Darwinian world-view was determined by intractable theological beliefs.
1) The earth is young.
2) Each species was specially created and did not change over time.
3) Variations are imperfections varying from a perfectly-adapted creation.
4) Observations are to substantiate the prevailing worldview.
b. Darwin, however, lived during a time of great change in scientific and social
realms.
c. Darwin’s ideas were part of a larger change in thought already underway among
biologists; this concept would eventually be known as evolution.
A. Mid-Eighteenth-Century Contributions
1. Carolus Linnaeus and Taxonomy
a. Taxonomy is the science of classifying organisms; taxonomy had been a main
concern of biology.
b. Carolus Linnaeus (1707–1778) was a Swedish taxonomist.
1) Linnaeus developed a binomial system of nomenclature (two-part names for
each species [e.g., Homo sapiens]).
2) He developed a system of classification for all known plants.
3) Like other taxonomists of his time, Linnaeus believed in the ideas of
a) special creation—each species had an “ideal” structure and function; and
b) fixity of species—each species had a place in the scala naturae, a sequential ladder of life.
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c. Linnaeus thought that classification should describe the fixed features of species
and reveal God’s divine plan.
d. His ideas reflected the ideas of Plato and Aristotle: the ideal form can be deduced,
and organisms can be arranged in order of increasing complexity.
e. His later work with hybridization suggested species might change with time.
2. Georges Louis Leclerc
a. Georges Louis Leclerc, known by his title, Count Buffon (1707–1788), was a
French naturalist.
b. He wrote a 44-volume natural history of all known plants and animals.
c. He also provided evidence of descent with modification.
d. His writings speculated on influences of the environment, migration, geographical
isolation, and the struggle for existence.
e. Buffon vacillated on whether he believed in evolutionary descent and he
professed to believe in special creation and the fixity of species.
3. Erasmus Darwin
a. Erasmus Darwin (1731–1802) was Charles Darwin’s grandfather.
b. He was a physician and a naturalist whose writings on both botany and zoology
contained many comments that suggested the possibility of common descent.
c. He based his conclusions on
1) changes undergone by animals during development,
2) artificial selection by humans, and
3) the presence of vestigial structures (structures or organs that are believed to
have been functional in an ancestor but are reduced and nonfunctional in a
descendant).
d. Erasmus Darwin offered no mechanism by which evolutionary descent might
occur.
B. Late Eighteenth/Early-Nineteenth Century Contributions
1. Cuvier and Catastrophism
a. George Cuvier (1769–1832), a French vertebrate zoologist, was the first to use
comparative anatomy to develop a system of classifying animals.
b. He founded the science of paleontology—the study of fossils—and suggested
that a single fossil bone was all he needed to deduce the entire anatomy of an
animal.
c. To explain the fossil record, Cuvier proposed that a whole series of catastrophes
(extinctions) and repopulations from other regions had occurred.
d. Cuvier was also a staunch advocate of special creation and fixity of species; this
presented him with a problem when geological evidence of a particular region
showed a succession of life forms in the Earth’s strata.
e. Catastrophism is the term applied to Cuvier’s explanation of fossil history: the
belief that catastrophic extinctions occurred, after which repopulation of surviving
species occurred, giving an appearance of change through time.
2. Lamarck and Acquired Characteristics
a. Lamarck (1744–1829) was the first to state that descent with modification occurs
and that organisms become adapted to their environments.
b. Lamarck, an invertebrate zoologist, held ideas at odds with Cuvier’s.
c. Lamarck mistakenly saw “a desire for perfection” as inherent in all living things.
d. Inheritance of acquired characteristics was Lamarck’s belief that organisms
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become adapted to their environment during their lifetime and pass these
adaptations to their offspring.
e. Experiments fail to uphold Lamarck’s inheritance of acquired characteristics; the
molecular mechanism of inheritance shows phenotypic changes do not result in
genetic changes that can be passed on to the next generation.
15.2 Darwin’s Theory of Evolution
A. Darwin’s Background
1. His nature was too sensitive to pursue medicine; he attended divinity school at
Cambridge.
2. He attended biology and geology lectures and was tutored by the Reverend John
Henslow.
3. Henslow arranged his five-year trip on the HMS Beagle; Darwin was an observant
student of nature.
B. Geology and Fossils
1. His study of geology and fossils caused him to concur with Lyell that the observed
massive geological changes were caused by slow, continuous processes.
a. Darwin took Lyell’s book on the voyage of the HMS Beagle.
b. In his book Principles of Geology, Charles Lyell presented arguments to support a
theory of geological change proposed by James Hutton.
c. In contrast to catastrophists, Hutton proposed that Earth was subject to slow but
continuous geological processes (e.g., erosion and uplifting) that occur at a
uniform rate, a theory called uniformitarianism.
d. The Argentina coast had raised beaches; he witnessed earthquakes raising Earth
several feet.
e. Marine shells occurred far inland and at great heights in the Andes.
f. Fossils of huge sloths and armadillo-like animals suggested modern forms were
descended from extinct forms with change over time; therefore species were not
fixed.
C. Biogeography
1. Biogeography is the study of the geographic distribution of life forms on Earth.
2. Patagonian hares replaced rabbits in the South American grasslands.
3. The greater rhea found in the north was replaced by the lesser rhea in the south.
4. Comparison of the animals of South America and the Galápagos Islands caused
Darwin to conclude that adaptation to the environment can cause diversification,
including origin of new species.
5. The Galápagos Islands
a. These volcanic islands off the South American coast had fewer types of
organisms.
b. Island species varied from the mainland species, and from island-to-island.
c. Each island had a variation of tortoise; long and short necked tortoises correlated
with different vegetation.
d. Darwin’s Finches
1) Finches on the Galápagos Islands resembled a mainland finch but there were
more types.
2) Galápagos finch species varied by nesting site, beak size, and eating habits.
3) One unusual finch used a twig or thorn to pry out insects, a job normally done
by (missing) woodpeckers (Darwin never witnessed this finch behavior).
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D.
E.
F.
G.
4) The variation in finches posed questions to Darwin: did they descend from
one mainland ancestor or did islands allow isolated populations to evolve
independently, and could present-day species have resulted from changes
occurring in each isolated population?
Natural Selection and Adaptation
1. Darwin decided that adaptations develop over time; he sought a mechanism by which
adaptations might arise.
2. Natural selection was proposed by both Alfred Russel Wallace and Darwin as a
driving mechanism of evolution caused by environmental selection of organisms most
fit to reproduce, resulting in adaptation.
3. Because the environment is always changing, there is no perfectly-adapted organism.
4. Natural selection is a process consisting of these conditions:
a. The members of a population have random but heritable variations.
b. In a population, many more individuals are produced each generation than the
environment can support.
c. Some individuals have adaptive characteristics that enable them to survive and
reproduce better. Darwin called the ability to have more offspring, differential
reproductive success.
d. The result of natural selection is a population adapted to its local environment.
5. Natural selection can only utilize variations that are randomly provided; therefore
there is no directedness or anticipation of future needs.
6. Extinction occurs when previous adaptations are no longer suitable to a changed
environment.
Organisms Have Inheritable Variations
1. In contrast to the previous worldview where imperfections were to be ignored,
variations were essential in natural selection.
2. Darwin suspected, but did not have today’s evidence, that the occurrence of variation
is completely random.
3. New variations are as likely to be harmful as helpful.
4. Variations that make adaptation possible are those that are passed on from generation
to generation.
5. Darwin could not state the cause of variations because genetics was not yet
established.
6. Natural selection only operates on variations that are already available in a
population’s gene pool.
Organisms Compete for Resources
1. Darwin and Wallace both read an essay by Thomas Malthus, a socioeconomist.
2. Malthus proposed that human populations outgrow food supply and death and famine
were inevitable.
3. Darwin applied this to all organisms; resources were not sufficient for all members to
survive.
Organisms Differ in Reproductive Success
1. Organisms whose traits enable them to reproduce to a greater degree have a greater
fitness.
a. Fitness is a measure of an organism’s reproductive success.
b. Black western diamondback rattlesnakes are more likely to survive on lava flows;
lighter-colored rattlesnakes are more likely to survive on desert soil.
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H.
I.
J.
K.
2. Darwin noted that humans carry out artificial selection.
a. Early humans likely selected wolf variants; consequently, desirable traits increase
in frequency in subsequent generations and produced the varieties of domestic
dogs.
b. Russian scientists have produced silver foxes that allow themselves to be petted
rather than running away from humans.
c. Many crop plant varieties can be traced to a single ancestor.
Organisms Become Adapted
1. An adaptation is a trait that helps an organism be more suited to its environment.
2. Unrelated organisms living in the same environment often display similar
characteristics.
3. Because of differential reproduction, adaptive traits increase in each succeeding
generation.
On the Origin of Species by Darwin
1. After the HMS Beagle returned to England in 1836, Darwin waited over 20 years to
publish.
2. He used the time to test his hypothesis that life forms arose by descent from a
common ancestor and that natural selection is a mechanism by which species can
change and new species arise.
3. Darwin was forced to publish Origin of Species after reading a similar hypothesis by
Alfred Russel Wallace.
Alfred Russel Wallace (Science Focus box)
1. Alfred Russel Wallace (1823-1913) was an English naturalist who independently and
simultaneously proposed natural selection as a mechanism for evolution.
2. He and the entomologist Henry Walter Bates took a collecting trip to the Amazon,
and then to the Malay Archipelago from 1854-1864).
3. After studying the animals from his trips, he divided the islands into a western and
eastern group. This sharp line dividing the two islands group is now known as
Wallace’s Line.
4. Wallace’s Line is located near a deep channel between the Oriental and Australian
regions. This area serves as an impassable barrier to animal dispersal.
5. In 1855 Wallace wrote an essay entitled, “On the Law Which Has Regulated the
Introduction of New Species.” At this point he saw that species share a common
ancestry and that species also change over time.
6. In 1858 Wallace concluded changes in species are due to changes in the environment
through natural selection.
7. Wallace wrote a manuscript of these findings and sent it to Charles Darwin for
review. Darwin was shocked that Wallace had the same theories that he had. Darwin
told Wallace to publish his manuscript at once.
8. Darwin published Origin of the Species one year later while Wallace was in the field.
9. Although Darwin overshadows Wallace, Wallace is still referred to as “England’s
Greatest Living Naturalist.”
Natural Selection Can Be Witnessed
1. Darwin formed his natural selection hypothesis by observing the distribution of
tortoises and finches on the Galápagos Islands.
2. Scientists are currently witnessing natural selection on the Galápagos Islands. Peter
and Rosemary Grant have been observing finch beak size change with rainfall.
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During wet seasons, the offspring have smaller beaks to eat the small seeds, however,
in drier seasons, offspring have larger beaks capable of breaking harder seeds that
survive the dry weather.
3. Other examples of natural selection can be seen in marine snails, and the scarlet
honeycreeper, plants and bacteria.
4. A common example of natural selection is industrial melanism. Prior to the industrial
revolution in Great Britain, light-colored peppered moths were more common (90%)
than the dark-colored peppered moths (10%).
a. Following the industrial revolution and increase in pollution, dark-colored
peppered moths reached 80% of the peppered moth population.
b. Once legislation regulated pollution reduction, dark-colored peppered moths
reduced to 16% in one of the collecting sites.
15.3 Evidence for Evolution
A. Common Descent
1. The hypothesis of common descent is supported by many lines of evidence.
2. The more varied the evidence, the more certain it becomes.
B. Fossils Evidence
1. Fossils are the remains and traces of past life or any other direct evidence of past life.
2. Fossils include skeletons, shells, seeds, insects trapped in amber, and imprints of
leaves.
3. Transitional fossils reveal links between groups.
a. Archeopteryx is an intermediate between reptiles and birds.
b. Ambulocetus natans is a whale with legs.
C. Biogeographical Evidence
1. Biogeography studies the distribution of plants and animals worldwide.
2. Distribution of organisms is explained by related forms evolving in one locale and
spreading to other accessible areas.
a. Darwin observed South America had no rabbits; he concluded rabbits originated
elsewhere.
b. Biogeography explains the abundance of finch species on the Galápagos Islands
lacking on the mainland.
3. Physical factors, such as the location of continents, determine where a population can
spread.
a. Cacti are restricted to North American deserts and euphorbia grow in African
deserts.
b. Marsupials arose when South America, Antarctica, and Australia were joined;
Australia separated before placental mammals arose, so only marsupials
diversified in Australia.
D. Anatomical Evidence
1. Organisms have anatomical similarities when they are closely related because of
common descent.
a. Homologous structures in different organisms are inherited from a common
ancestor.
b. Analogous structures are inherited from unique ancestors and have come to
resemble each other because they serve a similar function.
c. Vertebrate forelimbs contain the same sets of bones organized in similar ways,
despite their dissimilar functions.
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2. Vestigial structures are remains of a structure that was functional in some ancestors
but is no longer functional in the organism in question.
a. Most birds have well-developed wings; some bird species have reduced wings
and do not fly.
b. Humans have a tailbone but no tail.
c. Presence of vestigial structures is explained by the common descent hypothesis;
these are traces of an organism’s evolutionary history.
3. Embryological development reveals a unity of plan.
a. During development, all vertebrates have a post-anal tail and paired pharyngeal
pouches.
1) In fishes and amphibian larvae, the pouches become gills.
2) In humans, the first pair of pouches becomes a cavity of middle ear and
auditory tube; the second pair becomes tonsils, while the third and fourth pairs
become thymus and parathyroid glands.
3) The above features are explained if fishes are ancestral to other vertebrate
groups.
E. Biochemical Evidence
1. All living organisms use the same basic biochemical molecules, e.g., DNA, ATP, and
many identical or nearly identical enzymes.
2. Organisms utilize the same DNA triplet code and the same 20 amino acids in their
proteins.
3. Many organisms share the same introns and types of repeats, which is remarkable
since there is no obvious functional reason why these components need to be so
similar.
4. This is substantiated by the analysis of the degree of similarity in amino acids for
cytochrome c among organisms.
5. These similarities can be explained by descent from a common ancestor.
6. Life’s vast diversity has come about by only a slight difference in the same genes.
F. Because it is supported by so many lines of evidence, evolution is no longer considered a
hypothesis.
1. Evolution is one of the great unifying theories of biology, similar in status to the germ
theory of disease in medicine.
2. In science, theory is reserved for those conceptual schemes that are supported by a
large number of observations or a large amount of experimental evidence and have
not been found lacking.
Lecture Enrichment Ideas
Experience Base: Large changes in organisms over time is a long term process. This means that
much of the evidence for evolution, beyond microevolutionary shifts in bacterial cultures, etc.,
must be illustrated by reasoning. Likewise a substantial number of students will approach this
topic with speculation or animosity due to particular religious beliefs or prior misconceptions.
1. Clarify the fact that Lamarck was the first to try to develop a possible mechanism for
evolution and should be considered among the great biologists of history, rather than the one
who got evolution wrong.
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2. Speculate how Darwin would have reacted to Mendel’s work if he had read it. (Darwin had a
copy of the publication of Mendel’s papers, but the pages had never been cut so that it could
be read.) Darwin did not have a mechanism to explain the passage of traits from parents to
offspring, which seriously troubled him.
3. Consider why Darwin waited over twenty years from his seminal trip on the HMS Beagle to
publish his theory. One of the topics to consider is Darwin’s feelings for his wife, who was a
very religious woman, and the impact that the theory of natural selection and evolution had
on those who considered the theory of special creation to be absolute truth.
4. Note that both Darwin and Wallace had extensive travel experiences, where they saw a wide
range of organisms, before they formulated their theory of evolution by natural selection.
Critical Thinking
Question 1. If there is no variation in a population, or if the only variation is acquired and not
inherited, or if all of the progeny survive and equally reproduce, will evolution occur?
Answer: No.
Question 2.
Why are an insect wing and a bird wing not considered evidence of relatedness?
Answer: While both bear the name “wing” because they help each to fly, these wings are not
homologous structures. The bird wing is a forearm, similar in origin to the forearm of a frog,
lizard, or human. The insect wing is a completely different structure with no origin in common
with vertebrate wings or forearms, and independent even from insect legs.
Question 3. If you have a cut on your finger that leaves a permanent scar, this readily shows in
fingerprinting and does distinguish you from the rest of the human population. Will such a
variation become part of the evolutionary fate of your lineage through natural selection?
Answer: No, because the trait was acquired and will not exhibit itself in offspring.
Question 4. If a wolf loses a leg in a trap and this cripples its ability to hunt, it has far less
chance of leaving offspring. Will that variation become part of the evolutionary fate of its lineage
through natural selection?
Answer: No, again because the trait was acquired and will not exhibit itself in the offspring.
Certainly acquired characteristics can change survival and offspring potential but only for the
present generation. Since the traits will not be exhibited in any offspring that do survive, the
acquired trait is still inconsequential to the evolutionary fate of the lineage. Because it is not
inherited, it is “history.”
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
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CHAPTER
16 HOW POPULATIONS EVOLVE
This chapter presents sections on population genetics, microevolution, natural selection, and the
processes which govern each. The Hardy-Weinberg principle and its applications are described
in detail. Much terminology and many definitions used in the study of evolutionary processes are
presented.
Chapter Outline
16.1
Population Genetics
1. A population is all of the members of a single species occupying a certain area at the
same time.
2. Population genetics studies the variation in alleles in a gene pool.
A. Genetic Variation
1. Populations can have many phenotypic, and therefore genotypic, differences.
2. Investigators study DNA sequencing to discover the amount of genetic variation in a
population.
3. DNA sequencing has allowed scientists to discover various loci that exhibit single
nucleotide polymorphisms (SNPs).
a. SNPs are DNA sequences in an organism’s genome that differ by a single
nucleotide.
b. SNPs can cause changes in amino acid sequences or other regulatory differences,
and are therefore thought to be an important source of genetic variations in
populations of all organisms.
4. New research discovered that humans inherit patterns of base-pair differences called
haplotypes (from the terms haploid and genotype).
a. The HapMap Project catalogs sequence differences, called haplotypes, in humans.
b. Researchers want to link haplotypes to the risk of specific illnesses, in the hope it
will lead to new methods of preventing, diagnosing, and treating disease.
B. Microevolution
1. It was not until the 1930s that population geneticists were able to apply the principles
of genetics to populations and thus to recognize when evolution had occurred.
2. Evolution that occurs within a population is called microevolution.
3. The gene pool is the total of all the alleles in a population; it is described in terms of
gene frequencies.
4. Neither dominance nor sexual reproduction changes allele frequencies.
5. The Hardy-Weinberg principle
a. This principle states an equilibrium of allele frequencies in a gene pool (using a
formula p2 + 2pq + q2) remains in effect in each succeeding generation of a
sexually reproducing population if five conditions are met.
1) No mutation: no allelic changes occur, or changes in one direction are
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balanced by changes in the other direction.
2) No gene flow: migration of alleles into or out of the population does not
occur.
3) Random mating: individuals pair by chance and not according to their
genotypes or phenotypes.
4) No genetic drift: the population is large so changes in allele frequencies due to
chance are insignificant.
5) No selection: no selective force favors one genotype over another.
b. In real life, conditions of the Hardy-Weinberg law are rarely if ever met, and
allele frequencies in the gene pool of a population do change from one generation
to the next, resulting in evolution.
c. Any change of allele frequencies in a gene pool of a population signifies that
evolution has occurred.
d. The Hardy-Weinberg law tells us what factors cause evolution—those that violate
the conditions listed.
e. A Hardy-Weinberg equilibrium provides a baseline by which to judge whether
evolution has occurred.
f. Hardy-Weinberg equilibrium is a constancy of gene pool frequencies that remains
across generations.
6. Industrial Melanism
a. The case of the peppered moths provides a case study in a shift in phenotype
frequencies under selection.
b. Before trees became coated with soot from air pollution, the percentage of darkcolored moths was 10%.
c. With birds acting as a selective agent, the light colored moths were reduced while
dark-colored moths were better adapted to survive on the darkened trees.
d. The last generation observed has 80% dark-colored moths.
C. Causes of Microevolution
1. Mutations
a. Mutations are permanent genetic changes.
b. Without mutations, there could be no inheritable phenotypic variations among
members of a population.
c. Mutations are the primary source of genetic differences among prokaryotes that
produce asexually.
d. In sexual reproducing organisms, both mutations and sexual recombination are
important in generating phenotypic differences.
2. Nonrandom Mating and Gene Flow
a. Random mating involves individuals pairing by chance, not according to
genotype or phenotype.
b. Nonrandom mating occurs when certain genotypes or phenotypes mate with one
another.
c. Assortive mating is a type of nonrandom mating that occurs when individuals
tend to mate with those having the same phenotype with respect to a certain
characteristic.
d. Assortative mating divides a population into two phenotypic classes with reduced
gene exchange.
e. Homozygotes for gene loci that control a trait increase, and heterozygotes for
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these loci decrease.
f. Gene flow (gene migration) is the movement of alleles among populations by
migration of breeding individuals.
g. Gene flow can increase variation within a population by introducing novel alleles
produced by mutation in another population.
h. Continued gene flow decreases diversity among populations, causing gene pools
to become similar.
i. Gene flow among populations can prevent speciation from occurring.
3. Genetic Drift
a. Genetic drift refers to changes in allele frequencies of a gene pool due to chance
rather than selection by the environment.
b. Genetic drift occurs in both large and small populations; small populations are
more likely to show the effects of drift.
c. Genetic drift occurs when founders start a new population, or after a genetic
bottleneck with interbreeding.
d. The bottleneck effect prevents most genotypes from participating in production
of the next generation.
1) The bottleneck effect is caused by a severe reduction in population size due to
a natural disaster, predation, or habitat reduction.
2) The bottleneck effect causes a severe reduction in the total genetic diversity of
the original gene pool.
3) The cheetah bottleneck causes relative infertility because alleles were lost due
to intense inbreeding when populations were reduced in earlier times.
e. The founder effect is an example of genetic drift where rare alleles or
combinations occur in higher frequency in a population isolated from the general
population.
1) This is due to founding individuals containing a fraction of total genetic
diversity of the original population.
2) Which particular alleles are carried by the founders is dictated by chance
alone.
3) As an example, dwarfism is much higher in a Pennsylvania Amish community
due to a few German founders.
16.2 Natural Selection
1. Natural selection favores the phenotype that is the most adaptive under the present
environmental circumstances. In this context, there are three types of natural
selection: stabilizing, directional, or disruptive.
2. Stabilizing selection occurs when extreme phenotypes are eliminated and the
intermediate phenotype is favored.
a. The average number of eggs laid by Swiss starlings is four or five.
b. If the female lays more or less than this number, fewer survive.
c. Genes determining the physiology of yolk production and behavior are involved
in clutch size.
3. Directional selection occurs when an extreme phenotype is favored; the distribution
curve shifts that direction.
a. A shift to more colorful, later maturing male guppies with exposure to no
predators compared to drab colored, early maturing male guppies.
4. Disruptive selection occurs when extreme phenotypes are favored and can lead to
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more than one distinct form.
a. British snails (Cepaea nemoralis) vary because a wide range causes natural
selection to vary.
b. In forest areas, thrushes feed on snails with light bands.
c. In low-vegetation areas, thrushes feed on snails with dark shells that lack light
bands.
5. Sexual Selection
a.
Sexual selection refers to adaptive changes in males and females that lead to an increased ability
to secure a mate.
1) In males, this may result in an increased ability to compete with other males
for a mate.
2) Females may select a mate with the best fitness (ability to produce surviving
offspring), thereby increasing her own fitness.
6. Female Choice
a.
Two hypothesis regarding a female’s choice of a mate are:
1) The good genes hypothesis contends that females choose mates based on traits
for improving the survival of offspring.
2) The runaway hypothesis states that females chose mates on the basis of traits
that attract them to females; the trait can then become exaggerated until it is a
handicap.
b. The Raggiana Bird of Paradise is dimorphic (males and females differ in size and
other traits)
1) The ornateness of the male is a factor in selection by a female.
2) More feathery Raggiana are parasite-free, and their selection by females
would increase the chance for survival.
7. Male Competition
a. A cost-benefit analysis can be applied to determine if the benefit of access to
mating is worth the cost of competition among males.
b. Baboons have a dominance hierarchy.
1) A dominance hierarchy is a ranking within a group where the higher ranking
individuals acquire more resources.
2) Dominance is determined by confrontation where one animal gives way to the
other.
3) Baboons are dimorphic: males are larger and have large canine teeth; they
decide when the troop moves, and they defend it.
4) Females mate with dominant males when ovulation is near; the dominant
males then protect all young.
5) The drawbacks to being large and in danger are outweighed by the chance of
fathering young.
6) The subordinate males have less chance to mate but they do have avenues to
have some offspring.
c. Red deer stake out a territory, an area that is defended against competitors.
1) Territoriality involves the type of behavior needed to defend a particular
territory.
2) A stag competes for females that form a harem that mate only with him.
3) A stag remains at peak fighting ability for only a short time; one stag can only
father about two dozen offspring.
8. Sexual Selection in Humans (Science Focus box)
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a. Human males compete.
b. Humans, like many other mammals, are dimorphic.
c. Males are larger and more aggressive, perhaps due to past sexual selection by
females.
1) Females choose.
2) Male mating success correlates best with income—wealthy males attracted
mates better than nonwealthy males.
3) Apparently, females prefer to mate with a male who is wealthy and has a
successful career—this will ensure that the children will live to reproduce.
1. Men Also Have a Choice
a. Men prefer women who will present them with children: health, age,
“figure,” faithfulness are all factors in a male’s choice of a mate.
16.3
Maintenance of Variations
1. Populations always show some genotypic variation; populations that lack variation
may not be able to adapt to new conditions.
A. Natural Selection
1. Imperfections are common because compromises have had to be made. For example,
the success of humans is attributable to their dexterous hands, but the spine is subject
to injury because the vertebrate spine was not originally designed to stand erect.
2. If a feature has successfully evolved, then the benefit outweighs the cost.
3. The environment also plays a role in maintaining variations. For example, disruptive
selection promostes polymorphisms in a population.
4. The environment also includes specific selecting agents that help maintain variations.
B. Heterozygote Advantage
1. Heterozygote advantage occurs when the heterozygote is favored over the two
homozygotes.
2. Sickle-Cell Disease
a. In sickle-cell disease, heterozygotes are more fit in malaria areas because the
sickle-cell trait does not express unless the oxygen content of the environment is
low; but the malaria agent causes red blood cells to die when it infects them (loss
of potassium).
b. Some homozygous dominants are maintained in the population but they die at an
early age from sickle-cell disease.
c. Some homozygotes are maintained in the population for normal red blood cells,
but they are vulnerable to malaria.
C. Cystic Fibrosis
a. In cystic fibrosis the recessive allele causes the person to have a defective plasma
membrane protein.
b. The agent that causes typhoid fever can use the normal version of this protein, but
not the defective one, to enter cells.
c. Heterozygote superiority caused the recessive allele to be maintained in the
population.
Lecture Enrichment Ideas
Experience Base: Basic algebra skills are necessary for students to understand the HardyWeinberg formula and high school math requirements will vary by state. Nonetheless, college
111
biology majors should have adequate math background.
1. Describe how each of the conditions of the Hardy-Weinberg equilibrium retards evolution.
2.
Provide examples of directional selection, stabilizing selection, and disruptive selection—consider very tall and
very short humans, different breeds of dogs, etc. When examining human traits and evolution, consider the precivilization selective pressures. What is happening when “civilization” prevents poorly adapted (in terms of
survival in pre-historic times) humans from being eliminated from the gene pool and individuals with this trait
can now reproduce and pass the trait along. Note how easy it is to slide into eugenics.
3. Consider why pre-mating isolating mechanisms are more efficient than post-mating isolating
mechanisms in reproductive isolation leading to speciation, and why Mayr predicted that
post-mating isolation would occur before pre-mating isolation in geographic isolation and
speciation.
4. Although malaria was present and continued to be a selective force in the South in the 1800s,
there was no longer an advantage to the heterozygous condition in the 20th century. Ask for
reasoned speculation on the change in frequencies of the sickle-cell gene; why is the sicklecell allele unlikely to totally disappear?
Critical Thinking
Question 1. Most organisms have a surprising amount of genetic diversity, or potential for
variation, within their gene pool. Only some of them are best adapted for the exact
environmental conditions in your local area this year. Why does the population possess all of this
genetic variation?
Answer: Conditions change: early and late springs, harsh and mild winters, etc. One year one
group is favored, the next year another is favored. This keeps a rich stockpile of genetic variation
available for future changes. An organism that becomes too narrow in its gene pool risks having
no individuals able to survive a particular variation in climate or other conditions.
Question 2. Consider a time period of continual variation in environmental conditions that is
then followed by a time period of relative stability and lack of variation. Compare this to the
opposite, a time period of stability followed by a time period of continual environmental
fluctuation. In which case would you expect to see more species go extinct?
Answer: More extinctions would likely occur when organisms lived a long time in a stable
environment, and alleles for survival in a different environment had been selected out, and then
the environment became highly variable. In the case of original high variability, the onset of a
stable environment poses no problem to a population that continually reinforced first one allele,
then another.
Question 3. Consider the proposal that left-handedness is inherited as a recessive gene (rr) and
right-handedness is dominant (RR, Rr). If 16% of this class is left-handed, what proportion of the
right-handed students are likely to be carriers of a left-handedness gene and what is the
frequency of the hypothetical left-handed gene?
Answer: The Hardy-Weinberg formula is p2 + 2 pq + q2 = 1 where p = R and q = r. Thus q2 = .16
and q = .4 or the left-handedness gene has a frequency of 40%. The right-handedness gene
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therefore has a frequency of 60%. And the carriers (Rr = 2 pq) equal 2 x 0.4 x 0.6 = 0.48; so 48%
of the class is likely carriers leaving 36% of the class homozygous dominant for the hypothetical
right-handedness gene.
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CHAPTER
17 SPECIATION AND MACROEVOLUTION
This chapter presents sections on separation of the species, modes of speciation, and
macroevolution. Modes of speciation are described in detail. Much terminology and many
definitions used in the study of evolutionary processes are presented. A Science Focus box
entitled, “The Burgess Shale Hosts a Diversity of Life,” is presented.
Chapter Outline
17.1 Separation of the Species
A. What is a Species?
1. The evolutionary species concept states that members of a species share the same
distinct evolutionary pathway and that species can be recognized by morphological
trait differences.
a. An advantage of defining species according to this concept is that it applies to
both sexually and asexually reproducing organisms.
b. However, a disadvantage to this species concept is using morphological traits to
distinguish species. The presence of variation between individuals may categorize
them as two different species rather than one species.
2. The biological species concept says that the members of one species interbreed and
have a shared gene pool, and each species is reproductively isolated from every other
species.
a. An advantage to this species concept is that it can designate species even when
trait differences may be difficult to find.
b. A disadvantage is that this concept cannot be applied to asexually reproducing
organisms, organisms only known by the fossil record, or species that could
possibly interbreed if they lived near one another.
3. DNA base sequence data and protein differences can now be used to investigate how
closely species are related to one another.
B. Reproductive Isolating Mechanisms
1. For two species to be separate, gene flow must not occur between them.
2. A reproductive isolating mechanism is any structural, functional, or behavioral
characteristic that prevents successful reproduction from occurring.
3. Prezygotic (“before formation of a zygote”) isolating mechanisms are anatomical or
behavioral differences between the members of two species that prevent mating or
make it unlikely fertilization will take place if mating occurs.
a. Habitat isolation occurs when two species occupy different habitats, even within
the same geographic range, so that they are less likely to meet and to attempt to
reproduce.
b. Temporal isolation occurs when two species live in the same location, but each
reproduces at a different time of year, and so they do not attempt to mate.
c. Behavioral isolation results from differences in mating behavior between two
species.
d. Mechanical isolation is the result of differences between two species in
reproductive structures or other body parts, so that mating is prevented.
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e. Gamete isolation includes incompatibility of gametes of two different species so
they cannot fuse to form a zygote; an egg may have receptors only for the sperm
of its own species or a plant stigma prevents completion of pollination.
4. Postzygotic (“after formation of a zygote”) isolating mechanisms prevent
development of a hybrid after mating has taken place.
a. Zygote mortality is when hybrids (offspring of parents of two different species)
do not live to reproduce.
b. Hybrid sterility occurs when the hybrid offspring are sterile (e.g., mules).
c. In F2 fitness, the offspring are fertile but the F2 generation is sterile.
17.2
Modes of Speciation
1. Speciation is the splitting of one species into two more species or the transformation
of one species into a new species over time.
a. Researchers recognize two modes of speciation: geographic isolation and
reproductive isolation.
A. Allopatric speciation
1. Allopatric speciation occurs when new species result from populations being
separated by a geographical barrier that prevents their members from reproducing
with each other.
2. First proposed by Ernst Mayr of Harvard University.
3. While geographically isolated, variations accumulate until the populations are
reproductively isolated.
4. Examples of Allopatric Speciation
a. An ancestral population of Ensatina salamanders migrated from northern
California to southern California. The Central Valley prevented gene flow
between the eastern and western populations of these salamanders. Genetic
differences increased resulting in two distinct forms of Ensatina salamanders.
b. Green iguana of South America is believed to be the common ancestor for the
marine iguana on the Galápagos Islands (to the west) and the rhinoceros iguana
on Hispaniola (to the north). A few of these iguanas may have swam to the islands
and over time formed populations separate from each other and from the parent
population of South America.
c. Many sockeye salmon in Washington State were introduced into Lake
Washington when some colonized an area of the lake near Pleasure Point Beach
and others migrated into the Cedar River. Because of the difference in water
current, over time, the salmon differed in shape and size due to the demands of
reproducing in the different water currents.
5. Reinforcement of Reproductive Isolation
a. A side effect to adaptive changes involving mating is reproductive isolation.
b. As populations become reproductively isolated, postzygotic isolating mechanisms
may arise before prezygotic isolating mechanisms.
c. An example is a horse reproducting with a mule, a sterile donkey is produced.
d. Natural selection would favor any variation in populations that prevents the
occurrence of hybrids when they do not have offspring.
e. Reinforcement refers to the process of natural selection favoring variations that
lead to reproductive isolation.
B. Adaptive Radiation
1. Adaptive radiation is a type of allopatric speciation and occurs when a single
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ancestral species gives rise to a variety of species, each adapted to a specific
environment.
2. An ecological niche is where a species lives and how it interacts with other species.
3. The case of Darwin’s finches illustrates the adaptive radiation of 13 species from one
founder mainland finch.
4. On the Hawaiian Islands, a wide variety of honeycreepers descended from one
goldfinch-like ancestor; Hawaii is also the home of the silversword plants that
radiated from ancestral tarweeds.
C. Sympatric Speciation
1. Sympatric speciation would occur when members of a single population develop a
genetic difference (e.g., chromosome number) that prevents them from reproducing
with the parent type.
2. A polyploid is a eukaryote with three or more complete sets of chromosomes.
a. Polyploidy is predominantly seen in plants and makes a significant contribution to
the evolution of new plants.
b. A polyploid plant can reproduce with itself, but cannot reproduce with the 2n
population.
c. The two types of polyploidy are aneuploidy and alloploidy.
3. Aneuploidy is the condition in which an organism gains or loses one or more
chromosomes.
a. Monosomy (2n – 1) occurs when an individual has only one of a particular type
of chromosome.
b. Trisomy (2n + 1) occurs when an individual has three of a particular type of
chromosome.
c. Farmers may use produce sterile plants because their fruits do not have seeds and
tend to be more favorable for the consumer.
4. Alloploidy requires two different but related species of plants to hybridize.
a. When hybridization occurs, it is followed by chromosome doubling.
b. However the offspring that has parents with different numbered pairs of
chromosomes will be sterile.
c. An example of alloploidy can be seen in the wheat plant used to produce bread.
The parents of the present day bread wheat had 28 and 14 chromosomes. The
hybrid with 21 chromosomes is sterile, but bread wheat with 43 chromosomes is
fertile since the chromosomes can pair during meiosis.
D. The Burgess Shale Hosts a Diversity of Life (Science Focus box)
1. The Burgess Shale is a rock outcropping in Yoho National Park, British Columbia
discovered by Charles Doolittle Walcott in 1909.
2. In this area there is a large quantity of fossils.
3. At the beginning of the excavation, it was difficult to extract the fossils from their
matrix, but now new methods that involve UV light and diluted acetic acid make it
easy to free the fossils.
4. These marine fossils are thought to have lived around 540 million years ago (MYA),
during the Precambrian period.
5. Many fossils found in the Burgess Shale are soft-bodied invertebrates, which is
unusual since soft-bodied animals rarely fossilized.
6. When these organisms were alive, they all lived in the sea, and this area was
subjected to mudslides. The mud entered the sea, buried the animals. Later the mud
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turned into shale and over time the shale was raised. Mud particles filled the spaces
around the organisms, so the soft tissue was preserved and fossils became somewhat
three-dimensional.
7. Organisms that have been preserved at the Burgess Shale are vertebrates,
invertebrates, and unicellular organisms.
8. There is much dispute regarding the evolution of these species. Some scientists
believe that evolution of these species happened slowly, while others believed it
happened quickly and suddenly.
17.3 Principles of Macroevolution
A. Macroevolution is evolution of new species and higher levels of classification.
B. Some evolutionists support a gradualistic model of macroevolution, meaning that speciation
occurs after populations become isolated, with each group continuing slowly on its own
evolutionary pathway.
1. According to this model, ancestral species gradually gives rise to two separate
species.
2. This model suggests that it is difficult to indicate when speciation occurred because
there would be so many transitional links.
C. Other evolutionists support a punctuated equilibrium model to explain the pace of evolution.
a. According to this model, periods of equilibrium (no change) are punctuated (or
interrupted) by speciation.
b. This model suggests that transitional links are less likely to become fossils and
less likely to be found.
c. Speciation is more likely to involve only an isolated population at one locate,
because a favorable genotype could spread more rapidly within such a population.
D. These two models could both assist in interpretation of the fossil record. For example, some
species may fit into one model, and other species fit into the other model.
1. Developmental Genes and Macroevolution
a. Genes can bring about radical changes in body shapes and organs.
b. The Pax6 gene is involved in eye formation in all organisms.
c. Homeotic (Hox) genes determine the location of repeated structures in all
vertebrates.
2. Gene Expression Can Influence Development
a. Gene expression influences organisms’ developmental processes.
b. These genes can bring about changes in body shapes and organs.
c. Despite millions of years of divergent evolution, all animals share the same
control switches for development
3. Development of the Eye
a. Eyes of species vary in size, compound or simple, etc.
b. Despite these differences, there is one gene, Pax6, required for eye formation.
c. The gene Pax6 was discovered by Walter Gehrig and colleagues in 1994.
d. Interestingly, the mouse Pax6 gene can cause an eye to develop in the leg of a
fruit fly.
4. Development of Limbs
a. The Tbx5 gene helps in the development of limbs in humans and wings in birds.
b. Tbx5 triggers different genes in birds and humans, which may explain why the
same protein is used in developing limbs in humans and wings in birds.
5. Development of Overall Shape
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a. Hox genes control the number and appearances of repeated structures along the
main body axes of vertebrates.
b. Shifts when the Hox gene is expressed can explain why some vertebrates, like the
snake, have hundreds of vertebrae, and others, like the chick only have seven.
6. Pelvic-Fin Genes
a. An altered expression of a particular gene can reduce the pelvic-fin bud in the
embryo.
b. Natural selection can lead to major skeletal changes in a relatively short
period of time.
7. Human Evolution
a. Human DNA base sequencing is similar to that of chimpanzees, mice, and all
vertebrates.
b. Scientists predict that differential gene expression and/or new functions for
“old” genes will explain how humans evolved.
E. Macroevolution Is Not Goal-Oriented
1. The evolution of the horse, Equus, represents a gradual, straight-line evolution until
its goal, the modern horse, has been achieved.
a. The trends seen in the evolution of the horse are: overall size, toe reduction, and
change in tooth size and shape.
2. However, based on fossils, it is easier to see that the horse lineage is not a straightline evolution, but rather forming a thick bush of many equine species.
3. One may have deducted that since the only genus that remains is Equus and the other
genera have become extinct, that evolution was directed towards producing Equus.
However, each of the ancestral species was adapted to its environment.
4. Adaptation occurs because the members of a population with an advantage are able to
have more offspring than other members.
5. Natural selection is not goal-oriented, but rather opportunistic.
Lecture Enrichment Ideas
5. Discuss why adaptive radiation should be prevalent when there is a window of no
competition for an environmental habitat, such as after the death of the dinosaurs or when
finches arrived on the Galápagos Islands.
6. Consider why pre-mating isolating mechanisms are more efficient than post-mating isolating
mechanisms in reproductive isolation leading to speciation, and why Mayr predicted that
post-mating isolation would occur before pre-mating isolation in geographic isolation and
speciation.
7. Discuss why allopatric speciation would be more likely than sympatric speciation for
animals. Also note that in the case of polyploid plants, the evolution of a new “species” is
immediate.
Critical Thinking
Question 1.
Most organisms have a surprising amount of genetic diversity, or potential for
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variation, within their gene pool. Only some of them are best adapted for the exact
environmental conditions in your local area this year. Why does the population possess all of this
genetic variation?
Answer: Conditions change: early and late springs, harsh and mild winters, etc. One year one
group is favored, the next year another is favored. This keeps a rich stockpile of genetic variation
available for future changes. An organism that becomes too narrow in its gene pool risks having
no individuals able to survive a particular variation in climate or other conditions.
Question 2. Consider a time period of continual variation in environmental conditions that is
then followed by a time period of relative stability and lack of variation. Compare this to the
opposite, a time period of stability followed by a time period of continual environmental
fluctuation. In which case would you expect to see more species go extinct?
Answer: More extinctions would likely occur when organisms lived a long time in a stable
environment, and alleles for survival in a different environment had been selected out, and then
the environment became highly variable. In the case of original high variability, the onset of a
stable environment poses no problem to a population that continually reinforced first one allele,
then another.
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Chapter
18
Origin and History of Life
This chapter looks at the origin of life. Beginning with the development of organic molecules from the early
atmostphere, the various eras and the biological events occurring in them are outlined in detail.
Chapter Outline
18.1 Origin of Life
1. Chemical evolution is the increase in complexity of chemicals that led to the first
cells.
a. Today, we say that “life only comes from life.”
b. However, the first cells had to arise from an increased complexity of chemicals.
A. The Early Earth
1. The Earth came into being about 4.6 BYA.
2. Heat from gravitation and radioactivity formed the Earth in several layers with iron
and nickel in a liquid core, silicate minerals in a semi-liquid mantle, and upwellings
of volcanic lava forming the first crust.
3. The Earth’s mass provides a gravitational field strong enough to hold an atmosphere.
4. Early Earth’s atmosphere differed from the current atmosphere, consisting of:
a. water vapor;
b. nitrogen;
c. carbon dioxide; and
d. small amounts of hydrogen, methane, ammonia, hydrogen sulfide, and carbon
monoxide.
5. The early atmosphere was formed by volcanic out-gassing characteristic of the young
Earth.
6. The early atmosphere contained little free oxygen (O2) and was probably a reducing
atmosphere with little free oxygen; a reducing atmosphere lacks free O2 and allows
formation of complex organic molecules.
7. The early Earth was so hot that H2O only existed as a vapor in dense, thick clouds.
8. As the Earth cooled, H2O vapor condensed to form liquid H2O, and rain collected in
oceans.
9. The Earth’s distance from the sun allows H2O to exist in all phases: solid, liquid, and
gas.
10. NASA photos seem to confirm that Earth is bombarded by comets adding substantial
water vapor.
B. Monomers Evolve
1. There are three hypotheses that explain how organic monomers could have evolved.
2. Hypothesis one: monomers came from outer space.
a. Comets and meteorites, perhaps carrying organic chemicals, have pelted the Earth
throughout history.
b. A meteorite from Mars (ALH84001) that landed on Earth 13,000 years ago, may
have fossilized bacteria.
3. Hypothesis two: monomers came from reactions in the atmosphere.
a. Oparin/Haldane independently suggested organic molecules could be formed in
the presence of outside energy sources using atmospheric gases.
b. Experiments performed by Miller and Urey (1953) showed experimentally that
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these gases (methane, ammonia, hydrogen, water) reacted with one another to
produce small organic molecules (amino acids, organic acids).
c. Lack of oxidation and decay allowed organic molecules to form a thick, warm
organic soup.
4. Hypothesis three: monomers came from reactions at hydrothermal vents.
a. Ammonia may have been scarce in the early atmosphere; undersea thermal vents,
which line ocean ridges, might have been responsible for converting nitrogen to
ammonia.
C. Polymers Evolve
1. Newly formed organic molecules polymerized to produce larger molecules.
a. Wachtershauser and Huber formed peptides using iron-nickel sulfides under
ventlike conditions.
b. Such minerals have a charged surface that attracts amino acids and provides
electrons so they bond together.
2. Protein-first Hypothesis
a. Sidney Fox demonstrated amino acids polymerize abiotically if exposed to dry
heat.
b. Amino acids collected in shallow puddles along the rocky shore; heat of the sun
caused them to form proteinoids (i.e., small polypeptides that have some catalytic
properties).
c. When proteinoids are returned to water, they form cell-like microspheres
composed of protein.
d. This assumes DNA genes came after protein enzymes; DNA replication needs
protein enzymes.
3. The Clay Hypothesis
a. Graham Cairns-Smith suggests that amino acids polymerize in clay, with
radioactivity providing energy.
b. Clay attracts small organic molecules and contains iron and zinc atoms serving as
inorganic catalysts for polypeptide formation.
c. Clay collects energy from radioactive decay and discharges it if temperature or
humidity changes.
d. If RNA nucleotides and amino acids became associated so polypeptides were
ordered by and helped synthesize RNA, then polypeptides and RNA arose at the
same time.
4. RNA-first Hypothesis
a. Only the macromolecule RNA was needed at the beginning to lead to the first
cell.
b. Thomas Cech and Sidney Altman discovered that RNA can be both a substrate
and an enzyme.
c. RNA would carry out processes of life associated with DNA (in genes) and
protein enzymes.
d. Supporters of this hypothesis label this an “RNA world” 4 BYA.
D. A Protocell Evolves
1. Before the first true cell arose, there would have been a protocell or protobiont.
2. A protocell would have a lipid-protein membrane and carry on energy metabolism.
3. Sidney Fox showed that if lipids are made available to microspheres, lipids become
associated with microspheres producing a lipid-protein membrane.
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4. Oparin demonstrated a protocell could have developed from coacervate droplets.
a. Coacervate droplets are complex spherical units that spontaneously form when
concentrated mixtures of macromolecules are held in the right temperature, ionic
composition, and pH.
b. Coacervate droplets absorb and incorporate various substances from the
surrounding solution.
c. In a liquid environment, phospholipid molecules spontaneously form liposomes,
spheres surrounded by a layer of phospholipids; this is called the “membranefirst” hypothesis.
d. A protocell could have contained only RNA to function as both genetic material
and enzymes.
5. If a protocell was a heterotrophic fermenter living on the organic molecules in the
organic soup that was its environment, this would indicate heterotrophs preceded
autotrophs.
a. A heterotroph is an organism that cannot synthesize organic compounds from
inorganic substances and therefore must take in preformed organic compounds.
b. An autotroph is an organism that makes organic molecules from inorganic
nutrients.
6. If the protocell evolved at hydrothermal vents, it would be chemosynthetic and
autotrophs would have preceded heterotrophs.
7. The first protocells may have used preformed ATP, but as supplies dwindled, natural
selection would favor cells that could extract energy from carbohydrates to transform
ADP to ATP.
8. Since glycolysis is a common metabolic pathway in living things, it evolved early in
the history of life.
9. As there was no free O2, it is assumed that protocells carried on a form of
fermentation.
10. The first protocells had a limited ability to break down organic molecules; it took
millions of years for glycolysis to evolve completely.
11. Fox has shown that a microsphere has some catalytic ability; Oparin found that
coacervates incorporate enzymes if they are available in the medium.
E. A Self-Replication System Evolves
1. In living systems, information flows from DNA → RNA → protein; it is possible that
this sequence developed in stages.
2. The RNA-first hypothesis suggests that the first genes and enzymes were RNA
molecules.
a. These genes would have directed and carried out protein synthesis.
b. Ribozymes are RNA that acts as enzymes.
c. Some viruses contain RNA genes with a protein enzyme called reverse
transcriptase that uses RNA as a template to form DNA; this could have given rise
to the first DNA.
3. The protein-first hypothesis contends that proteins or at least polypeptides were the
first to arise.
a. Only after the protocell develops complex enzymes could it form nucleic acids
from small molecules.
b. Because a nucleic acid is complicated, the chance that it arose on its own is
minimal.
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c. Therefore, enzymes are needed to guide the synthesis of nucleotides and then
nucleic acids.
4. Cairns-Smith suggests that polypeptides and RNA evolved simultaneously.
a. The first true cell would contain RNA genes that replicated because of the
presence of proteins; they become associated in clay in such a way that the
polypeptides catalyzed RNA formation.
b. This eliminates the chicken-and-egg paradox; both events happen at the same
time.
5. Once the protocell was capable of reproduction, it became a true cell and biological
evolution began.
a. After DNA formed, the genetic code still had to evolve to store information.
b. Because the current code is subject to fewer errors than other possible codes, and
because it minimizes mutations, it likely underwent a natural selection process.
F. A Recap of the Steps
1. Most biologists suspect life evolved in basic steps.
a. Abiotic synthesis of organic molecules such as amino acids occurred in the
atmosphere or at hydrothermal vents.
b. Monomers joined together to form polymers at seaside rocks or clay, or at vents;
the first polymers could have been proteins or RNA or both.
c. Polymers aggregated inside a plasma membrane to make a protocell that had
limited ability to grow; if it developed in the ocean it was a heterotroph, if at a
hydrothermal vent, a chemoautotroph.
d. Once the protocell contained DNA genes or RNA molecules, it was a true cell.
18.2 History of Life
A. Fossils Tell a Story
1. A fossil is the remains or traces of past life, usually preserved in sedimentary rock.
2. Most dead organisms are consumed by scavengers or decompose.
3. Paleontology is the study of fossils and the history of life, ancient climates, and
environments.
4. Sedimentation has been going on since the Earth was formed; it is an accumulation
of particles forming a stratum, a recognizable layer in a stratigraphic sequence laid
down on land or in water.
5. The sequence indicates the age of fossils; a stratum is older than the one above it and
younger than the one below it.
B. Relative Dating of Fossils
1. Strata of the same age in England and Russia may have different sediments.
2. However, geologists discovered that strata of the same age contain the same fossils,
termed index fossils.
3. Therefore, fossils can be used for the relative dating of strata.
4. A particular species of fossil ammonite is found over a wide range and for a limited
time period; therefore, all strata in the world that contain this ammonite are of the
same age.
5. However, relative dating does not establish the absolute age of fossils in years.
C. Absolute Dating of Fossils
1. Absolute dating relies on radioactive dating to determine the actual age of fossils.
2. Radioactive isotopes have a half-life, the time it takes for half of a radioactive isotope
to change into a stable element.
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3. Carbon 14 (14C) is a radioactive isotope contained within organic matter.
a. Half of the carbon 14 (14C) will change to nitrogen 14 (14N) every 5,730 years.
b. Comparing 14C radioactivity of a fossil to modern organic matter calculates the
age of the fossil.
c. After 50,000 years, the 14C radioactivity is so low it cannot be used to measure
age accurately.
4. It is possible to determine the ratio of potassium 40 (40K) and argon 40 to date rocks
and infer the age of a fossil.
D. The Precambrian Time
1. Geologists have devised the geological timescale, which divides the history of Earth
into eras, and then periods and epochs.
2. Life arose in the Precambrian Era.
a. The Precambrian encompasses 87% of the geologic time scale.
b. Early bacteria probably resembled the archaea that live in hot springs today.
c. 3.8 BYA, the first chemical fingerprints of complex cells occur; at 3.46 BYA,
photosynthetic prokaryotic cells appear.
d. Boulders called stromatolites from this early time resemble living stromatolites
with cyanobacteria in the outer surface.
e. Oxygen-releasing photosynthesis by cyanobacteria in stromatolites caused the
atmosphere to become oxidizing rather than reducing.
f. By 2 BYA, oxygen levels were high enough that anaerobic prokaryotes were
declining.
g. Accumulation of O2 caused extinction of anaerobic organisms and the rise of
aerobic organisms.
h. O2 forms ozone or O3 in the upper atmosphere, contributing to the ozone shield
and blocking ultraviolet radiation from reaching the Earth’s surface; this allowed
organisms to live on land.
3. Eukaryotic Cells Arise
a. The eukaryotic cell, which arose 2.1 BYA, is always aerobic and contains a
nucleus and organelles.
b. The Endosymbiotic Hypothesis states that a nucleated cell engulfed prokaryotes,
which then became organelles. Evidence includes:
1) Present-day mitochondria and chloroplasts have a size that lies within the
range of that for bacteria.
2) Mitochondria and chloroplasts have their own DNA and make some of their
own proteins.
3) Mitochondria and chloroplasts divide by binary fission similar to bacteria.
4) The outer membrane of mitochondria and chloroplasts differ.
4. Multicellularity Arises
a. It is not known exactly when multicellular organisms appeared; they would have
been microscopic.
b. Separating germ cells from somatic cells may have contributed to the diversity of
organisms.
c. Fossils of the Ediacara Hills of South Australia, from about 600-545 MYA, were
soft-bodied early invertebrates.
1) These bizarre animals lived on mudflats in shallow marine waters.
2) They lacked internal organs and could have absorbed nutrients from the sea.
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E. The Paleozoic Era
1. The Paleozoic Era lasted over 300 million years and was a very active period with
three major mass extinctions.
a. An extinction is the total disappearance of a species or higher taxonomic group.
b. Mass extinction is the disappearance of a large numbers of species or higher
groups in a short geological time, just a few million years.
2. Cambrian Animals
a. The Cambrian Period saw invertebrates flourish; invertebrates lack a vertebral
column.
b. Today’s invertebrates all trace their ancestry to the Cambrian Period, and possibly
earlier.
c. A molecular clock, based on a fixed rate of changes in base pair sequences,
allows us to trace backward how long current species have evolved separately.
d. Why fossils are easy to find in the Cambrian but not before is a complex question;
most likely the animals evolved earlier but without outer skeletons.
e. Cambrian seafloors were dominated by trilobites, now extinct, that had armored
exoskeletons.
f. Perhaps the evolution of exoskeletons was due to the presence of plentiful O2 in
the atmosphere.
g. A skeleton may have been due to the increased pressures of predation.
3. Invasion of Land
a. Early in the Ordovician Period, marine algae expanded to freshwater.
b. In the Silurian Period, vascular plants invaded land and later flourished in warm
swamps in the Carboniferous Period.
c. Spiders, centipedes, mites and millipedes all preceded the appearance of insects
on land.
d. The appearance of wings on insects in the Carboniferous Period allowed insects to
radiate into a diverse group.
e. The vertebrate line of descent began in the early Ordovician Period.
f. The Devonian Period is called the Age of Fishes and saw jawless and then jawed
fishes, including both cartilaginous and ray-finned fishes.
g. The Carboniferous Period was an age of coal-forming forests with an abundance
of club mosses, horsetails, and ferns.
1) It is called the “Age of the Amphibians” because amphibians diversified at
this time.
2) Early vascular plants and amphibians were larger and more abundant during
the Carboniferous Period; a climate change to colder and drier began the
process that produced coal.
F. The Mesozoic Era
1. Although there was a mass extinction at the end of the Paleozoic, evolution of some
plants and animals continued into the Triassic, the first period of the Mesozoic Era.
2. The Triassic period
a. Gymnosperms flourished, especially cycads; the Triassic and Jurassic are called
the “Age of Cycads.”
b. One group of reptiles, the therapsids, had the first mammal features.
c. Reptiles, originating in the Permian, underwent adaptive radiation.
3. The Jurassic Period
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a. Many dinosaurs flourished in the sea, on land, and in air.
b. Controversy surrounds dinosaurs being ectothermic or endothermic.
4. The Cretaceous Period
a. A new Chinese fossil, Jeholodens, reveals an early mammal with a long snout but
sprawling reptile-like hind limbs.
b. The era of dinosaurs ended in a mass extinction in which dinosaurs, most reptiles,
and many marine organisms perished.
G. The Cenozoic Era
1. The Cenozoic Era is divided into the Tertiary and the Quaternary Periods.
2. During the Cenozoic Era, mammals with hair and mammary glands diversified and
human evolution began.
3. Mammalian Diversification
a. During the Paleocene Epoch, mammals were small and resembled rats.
b. In the Eocene Epoch, all of the modern orders of mammals had developed.
c. Many of the types of herbivores and carnivores of the Oligocene Epoch are
extinct today.
4. Evolution of Primates
a. Flowering plants were diverse and plentiful by the Cenozoic Era; primates were
adapted to living in flowering trees.
b. The first primates were small squirrel-like animals; from them evolved the first
monkeys and apes.
c. Apes diversified during the Miocene and Pliocene Epochs; this includes the first
hominids, the group that includes humans.
d. During the Tertiary Period, the world’s climate cooled with the last two epochs
known as the Ice Age.
e. The Pleistocene Epoch saw many large sloths, beavers, wolves, bison, woolly
rhinoceroses, mastodons, and mammoths; modern humans arose and may have
contributed to extinction.
18.3 Factors That Influence Evolution
A. Continental Drift
1. Earth’s crust is dynamic, not immobile as was once thought.
2. In 1920, German meteorologist Alfred Wegener presented data from across
disciplines supporting continental drift.
3. Continental drift was confirmed in the 1960s; the continents moved with respect to
one another.
4. During the Permean Period, the continents were joined to form one supercontinent
called Pangaea which later divided into Gondwana and Laurasia and then split to
form today’s configuration.
5. Continental drift explains why the coastlines of several continents (e.g., the outline of
the west coast of Africa and that of the east coast of South America) are mirror
images of each other.
6. The same geological structures (e.g., mountain ranges) are found in many areas where
continents once touched.
7. Continental drift explains unique distribution patterns of several fossils (e.g., species
of the seed fern Glossopteris).
8. Continental drift also explains why some fossils (e.g., reptiles Cynognathus and
Lystrosaurus) are found on different continents.
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9. Continental drift explains why Australia, South America, and Africa have distinctive
mammals; current mammalian biological diversity is the result of isolated evolution
on separate continents.
B. Plate Tectonics
1. Plate tectonics is the study of the behavior of the Earth’s crust in terms of moving
plates that are formed at ocean ridges and destroyed at subduction zones.
2. Ocean ridges are ridges on ocean floors where oceanic crust forms; regions in oceanic
crust where molten rock rises and material is added to the ocean floor result in
seafloor spreading.
3. Seafloor spreading is the lateral movement of oceanic crust away from ocean ridges
due to material added to the ocean floor.
4. Subduction zones are regions where oceanic crust collides with the continental crust,
causing the oceanic crust to descend into the mantle where it is melted.
5. Where the ocean floor is at the leading edge of a plate, a deep trench forms bordered
by volcanoes or volcanic island chains.
6. Two continents colliding form a mountain range (e.g., the Himalayas are the result of
the collision of India and Eurasia).
7. Transform boundaries are regions where two crustal plates meet and scrape past one
another resulting in relatively frequent earthquakes.
C. Mass Extinctions
1. Five mass extinctions occurred at the ends of the Ordovician, Devonian, Permian,
Triassic, and Cretaceous periods.
2. Mass extinctions have been attributed to tectonic, oceanic, and climatic changes.
3. Walter and Louis Alvarez proposed that the Cretaceous extinction was due to a bolide
(an asteroid that explodes producing meteorites) striking the Earth.
a. A layer of iridium soot has been identified in the Cretaceous clay, the correct
strata.
b. A huge crater near the Yucatan is the impact site.
c. The effect would have resembled a worldwide atomic explosion.
4. Continental drift contributed to Ordovician extinction; Gondwanaland arrived at the
south pole and glaciers chilled oceans and land until Gondwanaland drifted away
from the pole.
5. The Devonian extinction may have been a bolide event; this saw an end to 70% of the
marine invertebrates; other possibilities include drifting back toward the south pole.
6. The Permian extinction was very severe; 90% of ocean species and 70% of land
species disappeared perhaps due to an excess of carbon dioxide due to a change in
ocean circulation due to a lack of polar ice caps.
7. The Triassic extinction has been attributed to meteorite collision with Earth; a crater
in Central Quebec may have been the impact site.
Lecture Enrichment Ideas
Experience Base: Dinosaurs are a topic now covered heavily in K12 science curricula but new
developments have made many recent textbook concepts obsolete; therefore the college
instructor will have to deal with many misconceptions. The comet and meteorite impact theories
have gone from speculative to fairly well accepted in the last decade and the instructor will
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probably face students who were taught the previous uncertainties. Breaking news on recent
major fossil finds in China and Africa will, however, make this a timely topic to cover.
1.
Review the differences between reduction and oxidation as chemical processes. Examine why the reducing
atmosphere would support development of larger organic molecules, while an oxidizing atmosphere would
break down large molecules.
2. Discuss the presence of organic subunits such as amino acids (including some not produced
by organisms) in meteorites suggesting that these molecules probably arose in space (or at
least somewhere other than Earth). How would this information affect your ideas about
chemical evolution? Could some of these molecules have been brought to Earth this way?
3. The early discussion of “What is Life” from Chapter 1 can be reviewed here with hindsight
on the complexity of metabolism.
4. Examine the relatively new ideas concerning reverse transcriptase and ribozymes, as opposed
to the central dogma of molecular genetics as proposed by Crick. Why did it take so long to
find these exceptions to the central dogma, and what effect does their presence have on our
understanding of how life might have arisen and developed on Earth?
5. Discuss why it would be likely that a membrane would be necessary before a cell would
develop.
6. Describe the evolutionary significance of continental drift in the development of
monotremes, marsupials, and placental mammals. Discuss the fact that South America was
inhabited by marsupials until relatively recently (within the last 20,000 years or so), when the
land bridge formed between North and South America, which allowed placental mammals to
reach South America and led to the extinction of many marsupials. Consider whether the
same process is happening in Australia now, since humans have introduced dogs, rats,
rabbits, etc.
Critical Thinking
Question 1.
Evolutionary scientists contend that life cannot arise again as it first did. Why?
Answer: Photosynthetic life was itself responsible for building up the current atmosphere with
its protective ozone shield. The earliest life developed in a harsh unprotected atmosphere with
intense radiation and without advanced life forms already present and competing for nutrient
resources. Today any unlucky protocell that arose under these modern conditions would be
rapidly gobbled up by advanced life forms.
Question 2. There is a tendency to conceptualize the evolutionary record as: “all ‘lower’
organisms evolved early and all ‘higher’ organisms evolved recently.” When did the flowering
plants evolve (and along with them the pollinating bees) compared to the Age of Fish? Reword
the above quotation correctly.
Answer: The Age of Fish, a vertebrate, was the Devonian. This is much earlier than the rise of
pollinating insects that are “lowly” invertebrates. “Recent organisms are derived from earlier
early organisms” but all generally continue to evolve through time.
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CHAPTER
19
SYSTEMATICS AND PHYLOGENY
A survey of the history of the discipline of taxonomy precedes a description of taxonomic
classification. Phylogenetic trees and systematics are described in detail. The “new”
classification system (three domains) is outlined, as is a description of the domains. A Science
Focus box examines “DNA Bar Coding of Life.”
Chapter Outline
19.1
Systematics
1. Systematics is a field of biology dedicated to understanding the evolutionary history
of life on Earth
2. Taxonomy is the branch of biology concerned with identifying, naming, and
classifying organisms.
A. Linnean Systematics
1. A natural system of classification reflects the evolutionary history of organisms.
2. Naming and identifying organisms began with the Greeks and Romans.
3. In the Middle Ages, organisms were described using long Latin descriptions.
4. John Ray (1627–1705), a British naturalist, argued that each organism should have a
set name.
B. The Binomial System
1. The number of known organisms expanded greatly in mid-eighteenth century due to
European travel.
2. Carolus Linnaeus (1707–1778) developed the binomial system to name species.
3. The binomial system of nomenclature names organisms using a two-part Latin name.
a. First part is the genus; closely related species are assigned to the same genus.
b. Second part is the specific epithet; it usually provides something descriptive about
an organism.
c. A scientific name consists of both genus and specific epithet (e.g., Lilium
buibiferum and Lilium canadense).
d. Both names are italicized or underlined; the first letter of the genus name is
capitalized.
e. The genus can be abbreviated when used with a specific epithet if the full name
was given before.
4. Common names vary with different languages, lump many species under one name or
have various names for the same species, and the same name may refer to different
organisms in different regions.
5. The job of naming all species is far from finished.
a. There are estimated to be between 3 and 30 million species living on Earth.
b. Currently, one million species of animals and a half million plant and
microorganismic species have been named.
c. Some groups, such as birds, are nearly all known; some insect groups are mostly
unknown.
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C. Linnean Classification Categories
1. Aristotle classified life into 14 groups (e.g., mammals, birds, etc.), and subdivided
them by size.
2. Ray grouped animals and plants according to how he thought they were related.
3. Linnaeus grouped plants by flower parts; his categories were published in Systema
Naturae in 1735
4. Today, taxonomists use seven categories of classification: species, genus, family,
order, class, phylum, and kingdom.
a. A higher category, the domain, has recently been added to these seven categories.
b. The higher the category, the more inclusive it is.
c. Members of a kingdom share general characters; members of a species share quite
specific characters.
d. Since one taxonomic group exists inside another group, these categories are also
termed nested. A character is any structural, chromosomal, or molecular feature
that distinguishes one group from another.
e. Additional levels of classification can be added by adding super-, sub-, or infra(e.g., suborder); thus, there are more than 30 categories of classification.
19.2 Phylogenetic Trees
1. Classification reflects phylogeny; one goal of systematics is to create phylogenetic
trees.
2. Phylogeny is the evolutionary history of a group of organisms.
3. A phylogenetic tree indicates common ancestors and lines of descent or lineages.
4. A primitive character is a trait that is present in a common ancestor and all
members of a group.
5. A derived character is present only in a specific line of descent.
6. Different lineages diverging from a common ancestor have ancestral
characteristics—traits shared by the ancestor and the species in its lines of descent.
7. Because classification is hierarchical, it is possible to use classification categories to
construct a phylogenetic tree.
8. When we say that two species are related, we mean that they share a common
ancestor.
A. Cladistic Phylogenetic Trees
1. Cladistic systematics is based on the work of Willi Hennig.
2. Cladistics analyze primitive and derived characters and constructs cladograms on
the basis of shared derived characters.
3. A cladogram is a diagram showing relationships among species based on shared,
derived characters; a cladogram thus traces the evolutionary history of the group
being studied.
4. Constructing a Cladogram
a. First step: construct a table of characters of the taxa being compared.
b. Any character found in the outgroup as well as the ingroup is a shared primitive
character.
c. Homologies shared by certain lineages are shared derived characters, or
synapomorphies.
d. A clade is an evolutionary branch that includes a common ancestor and all its
descendent species.
5. How to Judge a Cladogram
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a. Cladists are guided by the principle of parsimony—the minimum number of
assumptions is most logical.
b. The best cladogram is one in which the fewest number of shared derived
characters are left unexplained or that minimizes the number of assumed
evolutionary changes.
c. This approach is vulnerable if convergent evolution produces what appears to be
common ancestry.
d. Statistical phylogenetic is a new branch of systematics that uses statistical tools
and not parisomy to help construct phylogenetic trees.
e. Reliability of cladograms is dependent on the knowledge and skill of a particular
investigator gathering data.
6. How to Judge a Clade
a. A Monophyletic group is a grouping of species that includes a common ancestor
and all the descendents of that ancestor.
b. A paraphyletic group contains a common ancestor and does not include all the
descendents.
c. A polyphyletic group contains some of the descendants of more than one common
ancestor and not all the common ancestors.
B. Tracing Phylogeny
1. Fossil Record Data
a. Because fossils can be dated, fossils can establish the age of a species.
b. It can be difficult to associate fossils with currently living groups; e.g., a new
view of turtle fossils could place them closer to crocodiles.
c. The fossil record is often incomplete because soft-bodied organisms do not
fossilize well.
d. Most organisms decay and the chances of becoming a fossil are low.
2. Morphological Data
a. Homology is character similarity that stems from having a common ancestor;
homology helps indicate when species belong to a related group.
b. Homologous structures are related to each other through common descent but
may differ in structure and function (e.g., the forelimbs of a horse and the wings
of a bat).
c. Convergent evolution is acquisition of similar traits in distantly related lines of
descent as a result of adaptation to similar environmental conditions; convergent
evolution may make it difficult to distinguish homologous from analogous
structures.
d. Analogous structures have the same function but are not derived from the same
organ in a common ancestor (e.g., the wings of an insect and the wings of a bat).
C. Behavioral Data
1. Since many different species may display some common behaviors, this may
substantiate the morphological data that some species are related through evolution.
D. Molecular Data
1. Speciation occurs when mutations bring about changes in base pair sequences of
DNA.
2. Each distinct lineage accumulates changes in DNA base pair sequences and amino
acid sequences in proteins over time.
3. Advances in analyzing nucleotide and amino acid sequences make abundant data
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available to researchers.
4. Protein Comparisons
a. Earlier studies used immunological reactions to antibodies, made by injecting a
rabbit with cells of one species, to determine the relatedness of two species.
b. Amino acid sequences are now used to determine the differences in proteins
between two species.
1) Cytochrome c is a protein found in all aerobic organisms; the amino acid
differences in cytochrome c between chickens and humans is 13 but between
chickens and ducks is only 3.
c. Since the number of universal proteins is limited, most new studies use RNA and
DNA.
5. DNA and RNA Comparisons
a. DNA differences can substantiate data, help trace the course of macroevolution,
and fill in the gaps of the fossil record.
e. Mitochondria DNA (mtDNA) mutates ten times faster than nuclear DNA;
mtDNA is often used for closely related species; North American songbirds were
found to have diverged well before retreating glaciation 250,000–100,000 years
ago.
6. Molecular Clocks
a. Nucleic acid changes are not tied to adaptation; the fairly constant changes
provide a molecular clock.
b. Comparison of mtDNA sequences equated a 5.1% nucleic acid difference among
songbird species to 2.5 million years.
c. The fossil record can then be used to calibrate the clock and confirm the
hypothesis drawn from molecular data.
E. DNA Bar Coding of Life (Science Focus box)
1. The Consortium for the Barcode of Life (CBOL) believes that any scientist will be
able to identify a species using a handheld scanner.
2. According to this Consortium, there will be a database of DNA sequences, and this
scanner would tap into the database and identify an organism.
3. Paul Hebert and colleagues at the University of Guelph in Canada believe that it is
possible to use the base sequence in DNA to develop a bar code for each living thing.
4. According to Herbert, the gene:
a. Should contain no more than 650 nucleotides so sequencing is easy and with no
mistakes.
b. Should be easy to extract from an organism’s complete genome.
c. Should have mutated to the degree that each species has its own sequence of
bases.
5. Herbert’s team uses a mitochondrial gene, cytochrome c oxidase subunit I (COI), for
a target gene in animals.
6. John Kress from the Smithsonian Institute in Washington, D.C. developed a potential
method for bar coding plant species.
19.3
The Three-Domain System
1. Recent research suggests one group of prokaryotes is so distantly related it should be
in a separate domain.
2. Sequencing of rRNA suggests all organisms evolved along three distinct lineages:
domains Bacteria, Archaea, and Eukarya.
133
3. Domain Bacteria
a. The bacteria are prokaryotic unicellular organisms that reproduce asexually.
b. Cyanobacteria are large photosynthetic prokaryotes.
c. Most bacteria are heterotrophic.
d. Bacteria are important in ecosystems because they break down organic remains,
thereby keeping chemical cycling going.
e. Some bacteria are parasitic and cause disease.
4. Domain Archaea
a. Like bacteria, archaea are prokaryotic unicellular organisms that reproduce
asexually.
b. The archaea live in extreme environments: methanogens in anaerobic swamps,
halophiles in salt lakes, and thermoacidophiles in hot acidic environments.
c. The archaea cell wall is diverse but not the same as the bacterial cell wall.
5. Domain Eukarya
a. Eukarytes are unicellular to multicellular organisms, always with a membranebound nucleus.
b. Sexual reproduction is common; various types of life cycles are seen.
c. Protists and fungi are eukarytes, as are plants and animals (these kingdoms will be
studied in detail in later chapters in the text and in this instructor’s manual).
Lecture Enrichment Ideas
Experience Base: As much as “classifying” is a natural intellectual activity that all people
engage in throughout their lives, the biological application of classification often appears to be
merely a matter of memorizing science conventions. To the contrary, grouping based on
similarity requires a full command of the full range of biological characteristics of organisms as
well as an understanding of genetic, reproductive, and ecological processes in order to deduce
past evolutionary patterns. The likely only effective method to understand cladistics is to work
through one simple set of animals or plants by developing a table of traits and a similarity matrix.
1. Describe the difference between “naming” (only one scientist gets to “name” a species);
“identifying” (anyone can “identify” with a key), and “classifying” (grouping a species with
its closest relatives). In everyday life, these are used almost interchangeably but they have
different meanings in biology. It is important for biology majors to correctly use these terms.
2. Students may ask how we can “know” there are many more species yet to be described and
named. From time to time, surveys of systematists are made asking how many species they
estimate are yet to be described based on their current rate of work and collecting. Why can’t
scientists arrive at one stable classification system where the taxa names no longer change?
3. Discussing the problems with Aristotle’s land-air-sea classification gives some idea of the
limitations of even modern classification schemes.
4. Describe how new research is constantly refining our understanding of the ever more
complex relationships as new techniques (such as rRNA analysis and DNA hybridization,
etc.) become available.
5. Ask students how drug research proceeds; why do they first test rats, then monkeys, before
moving to human trials? Describe the predictive power of a phylogeny that is closer to
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evolutionary similarity.
Critical Thinking
Question 1. In many states, you must have a fishing license to hunt frogs. Many stores post a
sign that says “No animals allowed.” These regulations are often written into state laws. Why
can’t biology use such common English names rather than scientific names?
Answer: Technically, a human is an animal and therefore could not enter the store. Likewise, a
frog is not a fish. Such definitions, albeit legal, vary from state to state and reflect local historical
usage. The northern “mudpuppy” is the southern “waterdog.” In contrast to common terms,
scientific terminology is not ambiguous, and the Latinized scientific name can be recognized in
the midst of a research article written in any language.
Question 2. Australia and the United States both have mouse-like, cat-like, and mole-like
animals. Yet the Australian versions of these mammals all have pouches (they are marsupials)
while the U.S. animals are mostly placental. The other features are very similar between
continents. Which of the three schools of classification might group the Australian and U.S.
look-alikes together? Why do they appear similar?
Answer: The animals from two different continents underwent convergent evolution as they
adapted to the same niches. This would not be a problem for the cladist who would separate the
primitive marsupials from the derived placentals, nor for the traditional phylogeneticist who
would weigh in this biogeography; however, the pheneticist would count similar characters and
likely group them together.
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135
PART
IV
MICROBIOLOGY AND EVOLUTION
The following three chapters describe microbiological organisms and evolution. Microbes have
existed on Earth for 3.5–3.8 billion years. Most are unicellular, and from these single cells came
multicellular forms such and plants and animals.
20 Viruses, Bacteria, and Archaea
21 Protist Evolution and Diversity
22 Evolution and Diversity of Fungi
CHAPTER
20
VIRUSES, BACTERIA AND ARCHAEA
This chapter presents a study of the microbes bacteria and archaea, and the nonliving-yetinfectious viruses. The methods of classification of these microbes is discussed, as is their
structure, and biological activities. Much terminology is presented in the chapter. Many diseases
are described. A Health Focus box discusses “Flu Pandemic.”
Chapter Outline
20.1 Viruses, Viroids, and Prions
A. Viruses
1. are associated with a number of plant, animal, and human diseases;
2. can only reproduce by using the metabolic machinery of the host cell;
3. are noncellular;
4. may have a DNA or RNA genome.
5. In 1884, Pasteur suspected something smaller than bacteria caused rabies; he chose a
Latin term for “poison.”
6. In 1892, Russian biologist Dimitri Ivanowsky, working with the tobacco mosaic
virus, confirmed Pasteur’s hypothesis that an infectious agent smaller than a
bacterium existed.
7. With the invention of the electron microscope, these infectious agents could be seen
for the first time.
B. Viral Structure
1. A virus is similar in size to a large protein, generally smaller than 200 nm in diameter.
2.
3.
Many viruses can be purified and crystallized, and the crystals stored for long periods of time.
Viral crystals become infectious when the viral particles they contain invade host cells.
4
The classification of viruses is based on
5.
a. their type of nucleic acid, including whether they are single-stranded or double-stranded;
b. their size and shape; and
c. the presence or absence of an outer envelope.
All viruses have at least two parts:
136
a. An outer capsid is composed of protein subunits.
b.
An inner core contains either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid), but not
both.
1) The viral genome at most has several hundred genes; a human cell, in
comparison, contains thousands of genes.
2) The viral envelope is usually partly host plasma membrane with viral glycoprotein spikes.
3) Viral particles have proteins, especially enzymes (e.g., polymerases), to produce viral DNA or
RNA.
4) Not all viruses have an envelope; such viruses are called naked viruses.
C. Parasitic Nature
1.
2.
3.
Viruses are obligate intracellular parasites that cannot multiply outside a living cell; they must infect a
living cell in order to reproduce.
a.
Animal viruses in laboratories are raised in live chick embryos or in cell tissue culture.
b.
Viruses infect all sorts of cells, from bacteria to human cells, but they are host specific.
1) The tobacco mosaic virus only infects certain plants.
2) The rabies virus infects only mammals.
3) The AIDS virus, HIV, infects only certain human blood cells.
4) The Hepatitis virus invades only liver tissues.
5) The Polio virus only reproduces in spinal nerve cells.
Virus Evolution
a.
Some believe that viruses originated from the very cells that they infect.
b.
For example, viral nucleic acids originated from the host cell genome.
c. Therefore, viruses evolved after cells came into existence; new viruses are probably evolving now.
d. Others suggest that viruses arose before the three domains.
Viruses often mutate; therefore, it is correct to say that they evolve.
a.
Those that mutate are troublesome; a vaccine effective today may not be effective
tomorrow.
b.
Influenza (flu) viruses mutate regularly.
D. Viral Reproduction
1.
2.
3.
4.
Viruses gain entry into and are specific to a particular host cell because portions of the capsid (or
spikes of the envelope) adhere to specific receptor sites on the host cell surface.
Viral nucleic acid then enters a cell, where viral genome codes for production of protein units in the
capsid.
A virus may have genes for a few special enzymes needed for the virus to reproduce and exit from a
host cell.
A virus relies on host cell enzymes, ribosomes, transfer RNA (tRNA), and ATP for its own replication.
E. Reproduction of Bacteriophages
1.
2.
3.
Bacteriophages (phages) are viruses that parasitize bacteria.
The lytic cycle is a bacteriophage’s “life” cycle consisting of five stages:
a. During attachment, portions of the capsid bind with receptors on the bacterial cell wall.
b. During penetration, a viral enzyme digests part of cell wall; the viral DNA is injected into a
bacterial cell.
c. Biosynthesis involves synthesis of viral components and begins after the virus brings about
inactivation of host genes not necessary to viral replication.
d. During maturation, viral DNA and capsids are assembled to produce several hundred viral
particles and lysozyme, coded by the virus, is produced.
e. When lysozyme disrupts the cell wall, release of the viral particles occurs and the bacterial cell
dies.
With the lysogenic cycle, the virus incorporates its DNA into the bacterium but only later is phage
produced.
a. Following attachment and penetration, viral DNA becomes integrated into bacterial DNA with no
destruction of the host DNA.
b. At this point, the phage is latent and the viral DNA is called a prophage.
c. The prophage is replicated along with host DNA; all subsequent cells (lysogenic cells) carry a
copy.
137
d.
Certain environmental factors (e.g., ultraviolet radiation) induce prophage to enter the biosynthesis
stage of the lytic cycle, followed by maturation and release.
F. Reproduction of Animal Viruses
1.
2.
Animal viruses replicate similarly to bacteriophages, but there are modifications.
a. If the virus has an envelope, glycoprotein spikes allow it to adhere to plasma membrane receptors.
b.
The virus genome covered by the capsid penetrates the host cell.
c.
Once inside, the virus is uncoated as the envelope and capsid are removed.
d.
Free of its covering, the viral genome (DNA or RNA) proceeds with biosynthesis.
e.
Newly assembled viral particles are released by budding.
f.
Components of viral envelopes (i.e., lipids, proteins, and carbohydrates) are obtained
from the plasma or nuclear membrane of the host cell as the viruses leave.
Retroviruses are RNA animal viruses that have a DNA stage.
a.
Retroviruses contain the enzyme reverse transcriptase that uses RNA as a template to
produce cDNA; cDNA is a copy of the viral genome.
b.
Viral cDNA is integrated into host DNA and is replicated as host DNA replicates.
c.
Viral DNA is transcribed; new viruses are produced by biosynthesis and maturation;
release is by budding.
G. Viral Infections of Special Concern
1.
2.
3.
4.
5.
6.
7.
8.
Viruses cause infectious diseases in plants and animals, including humans.
Some animal viruses are specific to human cells: papilloma virus, herpes virus, hepatitis virus, and
adenoviruses, which can cause specific cancers.
Retroviruses include the AIDS viruses (e.g., HIV) and also cause certain forms of cancer.
Emerging Viruses
HIV is an example of an emerging virus: the causative agent of a disease that has only recently arisen
and infected people.
In some cases of emerging diseases, the virus is simply transported from one location to another; e.g.,
West Nile virus and severe acute respiratory syndrome (SARS).
The high mutation rate of viruses also causes infectious viruses to emerge; e.g., AIDS and Ebola fever.
A change in the mode of transmission is yet another way infectious viruses could emerge.
H. Viroids and Prions
1. Viroids are naked strands of RNA, a dozen of which cause crop diseases.
2. Like viruses, viroids direct the cell to produce more viroids.
3. Prions (proteinaceous infectious particles) are newly discovered disease agents that
differ from viruses and bacteria.
I. Flu Pandemic (Health Focus box)
1. Flu Viruses
a. A flu virus has an H (hemagglutinin) spike, which allows the virus to bind to its
receptor. There are 16 known types of H spikes.
b. The N (neuraminidase) spike attaches host plasma membranes so the mature
viruses can exit the cell. There are nine known types of N spikes.
c. These viral spikes can occur in different varieties called subtypes.
d. Many flu viruses are assigned specific codes based on the type of spike.
1) H5N1 (bird flu) virus gets its name from its variety of H5 spikes and its
variety of N1 spikes.
e. Our immune system only recognizes a particular variety of H and N spikes it has
been exposed to.
f. When a new flu virus arises and there is little or no immunity in the human
population, a flu pandemic may occur.
2. Possible Bird Flu Pandemic of the Future
a. The H5N1 subtype of the bird flu virus is of concern because it has the
potential to reach pandemic proportions.
b. H5N1 first infected waterfowl, then chickens, and now humans.
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c. This virus can attach to both a bird flu receptor and a human flu receptor.
d. The virus can transfer from poultry to human, but has rarely been transmitted
from human to human.
e. However, with additional mutations, human to human transmission could be
more common, and spread around the world.
3. How to Be Prepared
a. One of the easiest practices to prevent the spread of a flu virus is cleaning
hands thoroughly and often using soap and water or alcohol-based sanitizer.
b. Keeping your hands away from your eyes, nose, and mouth can help prevent
the virus from entering your body.
c. Education will help prepare for a pandemic.
d. For more information, visit www.pandemicflu.gov.
20.2 The Prokaryotes
A. Prokaryotes include the bacteria and archaea.
1.
2.
3.
4.
Bacteria were discovered in the seventeenth century when Antonie van Leeuwenhoek examined
scrapings from his teeth.
The “little animals” Leeuwenhoek observed were thought by him and others to arise spontaneously
from inanimate matter.
Around 1850, Pasteur devised an experiment showing that the bacteria present in air contaminated the
media.
A single spoonful of soil contains 1010 prokaryotes; these are the most numerous life forms.
B. Structure of Prokaryotes
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Prokaryotes range in size from 1–10 µm in length and from 0.7–1.5 µm in width.
“Prokaryote” means “before a nucleus”—their cells lack a eukaryotic nucleus.
Prokaryotic fossils date back as far as 3.5–3.8 billion years ago.
Fossils indicate prokaryotes were alone on earth for 2 billion years; they evolved very diverse
metabolic capabilities.
Prokaryotes adapted to most environments because they differ in the many ways they acquire and
utilize energy.
Outside the plasma membrane of most cells is a rigid cell wall that keeps the cell from bursting or
collapsing due to osmotic changes by peptidoglycan, a complex molecule containing a unique amino
disaccharide and peptide fragments.
a. The cell wall may be surrounded by an organized capsule called a glycocalyx and/or by a loose
gelatinous sheath called a slime layer.
b. In parasitic forms, these outer coverings protect the cell from host defenses.
Some prokaryotes move by means of flagella.
a. The flagellum has a filament composed of three strands of the protein flagellin wound in a helix
and inserted into a hook that is anchored by a basal body.
b. The flagellum is capable of 360o rotation which causes the cell to spin and move forward.
Many prokaryotes adhere to surfaces by means of fimbriae.
a. Fimbriae are short hairlike filaments extending from the surface.
b. The fimbriae of Neisseria gonorrhoeae allow it to attach to host cells and cause gonorrhea.
Prokaryotic cells lack the membranous organelles of eukaryotic cells.
Various metabolic pathways are located on the plasma membrane.
A nucleoid is a dense area in prokaryotes where the chromosome is located; it is a single circular
strand of DNA.
Plasmids are accessory rings of DNA found in some prokaryotes; they can be extracted and used as
vectors to carry foreign DNA into bacteria during genetic engineering procedures.
Protein synthesis in prokaryotic cells is carried out by thousands of ribosomes, which are smaller than
eukaryotic ribosomes.
C. Reproduction in Prokaryotes
1.
Binary fission is the splitting of a parent cell into two daughter cells; it is asexual reproduction in
prokaryotes.
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a.
b.
c.
d.
2.
A single circular chromosome replicates; the two copies separate as the cell enlarges.
Newly formed plasma membrane and the cell wall separate the cell into two cells.
Mitosis, which involves formation of a spindle apparatus, does not occur in prokaryotes.
Because prokaryotes have a short generation time, mutations are generated and distributed through
a population more rapidly.
e. Prokaryotes are haploid; mutations are therefore immediately subjected to natural selection.
In bacteria, genetic recombination can occur in three ways.
a. Conjugation occurs when a bacterium passes DNA to a second bacterium through a tube (sex
pilus) that temporarily joins two cells; this occurs only between bacteria in the same or closely
related species.
b. Transformation involves bacteria taking up free pieces of DNA secreted by live bacteria or
released by dead bacteria.
c. In transduction, bacteriophages transfer portions of bacterial DNA from one cell to another.
d. Plasmids can carry genes for resistance to antibiotics and transfer them between bacteria by any of
these processes.
20.3 The Bacteria
A. Characteristics of Bacterial Cells
1. Bacteria are the more common type of prokaryote
2. Bacterial cell walls are protected by peptidoglycan, a complex of polysaccharides
linked by amino acids.
3.
The Gram stain procedure (developed in the late 1880s by Hans Christian Gram) differentiates
bacteria.
a. Gram-positive bacteria stain purple, whereas Gram-negative bacteria stain pink.
b. This difference is dependent on the thick or thin (respectively) peptidoglycan cell wall.
4. Bacteria and archaea have three basic shapes.
a. A spirillum is spiral-shaped.
b. A bacillus is an elongated or rod-shaped bacteria.
c. Coccus bacteria are spherical.
d. Cocci and bacilli tend to form clusters and chains of a length typical of the particular species.
B. Bacterial Metabolism
1. Bacteria differ in their need for, and tolerance of, oxygen (O 2).
a. Obligate anaerobes are unable to grow in the presence of O 2; this includes anaerobic bacteria that
cause botulism, gas gangrene, and tetanus.
b. Facultative anaerobes are able to grow in either the presence or absence of gaseous O2.
c. Aerobic organisms (including animals and most prokaryotes) require a constant supply of O 2 to
carry out cellular respiration.
2. Autotrophic Bacteria
a. Photoautotrophs are photosynthetic and use light energy to assemble the organic molecules they
require.
1) Primitive photosynthesizing bacteria (e.g., green sulfur bacteria and purple sulfur bacteria)
use only photosystem I that contains bacteriochlorophyll; they do not give off O2 because
hydrogen sulfide (H2S) is used as an electron and H+ donor instead of H2O.
2) Advanced photosynthesizing bacteria (e.g., cyanobacteria) use both photosystem I and II that
contain the same types of chlorophylls found in plants; they do give off O 2 because H2O is
used as an electron and H+ donor.
b. Chemoautotrophs make organic molecules by using energy derived from the oxidation of
inorganic compounds in the environment.
1) Deep ocean hydrothermal vents provide H2S to form chemosynthetic bacteria.
2) The methanogens are chemosynthetic bacteria that produce methane (CH4) from hydrogen gas
and CO2; ATP synthesis and CO2 reduction are linked to this reaction and methanogens can
decompose animal wastes to produce electricity as an ecological friendly energy source.
3) Nitrifying bacteria oxidize ammonia (NH3) to nitrites (NO2) and nitrites to nitrates (NO3).
3. Heterotrophic Bacteria
a. Most free-living bacteria are chemoheterotrophs that take in pre-formed organic nutrients.
b. As aerobic saprotrophs, there is probably no natural organic molecule that cannot be broken
down by some prokaryotic species.
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c. Bacteria produce chemicals including ethyl alcohol, acetic acid, butyl alcohol, and acetones.
d. Bacteria action produces butter, cheese, sauerkraut, rubber, cotton, silk, coffee, and cocoa.
e. Antibiotics are produced by some bacteria.
C. Symbiotic Relationships
1. Bacteria and archaea form symbiotic relationships, forming relationships with members of other
species; forms of symbiosis include mutualistic, commensalistic, and parasitic relationships.
Commensalism occurs when one population modifies the environment in such a way that a second
population benefits. Commensalistic bacteria live in or on organisms of other species and cause
them no harm.
Mutualistic bacteria that live in the intestines of humans benefit from undigested material and release
vitamins K and B12, which we use to produce blood components.
Parasitic bacteria are responsible for a wide variety of infectious plant, animal and human diseases.
Parasitic bacteria that cause disease are called pathogens.
Some bacteria form resistant endospores in response to unfavorable environmental conditions.
1) Some cytoplasm and the chromosome dehydrate and are encased by three heavy, protective
spore coats.
2) The rest of the bacterial cell deteriorates and the endospore is released.
3) Endospores survive in the harshest of environments: desert heat and dehydration, boiling
temperatures, polar ice, and extreme ultraviolet radiation.
4) Endospores also survive very long periods of time; anthrax spores 1,300 years old can cause
disease.
5) When environmental conditions are again suitable, the endospore absorbs water and grows
out of its spore coat.
6) In a few hours, newly emerged cells become typical bacteria capable of reproducing by binary
fission.
7) Endospore formation is not reproduction--it is a means of survival and dispersal to new
locations.
Pathogens may be able to produce a toxin, and or adhere to surfaces and sometimes
invade organs or cells.
1) Toxins are small organic molecules, or small pieces of protein or parts of the bacterial cell
wall, that are released when bacteria die.
2) In almost all cases, the growth of the bacteria does not cause disease but instead the toxins
they release cause the disease. Example: Clostridium tetani, the causative agent of tetanus.
3) Adhesion factors allow a pathogen to bind to certain cells, which determines which tissue in
the body will be the host. Example: Shigella dysentariae releases a toxin and also sticks to
the intestinal wall, making it a life-threatening form of dysentary.
4) Antibacterial compounds either inhibit cell wall synthesis or protein biosynthesis;
increasingly, many pathogenic bacteria are becoming resistant to bacteria.
D. Cyanobacteria
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Cyanobacteria are Gram-negative bacteria with a number of unusual traits.
They photosynthesize in the same manner as plants, and thus are responsible for introducing O 2 into
the primitive atmosphere.
They were formerly mistaken for eukaryotes and classified with algae.
Cyanobacteria have pigments that mask chlorophyll; they are not only blue-green but also red, yellow,
brown, or black.
They are relatively large (1–50 µm in width).
They can be unicellular, colonial, or filamentous.
Some move by gliding or oscillating.
Some possess heterocysts, thick-walled cells without a nucleoid, where nitrogen fixation occurs.
Cyanobacteria are common in fresh water, soil, on moist surfaces, and in harsh habitats (e.g., hot
springs).
Some species are symbiotic with other organisms (e.g., liverworts, ferns, and corals).
Lichens are a symbiotic relationship where the cyanobacteria provide organic nutrients to the fungus
and the fungus protects and supplies inorganic nutrients.
Cyanobacteria were probably the first colonizers of land during evolution.
Cyanobacteria “bloom” when nitrates and phosphates are released as wastes into water; when they die
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off, decomposing bacteria use up the oxygen and cause fish kills.
20.4 The Archaea
1.
2.
3.
Archaea are prokaryotes with molecular characteristics that distinguish them from bacteria and
eukaryotes; their rRNA base sequence is different from that in bacteria.
Because archaea and some bacteria are both found in extreme environments (hot springs, thermal
vents, salt basins), they may have diverged from a common ancestor.
Later, the eukarya split from the archaea; archaea and eukarya share some ribosomal proteins not
found in bacteria; initiate transcription in the same manner, and have similar types of tRNAs.
A. Structure of Archaea
1.
Archaea have unusual lipids in their plasma membranes that allow them to function at high
temperatures: glycerol linked to hydrocarbons rather than fatty acids.
2. Cell walls of archaea do not contain the peptidoglycan found in bacterial cell walls.
B. Types of Archaea
1. Methanogens live under anaerobic environments (e.g., marshes) where they produce methane.
a. Methane is produced from hydrogen gas and carbon dioxide and is coupled to formation of ATP.
b. Methane released to the atmosphere contributes to the greenhouse effect.
c. About 65% of the methane found in our atmosphere is produced by methanogenic archaea.
2. Halophiles require high salt concentrations (e.g., Great Salt Lake).
a. Their proteins have unique chloride pumps that use halorhodopsin to synthesize ATP in the
presence of light.
b. They usually require 12–15% salt concentrations; the ocean is only 3.5% salt.
3. Thermoacidophiles live under hot, acidic environments (e.g., geysers).
a. They survive best at temperatures above 80oC; some survive above boiling temperatures.
b. Metabolism of sulfides forms acidic sulfates; these bacteria grow best at pH of 1 to 2.
Lecture Enrichment Ideas
Experience Base: Most students have grown up with social expectations of hygiene: “wash your
hands before coming to supper,” “don’t eat directly from the serving bowl,” etc. Some practices,
though, such as pressure cooking, surgical instrument sterilization, etc., are less understood
outside of the sciences. Relating these practices to the information in this chapter will make this
material more relevant to many students. College is also a time for students to discard the term
“germ” except in the historical aspect of “germ theory” for more exact terms that designate the
disease agent.
1. The size difference between viruses and prokaryotes and eukaryotic cells can be illustrated:
If a eukaryotic cell were the size of a large lecture room, a bacterium would approximate the
size of a chair or desk, and a virus would be smaller than a pencil.
2. Diagrams of the entry, replication, and release of DNA and RNA viruses in host cells are best
illustrated by drawing the sequences and providing line diagrams. Discuss the possible
origins of viruses, with emphasis on the fact that bacterial, plant, and animal viruses have
genes that are more like the genes of their host cells than like other kinds of viruses. The
most logical explanation is that viruses represent parts of the host cell genome that became
semi-independent. Explore whether or not this would work for RNA viruses.
3.
Examine the products that a provirus (integrated virus) makes that cause human diseases when the human is
infected by a lysogenic bacterium, such as diphtheria toxin or the factor in Streptococcus that produces scarlet
fever or rheumatic fever.
4.
Discuss the reproductive pathway for the AIDS virus, pointing out the areas in its replication that might be
attacked by antiviral drugs.
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5.
Biographical readings from books such as The Virus Hunters or The Microbe Hunters provide the context for
early work by Koch and Pasteur. Such reading excerpts reveal a very human set of researchers who knew their
work meant life or death for thousands.
6.
Describe why the production of endospores is important in the survival of some forms of bacteria. Ask where a
student could point out a sterile location in a house. (The oven is probably the closest to sterile.) What can we
do to maintain a sterile field in an operating room where not all objects can be boiled?
7.
The recent discovery of prions as infectious agents was highly controversial. Discuss why a disease agent that
was not a living cell or active virus, and lacked any RNA or DNA, would be suspect and require definitive
proof for the science community to accept it. Bovine spongiform encephalopathy (BSE) remains a “hot” news
topic and has implications for international trade and world community politics.
Critical Thinking
Question 1.
Why do we refer to viruses as “activated” or “inactivated” rather than “living” or “dead”?
Answer: “Living” organisms should have the ability to independently grow, reproduce, metabolize and respire, etc.
Virus are reproduced only when taken into other cells. Outside of host cells, they demonstrate no life properties.
Their ability to be propagated and to replicate mutations is dependent upon the machinery of the host cell. They
cannot be proclaimed “dead” because alone, they were never alive.
Question 2.
Why is a chemical agent, such as sulfa or phenol, not considered an “antibiotic”?
Answer: Antibiotics are substances that were originally derived from living organisms, such as molds, etc., and
include penicillin, streptomycin, tetracycline, and erythromycin. They either block synthesis of bacterial cell walls
or block protein synthesis. Chemical agents came from the chemistry lab rather than from organisms.
Question 3.
Why are bacterial 16S ribosomal RNA sequences used rather than the circular chromosome loop
in research that attempts to work out the phylogenetic relationships of prokaryotes?
Answer: Because of the lack of a nucleus, there is a substantial amount of gene transfer among prokaryotes for traits
that affect variable metabolism, antibiotic resistance, etc. By selecting sequences that are bound to a basic function,
any changes are likely to avoid the continuous extracellular flow of genes, and provide a more reliable “clock.”
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
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CHAPTER
21 PROTIST EVOLUTION AND DIVERSITY
The many different types of protists are described in this chapter. Emphasis is placed on their
classification, organizational form, and reproductive mechanisms. The commercial uses of
various protists are discussed, as are their importance to the ecosystem. The involvement of
certain protists in human disease is outlined. A Science Focus box describes the various life
cycles found among the different species of algae.
Chapter Outline
21.1 General Biology of Protists
A. Protists are classified in the domain Eukarya (they have eukaryotic cells) and the kingdom
Protista.
1.
The endosymbiotic hypothesis suggests how the eukaryotic cells arose.
a. It proposes that aerobic bacteria became mitochondria.
b. Cyanobacteria became chloroplasts after being taken up by eukaryotic cells.
c. Giardia lamblia has two nuclei but no mitochondria, suggesting that a nucleated cell preceded the
acquisition of mitochondria.
2. Although many protists are unicellular, they are highly complex.
a. Amoeboids and ciliates possess unique organelles, such as contractile vacuoles.
3. Most protists are free-living; some are parasitic, some (e.g., slime molds) are saprophytic (feed on decaying
plant material), and some are mixotrophic (combining autotrophic and heterotrophic nutrition modes).
4. Some protists are photoautotrophic; some are heterotrophic.
5. Most protists use asexual reproduction, but sexual reproduction occurs in some species.
a. Formation of spores allows free-living and parasitic protists to survive hostile environments.
b. A cyst is a dormant cell with a resistant outer covering; the cyst allows a free-living species to
overwinter and helps certain parasitic species survive the host’s digestive juices.
6. Some protists are of great medical importance because they cause disease; others are ecologically
important.
7. Aquatic plankton serve as food for heterotrophic protists and animals.
8. Photosynthetic plankton produce much of the oxygen in the atmosphere.
9. Many protists enter symbiotic relationships; coral reefs rely on symbiotic photosynthetic protists.
B. Evolution and Diversity of Protists
1. Classification of protests has been based on modes of nutrition.
a. Algae are autotrophic, similar to land plants
b. Protozoans and slime molds are heterotrophic by ingestion, similar to animals
c. Water molds are heterotrophic by absorption, similar to fungi.
2. Protozoans include photosynthetic, heterotrophic, and mixotrophic organisms and have some form of
locomotion (flagella, pseudopods, cilia).
3. Currently, the most widely accepted method of categorizing protests is by assigning them to
supergroups, which is a major eukarotic group. There are six protist supergroups.
22.2 Diversity of Protists
A. Supergroup Archaeplastids
1. Archaeoplasts include land plants, green and red algae.
2. Green Algae
a.
b.
c.
Green algae contain both chlorophyll a and b.
They live in the ocean, in fresh water habitats, snowbanks, and on moist land.
The majority of green algae are unicellular, but filamentous (Spirogyra), colonial (Volvox), and
multicellular forms exist (Ulva).
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d. Green algae are not always green; some have pigments that give them an orange, red, or rust color.
e. Plants are considered to be most closely related to the green algae.
f. Green algae are divided into chlorophytes and charophytes.
B. Chlorophytes
1. Chlamydomonas is a minute, unicellular green alga less than 25 m long.
2. It has a cell wall and a single, large, cup-shaped chloroplast with a pyrenoid for starch synthesis.
3. The chloroplast contains a light-sensitive eyespot (stigma) that directs the cell to light for
photosynthesis.
4. Two long whiplike flagella project from the anterior end to propel the cell toward light.
5. When growth conditions are favorable, Chlamydomonas reproduces asexually with flagellated spores
called zoospores.
6. When growth conditions are unfavorable, Chlamydomonas reproduces sexually.
a. Gametes from two different mating types join to form a zygote.
b. A heavy wall forms around the zygote; a resistant zygospore survives until conditions are
favorable.
7. Volvox
a. Volvox is a Colonial Green Algae. A colony is a loose association of independent cells.
b. Volvox is a hollow sphere with thousands of cells arranged in a single layer.
c. Volvox cells resemble Chlamydomonas cells; a colony arises if the daughter cells fail to separate.
d. Volvox cells cooperate when flagella beat in a coordinated fashion.
e. Some cells are specialized forming a new daughter colony within the parental colony.
f. Daughter colonies are inside a parent colony until an enzyme dissolves part of a wall so they can
escape.
8. Ulva, a Multicellulr Chlorophyte
a. The multicellular Ulva is called sea lettuce because of its leafy appearance.
b. The thallus (body) is two cells thick but can be a meter long.
c. Ulva has an alternation of generations life cycle, as do plants, but the generations look alike.
d. The gametes look alike (isogametes) and the spores are flagellated.
9. Charophytes
a. Cell division in one plane produces end-to-end chains of cells or filaments.
b. Spirogyra is a filamentous algae found on surfaces of ponds and streams.
1)
It has ribbonlike spiral chloroplasts.
2) Two strands may unite in conjugation and exchange genetic material, forming a diploid
zygote.
3)
The zygotes withstand winter; in spring they undergo meiosis to produce haploid
filaments.
c. Chara (stoneworts) are green algae that live in freshwater lakes and ponds.
d. The stonewort Chara forms a cell plate during cell division and has multicellular sex organs
making plants most closely related to this group.
e. Chara also has a stemlike body with nodes and internodes; the cells of the body originate from
apical meristem features that are homologous with plants.
10. The Red Algae
a. Red algae are chiefly marine multicellular algae that live in warmer seawater.
b. Red algae posses a red and a blue pigment in addition to chlorophyll.
c. They are generally much smaller and more delicate than brown algae.
d. Some are filamentous, but most are branched, having a feathery, flat, or ribbonlike appearance.
e. Coralline algae are red algae with cell walls with calcium carbonate; they contribute to coral reefs.
f. Red algae are economically important.
1) Mucilaginous material in cell walls of Gelidium and Gracilaria is the source of agar used in
drug capsules, dental impressions, and cosmetics.
2) In the laboratory, agar is a major microbiological media, and when purified, is a gel for
electrophoresis.
3) Agar is used in food preparation to keep baked goods from drying and to set jellies and
desserts.
4) Carrageen, an emulsifying agent extracted from Chondrus crispus, is used in production of
chocolate and cosmetics.
C. Life Cycles Among the Algae (Science Focus box)
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1.
Depending on the species and environmental conditions, asexual and sexual reproduction occur in
algae.
2. Asexual Reproduction
a. Asexual reproduction occurs frequently when the environment is favorable to growth.
b. Asexual reproduction only requires one parent.
c. The offspring are genetically identical to parent.
3. Sexual Reproduction
a. Sexual reproduction occurs when the environment is changing and unfavorable to growth.
b. Genetic recombinations may produce individuals that are more likely to survive environmental
extremes such as high or low temperature, acidic or basic pH.
D. Supergroup Chromaleolates
1. This supergroup includes the brown algae, diatoms, golden brown algae, and watermolds
a. The Brown Algae
1) Brown algae are stramenopiles (having either a flagella or descended from an ancestor that
had flagella).
2) Brown algae have chlorophylls a and c and fucoxanthin that give them their color.
3) Their reserve food is a carbohydrate called laminarin.
4) Seaweed refers to any large, complex alga.
5) Their cell walls contain a mucilaginous water-retaining material that inhibits desiccation.
6) Laminaria is an intertidal kelp that is unique among protists; this genus shows tissue
differentiation.
7) Nereocystis and Macrocystis are giant kelps found in deeper water anchored to the bottom by
their holdfasts.
8) Individuals of the genus Sargassum sometimes break off from their holdfasts and form
floating masses.
9) Brown algae provide food and habitat for marine organisms, and they are also important to
humans.
a. Brown algae are harvested for human food and for fertilizer in several parts of the world.
b. Macrocystis is a source of algin, a pectinlike substance added to give foods a stable,
smooth consistency.
10)
Most have an alternation of generations life cycle.
11) Fucus is an intertidal rockweed; meiotic cell division produces gametes and the adult is
always diploid.
2. Diatoms
a. Diatoms are stramenopiles that have cell walls consisting of two silica-impregnated halves or
valves.
b.
When diatoms reproduce asexually, each received one old valve.
c.
The new valve fits inside the old one; therefore, the new diatom is smaller than the
original one.
d.
This continues until they are about 30% of their original size.
e. Then they reproduce sexually; a zygote grows and divides mitotically to form diatoms of normal
size.
f. They are yellow-orange in color because they contain a carotenoid pigment in addition to
chlorophyll.
g. Diatoms are an important part of the phytoplankton, photosynthetic organisms that are a source
of food and oxygen for heterotrophs.
h. The cell wall has an outer layer of silica (glass) with a variety of markings formed by pores.
i. Diatom remains accumulate on the ocean floor and are mined as diatomaceous earth for use as
filters, abrasives, etc.
3. Golden Brown Algae
a. Golden brown algae derive their color from yellow-brown carotenoid pigments.
b. They are unicellular or colonial protists, with two flagella.
c. The cells of golden brown algae may be naked, covered with organic or silica scales, or enclosed
in a secreted cagelike structure called a lorica.
d. Ochromonas is capable of photosynthesis as well as phagocytosis.
e. Golden brown algae contribute to freshwater and marine phytoplankton.
4. The Water Molds
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a.
b.
Water molds usually live in water, form furry growths, and parasitize fish or insects and
decompose remains.
Terrestrial water molds parasitize insects and plants; a water mold caused the 1840s Irish potato
famine.
Most water molds are saprotrophic, living off dead organic matter.
Water molds have a filamentous body but cell walls are composed largely of cellulose.
During asexual reproduction, they produce diploid motile spores (2n zoospores) with flagella.
Unlike fungi, the adult is diploid; gametes are produced by meiosis.
Eggs are produced in enlarged tips called oogonia.
c.
d.
e.
f.
g.
E. Alveolates
 The alveolates have alveoli (small sacs) just beneath the plasma membrane.
 The function of the alveoli may be to support the cell surfaces.
 Alveolates are unicellular protists.
1. The Dinoflagellates
a. The unicellular dinoflagellates are usually bounded by protective cellulose plates.
b. Most have two flagella.
1)
One lies in a longitudinal groove and acts as a rudder.
2) The other is located within a transverse groove; beating causes the cell to spin as it moves
forward.
c. Some species of dinoflagellates are heterotrophic; some are parasitic.
d. They are extremely numerous (30,000 per ml) and an important source of food in the ecosystem.
e. Under certain conditions, Gymnodinium and Gonyaulax increase in number enormously and cause
a “red tide”; they produce a powerful neurotoxin killing fish and causing paralytic shellfish
poisoning.
f. They usually reproduce asexually by using mitosis and longitudinal cell division.
2. The Ciliates
a. Ciliates are unicellular protists that move by coordinated strokes of hundreds of cilia projecting
through holes in a semirigid pellicle.
b. They discharge long, barbed trichocysts for defense and for capturing prey; toxicysts release a
poison.
c. Most are holozoic and ingest food through a gullet and eliminate wastes through an anal pore.
d. During asexual reproduction, ciliates divide by transverse binary fission.
e. Ciliates possess two types of nuclei—a large macronucleus and one or more small micronuclei.
f.
The macronucleus controls the normal metabolism of the cell.
g.
The micronucleus is involved in sexual reproduction.
1) The macronucleus disintegrates and the micronucleus undergoes meiosis.
2) Two ciliates then exchange a haploid micronucleus.
3) The micronuclei give rise to a new macronucleus containing only housekeeping genes.
h. Ciliates are diverse with over 8,000 known species.
1)
Members of the genus Paramecium are complex.
2)
The barrel-shaped didiniums expand to consume paramecia much larger than themselves.
3)
Suctoria rest on a stalk and paralyze victims, sucking them dry.
4)
Stentor resembles a giant blue vase with stripes.
3. Apicomplexans
a. Apicomplexans are also known as sporozoans.
b. All apilcomplexans are parasites
c. Plasmodium vivax causes one type of malaria; it is the most widespread human parasite.
1) After the bite of an infected female Anopheles mosquito, the parasite eventually invades red
blood cells.
2)
Chills and fever appear as red blood cells burst and release toxin into the blood.
d. Malaria remains a major world disease due to insecticide-resistant strains of mosquitoes and drugresistant strains of Plasmodium.
e. Pneumocystis carinii causes the pneumonia seen primarily in AIDS patients.
1)
During sexual reproduction, thick-walled cysts form in the lining of pulmonary air sacs.
2)
Cysts contain spores that successively divide until the cyst bursts and the spores are
released.
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3) Each spore becomes a new organism, reproduces asexually and can enter an encysted sexual
stage.
F. Supergroup Excavates
 The excavates include zooflagellates.
 Zooflagellates have atypical or absent mitochondria, and distinctive flagella and/or deep oral groves.
1. Euglenids
a. Euglenids are small (10–500 µm) freshwater unicellular organisms.
b. One-third of all genera have chloroplasts; those that lack chloroplasts ingest or absorb their food.
c. Their chloroplasts are surrounded by three, rather than two, membranes.
1)
Their chloroplasts resemble those of green algae.
2)
They are probably derived from a green algae through endosymbiosis.
d. The pyrenoid outside the chloroplast produces an unusual type of carbohydrate polymer
(paramylon) not seen in green algae.
e. They possess two flagella, one of which typically is much longer than the other and projects out of
a vase-shaped invagination; it is called a tinsel flagellum because it has hairs on it.
f. Near the base of the longer flagellum is a red eyespot that shades a photoreceptor for detecting light.
g. They lack cell walls, but instead are bounded by a flexible pellicle composed of protein bands side-byside.
h. A contractile vacuole, similar to certain protozoa, eliminates excess water.
i. Euglenoids reproduce by longitudinal cell division; sexual reproduction is not known to occur.
2. Parabasalids and Diplomonads
 Both parabasalids and diplomonads are single-celled, flagellated protozoans that can survive in low
oxygen environments.
a. Parabasalids have a fibrous connection between the Golgi apparatus and flagella.
1) Trichonomas vaginalis is a sexually transmitted organism that infects the vagina and urethra
of women and prostate, seminal vesicles and urethra of men.
b. A diplomonad cell has two nuclei and two sets of flagella.
1) Giardia lamblia cysts are transmitted through contaminated water; they cause severe diarrhea.
3. Kinetoplastids
a. The kinetoplastids are single-celled, flagellated protozoans named for their kinetoplasts, which
are large masses of DNA found in their mitochondria.
1) Trypanosoma bruce, are parasitic kinetoplastids transmitted by the bite of a tsetse fly, is the
cause of African sleeping sickness.
G. Supergroup Amoebozoans
 The amoebozoans are comprised of protozoans that move by pseudopods.
 Amoebozoans live in aquatic environments, oceans and freshwater lakes and ponds, and often times
part of the plankton.
1. Amoeboids
a. Amoeboids engulf prey with pseudopods, cytoplasmic extensions formed as cytoplasm streams in
one direction.
b. Amoeba proteus is a commonly studied member.
c. Amoeboids phagocytize their food; pseudopods surround and engulf prey.
d. Food is digested inside food vacuoles.
e. Freshwater amoeboids have contractile vacuoles to eliminate excess water.
f. Entamoeba histolytica is an amoebic parasite that invades the human intestinal lining.
2. Slime Molds
a. These organisms resemble fungi but all have flagellated cells that fungi never have.
b. The plasmodial slime molds exist as a plasmodium.
1) This diploid multinucleated cytoplasmic mass creeps along, phagocytizing decaying plant
material.
2) Fan-shaped plasmodium contains tubules of concentrated cytoplasm in which liquefied
cytoplasm streams.
3) Under unfavorable environmental conditions (e.g., drought), the plasmodium develops many
sporangia, called a fruiting body, that produce spores by meiosis.
4) Mature spores are released and survive until more favorable environmental conditions return;
then each releases a haploid flagellated cell or an amoeboid cell.
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5) Two flagellated or amoeboid cells fuse to form a diploid zygote that produces a
multinucleated plasmodium again.
c. The cellular slime molds exist as individual amoeboid cells.
1)
They live in soil and feed on bacteria and yeast.
2) As food runs out, amoeboid cells release a chemical that causes them to aggregate into a
pseudoplasmodium.
3)
The pseudoplasmodium stage is temporary; it gives rise to sporangia that produce spores.
4) Spores survive until more favorable environmental conditions return, at which time they
germinate.
5) Spores germinate to release haploid amoeboid cells, which is again the beginning of the
asexual cycle.
H. Supergroup Rhizarians
1. The rhizarians is comprised of the foraminiferans and the radiolarians, organized with a fine,
thread-like pseudopods.
2. They were previously classified with Amoebozoans, but are now assigned to a different subgroup due
molecular data indicating the two groups are not closely related.
3.
Foraminiferans and radiolarians have a skeleton called a test.
a. Foraminiferans have a multi-chambered CaCO3 shell; thin pseudopods extend through holes.
b. Radiolaria have a test composed of silica or strontium sulfate.
1)
Most have a radial arrangement of spines.
2) Pseudopods (actinopods) project from an external layer of cytoplasm and are supported by
rows of microtubules.
.
c. Tests of dead foraminiferans and radiolarians form deep layers of ocean floor sediment.
d. Dating back to the Precambrian, each layer has distinctive foraminiferans which helps date rocks.
e. Over hundreds of millions of years, the CaCO3 shells have contributed to the formation of chalk
deposits (i.e., White Cliffs of Dover, limestone of the great Egyptian pyramids).
I. Supergroup Opisthokonts
1. Animals and fungi are included in the supergroup opisthokonts in addition to several unicellular and
multicellular protists.
2. The choanoflagellates are animal-like protozoans that are near relatives to sponges.
a. They include unicellular and colonial forms, and are filter feeders.
3. Nucleariids are opisthokonts with a rounded or slightly flattened cell body and threadlike pseudopods
called filopodia.
a. Most feed on algae or cyanobacteria.
b. They appear to be close fungal relatives due to molecular similarities.
Lecture Enrichment Ideas
Experience Base: Unfortunately, students’ woodland experiences are becoming more and more
limited. Likely, relatively few students will have observed molds on rotten logs. More may have
maintained a home aquarium and seen water mold on fish. Therefore, visuals will be important
to illustrate most groups. Likewise, the complex diversity and variation in cycles will require
diagrams.
1. “Algae” and “protozoa” are solid examples of how common terms that appear to group
similar organisms do not match up with our understanding of their actual phylogeny.
Nevertheless, it remains used as a general term in the absence of other common terms
referring to one-celled eukaryotes.
2. Plasmodium is easily and safely grown and passed around in a Petri dish. Time-lapse video
can show the very slow movement that is usually unnoticed in nature. Media are necessary to
show students what protists look like and how they move and reproduce, if actual culture
specimens are not available.
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3. Since many of the parasitic forms of kingdom Protista are tropical and subtropical,
Americans are often unfamiliar with the ravages of malaria, dysentery, etc. Descriptions and
visuals can emphasize the seriousness of such major diseases.
4. Keep students aware that all the organisms placed in the kingdom Protista are not really
closely related to each other. Some have suggested that it would be more “reasonable” to
have as many as nineteen kingdoms.
5. Emphasize the importance of the algae as the base of the food chain in both freshwater and
marine environments.
6. Point out that lichens can be made up of fungi and either green algae or cyanobacteria, and
examine how they could be symbiotic or how the fungus could be parasitic on the alga or the
bacterium. What experiments would distinguish this?
7. Describe industrial and commercial uses of protists (mainly algae).
Critical Thinking
Question 1. Euglena are positioned between animallike and plantlike organisms. If they have
chloroplasts, why aren’t they solidly algaelike, or grouped with green algae?
Answer: Two-thirds of euglenoid genera lack chloroplasts, and some can lose chloroplasts if grown in the absence
of light. And the chloroplast, when present, has a three-layer membrane, suggesting it was derived from engulfing of
green algae through endosymbiosis.
Question 2.
Water molds are filamentous and saprotrophic. Even in fairly recent biology textbooks, they were
classified with the fungi. Why are they no longer classified with fungi when their predominant features are
funguslike?
Answer: During asexual reproduction, they produce motile 2n zoospores. Adults are diploid, not haploid as is the
fungi, and meiosis produces the gametes. This is a complex form of reproduction that is not likely to be
independently evolved by a fungus, while the funguslike features could be evolved by a protist.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
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CHAPTER
22
EVOLUTION AND DIVERSITY OF FUNGI
This chapter studies the Kingdom Fungi. The morphology of fungi is described, and the modes
of reproduction. Much terminology is presented in this chapter. Emphasis is placed on the sac
fungi (formerly called imperfect fungi—this section has been rewritten from the previous edition
of the text). The commercial and food uses of fungi are discussed, as are many fungal diseases of
humans. A Health Focus box discusses “Deadly Fungi.”
Chapter Outline
22.1 Evoluation and Characteristics of Fungi
1.
The 80,000 species of the Kingdom Fungi are mostly multicellular eukaryotes that share a common
mode of nutrition.
2. Like animals, fungi are heterotrophic and consume preformed organic matter.
3. Animals, however, are heterotrophic by ingestion while fungi are heterotrophic by absorption.
4. Fungal cells secrete digestive enzymes; following breakdown of molecules, the nutrients are absorbed.
5. Most fungi are saprotrophic decomposers, breaking down wastes or remains of plants and animals.
A. Evolution of Fungi
1. Fungi include club fungi, sac fungi, AM fungi, zygospore fungi, and chytrids.
2. Chytrids are unlike all the other fungi because they are aquatic and they have flagellated spores and
gamets.
3. The AM fungi only exist as mycchorizae in association with plant roots.
4. Molecular data shows that animals and fungi share a common ancestor after plants evolved.
5. The earliest fossil of fungi is dated 450 MYA.
B. Structure of Fungi
1. Fungi can be unicellular (e.g., yeasts).
2. Most fungi are multicellular in structure.
a. The thallus (body) of most fungi is called a mycelium.
b. A mycelium is a network of hyphae comprising the vegetative body of a fungus.
c. Hyphae are filaments that provide a large surface area and aid absorption of nutrients.
d. When a fungus reproduces, a portion of the mycelium becomes a reproductive structure.
3. Fungal cells lack chloroplasts and have a cell wall made of chitin, not cellulose.
a. Chitin, like cellulose, is a polymer of glucose molecules organized into microfibrils.
b. In chitin, unlike cellulose, each glucose has an attached nitrogen containing amino group.
4. The energy reserve of fungi is not starch, but glycogen, as in animals.
5. Except for aquatic chytrids, fungi are nonmotile; their cells lack basal bodies and do not have flagella at
any stage in their life.
6. Fungi move to a food source by growing toward it; hyphae can grow up to a kilometer a day.
7. Nonseptate fungi lack septa, or cross walls, in their hyphae; nonseptate hyphae are multinucleated.
8. Septate fungi have cross walls in their hyphae; pores allow cytoplasm and organelles to pass freely.
9. The septa that separate reproductive cells, however, are complete in all fungal groups.
C. Reproduction of Fungi
1. In general, fungal sexual reproduction involves the following:
haploid hyphae → dikaryotic stage → diploid zygote
↑--------------------- meiosis---------------↓
2. During sexual reproduction, haploid hyphae from two different mating types fuse.
3. If nuclei do not fuse immediately, the resulting hypha is dikaryotic (contains paired haploid nuclei,
n + n).
a. In some species, nuclei pair but do not fuse for days, months, or even years.
b. The nuclei continue to divide in such a way that every cell has at least one of each type of nucleus.
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4.
5.
6.
7.
When the nuclei fuse, the resulting zygote undergoes meiotic cell division leading to spore formation.
Fungal spores germinate directly into haploid hyphae without embryological development.
Fungal Spore Formation
a. Spores are an adaptation to life on land and ensure that the species will be dispersed to new
locations.
b. A spore is a reproductive cell that can grow directly into a new organism.
c. Fungi produce spores both during sexual and asexual reproduction.
d. Although nonmotile, the spores are readily dispersed by wind.
Asexual reproduction can occur by three mechanisms:
a. Production of spores by a single mycelium is the most common mechanism.
b. Fragmentation is when a portion of a mycelium becomes separated and begins a life of its own.
c. Budding is typical of yeasts; a small cell forms and gets pinched off as it grows to full size.
22.2 Diversity of Fungi
1.
6.
In 1969, R. H. Whittaker argued for their own kingdom based on their multicellular nature and mode
of nutrition.
Not knowing phylogeny, fungal groups are classified according to differences in life cycles and the
types of structure that produces spores.
A. Chytrids
1. The chytrids (chytridiomycota) include 790 species of the simplest fungi that may resemble the first
fungi to have evolved.
2. Some chytrids are single cells while others formed branched nonseptate hyphae.
3. They are the only fungi who have flagellated sperm, eggs, and spores.
4. The zoospores links fungi to choanoflagellates and other animals and helps place fungi in the
supergroup Opisthokonta.
5. Asexual reproduction is common through the production of zoospores within a single cell that grow
into new chytrids.
6. Some chytrids reproduce sexually and undergo alternation of generations, like green plants and certain
algae.
7. Chytrids play a role in the decay and digestion of dead aquatic organisms, some chytrids are parasitic
on plants, animals, and protists.
a. The parasitic chytrid, Batrachochytrium dendrobatidis has recently decimated populations of
harlequin frogs in central and South America.
b. The parasitic chytrid grows inside the frog’s skin cells and disrupt the ability of frogs to aquire
oxygen though their skin.
B. Zygospore Fungi
1. Phylum Zygomycota contains about 1,050 species of zygospore fungi.
2. Most are saprotrophs living off plant and animal remains in the soil or bakery goods in a pantry.
3. Some are parasites of small soil protists, worms, or insects.
4. The black bread mold, Rhizopus stolonifer, is a common example.
a. With little cellular differentiation among fungi, hyphae specialize for various functions.
b. Stolons are horizontal hyphae that exist on the surface of the bread.
c. Rhizoids are hyphae that grow into the bread, anchor the mycelium, and carry out digestion.
d. Sporangiophores are stalks that bear sporangia.
e. A sporangium is a capsule that produces spores called sporangiospores.
f. During asexual reproduction, all structures are haploid.
5. The phylum name refers to the zygospore seen during sexual reproduction.
a. Hyphae of different mating types (+ and –) are chemically attracted and grow toward each other.
b. Ends of hyphae swell as nuclei enter; cross walls develop behind each end, forming gametangia.
c. Gametangia merge into a large multinucleate cell in which nuclei of two mating types pair and
fuse.
d. A thick wall develops around the cell, forming a zygospore.
e. The zygospore undergoes a period of dormancy before meiosis and germination takes place.
f. Germination involves the development of one or more sporangiophores, with sporangia at their
tips.
g. The spores are dispersed by air currents and give rise to new haploid mycelia.
C. AM Fungi
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1.
2.
3.
4.
The AM fungi (arbuscular mycorrhizal) in Phylum glomeromycota consists of 160 species.
Arbuscules are branching invaginations that the fungus makes when it invades plant roots.
Mycchorizae are a mutualistic association that benefits both the fungus and the plant.
AM fungi were classified as zoogospore fungi, but are now classified as a separate group based on
molecular data.
D. Sac Fungi
1. Phylum Ascomycota contains about 60,000 species of sac fungi.
2. Two main groups are recognized: the sexual ascomycetes (sexual reproduction) and asexual
ascomycetes (sexual reproduction not yet observed).
3. Sexual ascomycetes include the yeasts, the unicellular fungi, red bread molds, morels, and truffles.
a. Many sexual ascomycetes are plant parasites and include the powdery mildews that grow on
leaves, leaf curl fungi, chestnut blight, and Dutch elm disease.
b. Ergot is a parasitic fungus on rye; it produces a toxin that can cause hysteria and death.
4. Asexual ascomycetes used to be in the phylum Deuteromycota (imperfect fungi), but are now
recognized as sac fungi.
a. The molds Aspergillus and Penicillium (renamed Talaromyces) and yeast Candida are in this
group.
5. Biology of the Sac Fungi
a. The body of the ascomycetes can be a single cell (e.g., yeasts), but more often it is a mycelium
composed of septate hyphae.
b. They are distinguished by the structures they form when they reproduce asexually and sexually.
6. Asexual Reproduction
a. Asexual reproduction is the norm among ascomycetes.
1) There are no sporangia in ascomycetes.
2) Conidiospores (conidia) develop directly on tips of conidiophores, modified aerial hyphae,
and are windblown when released.
3) Yeasts reproduce asexually by budding.
7. Sexual Reproduction
a. The ascus, contained within saclike ascocarp,is a fingerlike sac that develops during sexual
reproduction; the asci are protected by sterile hyphae within a fruiting body called the ascocarp.
1) Each ascus contains eight haploid nuclei and produces eight ascospores.
2) In most ascomycetes, the asci become swollen and burst, expelling the ascospores.
3) If released into the air, the spores are windblown.
8. The Benefits and Drawbacks of Sac Fungi
a. Sac fungi digest resistant materials containing cellulose, lignin, or collagen.
b. Some species can consume jet fuel and wall paint.
c. Others form symbiotic relationships with algae, forming lichens, and plant roots, forming
mycorrhizae.
d. Sac fungi accounts for most of the known fungal pathogens causing plant diseases: powdery
mildew, leaf curl fungus, chestnut blight, Dutch elm disease, and Ergot.
e. Many sac fungi produce substances including penicillin, cyclosporin, steroids.
f. Some cause diseases (mycoses), e.g., ringworm.
9. Yeasts
a. Saccharomyces cerevisiae is brewer’s yeast; because it produces CO2, yeast fermentation is
important in the production of bread.
b. Yeasts are also used in genetic engineering experiments requiring a eukaryote.
c. Candida albicans is the yeast which causes the widest variety of fungal infections, e.g., vaginal
infections, oral thrush, or, in immunocompromised individuals, systemic infections.
10. Molds
a. Aspergillus is a group of green molds that is sometimes pathogenic to humans.
b. It is used to produce soy sauce by fermentation of soybeans; it is used in the production of many
other food and cosmetic additives.
c. Aspergillus flavus secretes a potent carcinogen, and it also causes disease of the respiratory tract.
d. Other molds grow in building materials, causing the “sick-building” syndrome.
e. Penicillium produces the important antibiotic penicillin.
f. Moldlike fungi in the genus Tinea cause diseases of the skin called tineas; examples are athlete’s
foot and ringworm.
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Histoplasma capsulatum, which grows in both mold form and yeast form, causes the mild “fungal
flu”; the fungus grows within cells of the immune system and causes systemic disease.
11. Control of Fungal Infections
a. Because fungal cells are similar to human cells, antifungal medications are difficult to design.
b. Some fungicides are directed against steroid biosynthesis in the fungus; some are based on heavy
metals.
E. Deadly Fungi (Health Focus box)
1. Red and yello Amanita
a. Poisinous red and yellow mushroom also known as fly agaric.
b. Toxins include muscarin and muscaridine, which produce symptoms similar to those of acute
alcoholic intoxication.
c. Symptoms begin at one to six hours and include staggering, losing consciousness, becoming
delirious, suffering from hallucinations, manic conditions, and stupor.
d. Another symptom, vomiting, is beneficial since the poisoning may be ridden from the system.
e. Deal occurs in less than 1% of cases.
2. Amanita phalloiddes
a. Also known as death angel mushroom, death occurs in 90% of victims.
b. Symptoms begin 10-12 hours after consumption and include abdominal pain, vomiting, delirium,
and hallucinations.
c. The poison also interferes with RNA transcription by inhibiting RNA polymerase, and victim dies
from kidney and liver damage.
3. Psilocybe mexicana
a. This mushroom is a hallucinogenic mushroom used in religious ceremonies, particularly among
Mexican Indians.
b. This species of mushroom contains a chemical called psilocybin that is a structural analogue of
LSD with mescaline
4. Claviceps purpurea
a. This species of sac fungus is called the ergot fungus that infects rye and replaces the grain with
ergot.
b. Ergots are hard, purple-black bodies consisting of tightly cemented hyphae.
c. After the ergot is ground with the rye to make bread, the fungus releases toxic alkaloids that cause
the disease ergotism.
d. Symptoms of ergotism include vomiting, feelings of intense heat or cold, muscle pain, a yellow
face, and lesions on hands and feet, and accompanied by hysteria and hallucinations.
e. Ergotism was common in Europe during the Middle Ages. Ergotism may have been the cause of
the individuals who were accused of witchcraft in Salem Massachusetts in the 17 th century. A
recent episode occurred in 1951 in Pont-Saint-Esprit, France. Over 150 people became hysterical
and four died.
f. Since the alkaloids that cause ergotism stimulate smooth muscle and selectively block the
sympathetic nervous system, they can be used in medicine to cause uterine contractions and to
treat certain circulatory disorders.
F. Club Fungi
1. Club fungi are in the phylum basidiomycota and include over 22,000 species.
2. They have septate hyphae and include mushrooms, bracket fungi, puffballs, bird’s nest fungi, and
stinkhorns.
3. Biology of Club Fungi
a. Sexual reproduction involves production of basidiospores within fruiting bodies in the basidium.
1) Sexual reproduction begins when monokaryotic hyphae of two different mating types meet
and fuse to form a dikaryotic (n + n) mycelium.
2) The dikaryotic mycelium continues its existence for years (perhaps even hundreds of years).
3) A basidium contains four projections; cytoplasm and a haploid nucleus enter to form four
basidiospores.
4) Released basidiospores are windblown; when they germinate, a new haploid mycelium forms.
5) In a puffball, spores inside parchmentlike membranes are released through a pore or when the
parchment breaks down.
6) In bird’s nest fungi, raindrops splatter basidiospore-containing “eggs” through the air.
7) Stinkhorns have a slimy cap and attract flies by their bad odor to pick up and distribute
g.
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4.
spores.
Smuts and Rusts
a. Smuts and rusts are club fungi that parasitize cereal crops (e.g., corn, wheat, oats, and rye) and
cause great economic crop losses every year.
b. Rusts and smuts do not form basidiocarps; their spores are small and numerous, resembling soot.
c. Some smuts enter seeds and exist inside the plant, becoming visible only at maturity.
d. Corn smut mycelia grow between the corn kernels and secrete substances to cause tumors on ears.
e. Rusts have a complex life cycle that may involve more than one host; thus, control measures may
center on eradicating the alternate hosts.
22.3 Symbiotic Relationships of Fungi
A. Lichens
1. Lichens are a symbiotic association between a fungus and a cyanobacterium or a green alga.
2. The body of a lichen is composed of three layers:
a. a thin, tough upper layer and a loosely packed lower layer that shield the photosynthetic cells in
the middle layer.
3. Special fungal hyphae penetrate or envelope the photosynthetic cells and transfer nutrients directly to
the rest of the fungus.
4. Lichens can reproduce asexually by releasing fragments that contain hyphae and an algal cell.
5. This association was considered mutualistic, but experimentation suggests a controlled parasitism by
the fungus of the alga.
a. The algae grow faster when they are alone rather than when they are part of a lichen.
b. On the other hand, it is difficult to cultivate the fungus, which does not grow naturally alone.
c. Different lichen species are identified based on the fungal partner.
6. Three types of lichens are recognized.
a. Compact crustose lichens are often seen on bare rocks or tree bark.
b. Foliose lichens are leaflike.
c. Fruticose lichens are shrublike.
7. Lichens are efficient at acquiring nutrients; they survive with low moisture, temperature, or poor soil.
8. Lichens may live in extreme environments and on bare rocks; they help form soil.
9. Lichens also take up pollutants and cannot survive where the air is polluted.
B. Mycorrhizae
1. Mycorrhizae are mutualistic relationships between soil fungi and roots of most plants.
2. The fungus enters the cortex of roots but does not enter the cytoplasm of plant cells.
3. Ectomycorrhizae, such as AM fungi, form a mantle that is exterior to the root, growing between cell
walls.
4. It helps the roots absorb more minerals; in turn, the plant passes on carbohydrates to the fungus.
5. The truffle lives in association with oak and beech tree roots; it can be inoculated with the fungus.
6. The fossil record indicates that the earliest plants had mycorrhizae associated with them; mycorrhizae
helped plants adapt to and flourish on land.
Lecture Enrichment Ideas
Experience Base: Most students will be familiar with “front yard” mushrooms, and certainly
most will have eaten mushrooms. These experiences provide a limited basis for extending the
lesson back from the sexual stages to the pervasive mycelial stages. The use of yeast in beer-,
wine-, and bread-making will provide more relevant examples.
1. Use local specimens, or films or slides to examine what fungal organisms look like and how
they reproduce.
2. Discuss how the parasitic forms of kingdom Fungi have adapted to their lifestyle.
3. Clarify the similarities and distinctions among slime molds, water molds, and fungi.
4. Discuss the industrial and commercial uses of fungi.
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5. Mildews, rusts, and smuts may be familiar to students who are around Midwest farms, etc.
6. Describe the symptoms and mode of action of the human diseases caused by fungi. Many
students will have had experiences with these disease (ringworm, athlete’s foot, etc.).
7. Clarify that lichens can be made up of fungi and either green algae or cyanobacteria; detail
the experiments that probe how they could be symbiotic or how the fungus could be parasitic
on the alga or the bacterium.
Critical Thinking
Question 1. Some evolutionary botanists propose that deciduous trees do not lose their leaves as a natural
consequence of winter and water freezing, but in order for the tree to discard a season of fungus-infected leaves. Is
this a reasonable hypothesis and what evidence might support it?
Answer: Many species of fungi have the ability to grow mycelia into leaves through the stomates, and this fungal
load certainly builds up as the leaves age. The fact that many trees in tropical and subtropical forests still shed their
leaves, usually during the driest season, during a continuous growing year is evidence for this hypothesis, and
evergreen trees have the ability to survive winters without shedding their leaves.
Question 2.
Stinkhorns have a resemblance to morel mushrooms, although the stinkhorns soon become slimy.
Why are they not in the same division if they look so much alike morphologically?
Answer: Morels are sac fungi that form eight ascospores per ascus. Stinkhorns are a basidiospore producer. The
reproductive method is more important in classifying fungi and is considered less likely to have arisen
independently than the similar morphology.
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PART
V
PLANT EVOLUTION AND BIOLOGY
Five chapters are devoted to flowering plant anatomy and physiology. The first chapter provides
a foundation for the others on nutrition and transport, growth and development, and
reproduction.
23
24
25
26
27
Plant Evolution and Diversity
Flowering Plants: Structure and Organization
Flowering Plants: Nutrition and Transport
Flowering Plants: Control of Growth and Responses
Flowering Plants: Reproduction
CHAPTER
23
PLANT EVOLUTION AND DIVERSITY
This chapter presents a study of the evolution and diversity of plants. Beginning with the
evolutionary history of plants, the chapter moves into a discussion of alternation of generations,
vascular plants, seed plants, gymnosperms, and angiosperms. Two Ecology Focus boxes discuss
“Carboniferous Forests,” and “Plants: Could We Do Without Them?”.
Chapter Outline
23.1 The Green Algal Ancestor of Plants
1.
Plants are multicellular photosynthetic eukaryotes, whose evolution is marked by adaptations to a land
existence.
2. Living on land requires adaptations, primarily dealing with the threat of desiccation.
3. The most successful land plants are those that protect all phases of reproduction from drying out and
have an efficient means of dispersing offspring on land.
4. To conserve water, the land plant body is covered by a waxy cuticle that is impervious to water while
still allowing carbon dioxide to enter so photosynthesis can continue.
5. In many plants, roots absorb water from the soil, and vascular system transports water in the body of
the land plant.
6. Flowering plants have evolved in a way that employs animals to assist with reproduction and seed
dispersal.
A. The Ancestry of Plants
1. Plants are believed to have evolved from a freshwater green algal ancestor over 450 million years ago.
a. Both utilize chlorophylls a and b and various accessory pigments.
b. In both, the food reserve is starch.
c. The cell walls of both contain cellulose.
d. DNA base codes for rRNA suggest plants are most closely related to green algae known as
charophytes.
2. The two group of charophytes (Carales and Coleochate) have several features that would have
promoted the evolution of multicellular land plants:
a. Cellulose cell walls of charophytes and land plant lineage are laid down by the same unique type
of cellulose synthesizing complexes. The cell walls of both types of organisms are similar.
b. In charophytes, apical cells produce cells that allow their filaments to increase in length. At the
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nodes, other cells can divide asymmetrically to produce reproductive structures. Land plants have
apical tissue that produces specialized tissues that add to or develop into new organs.
c. Plasmodesmata between cells provide a means of communication between neighboring cells and
may have played a role in the evolution of specialized tissues in land plants.
d. A placenta (designed cells) transfers nutrients from haploid cells of the previous generation to the
diploid zygote. Both charophytes and land plants retain and care for the zygote.
B. Alternation of Generations Life Cycle
1. Land plants have a two-generation life cycle called alternation of generations.
a.
Embryophyta is an alternate name for the land plant clade since the 2n embryo are
retained and protect the embryo from drying out.
b.
The sporophyte is a diploid (2n) generation producing haploid spores by meiotic cell
division. Spores have a wall that contains sporopollenin, a molecule that prevents drying out.
c.
The gametophyte is a haploid (n) generation producing haploid gametes by mitotic
division. A male gametangium is called an antheridium, and a female gametangium is called an
archegonium.
d.
In the plant life cycle, a spore undergoes mitosis and becomes a gametophyte.
d.
Note that meiosis produces haploid spores.
e.
Mitosis occurs as a spore becomes a gametophyte, and also as a zygote becomes a
sporophyte.
f.
The charophytes have a haploid life cycle. The zygote undergoes meiosis, and only four
zoospores are produced per zygote.
2. Plants differ in which generation—gametophyte or sporophyte—is dominant.
a.
In nonvascular plants, the gametophyte is dominant.
b.
In the vascular groups, the sporophyte is dominant.
c.
The shift to sporophyte dominance is an adaptation to life on land.
d.
As the sporophyte gains dominance, the gametophyte becomes microscopic and
dependent on the sporophyte.
C. Other Derived Traits of Land Plants
1. Leaves and stems are covered by a waxy cuticle that holds in water but limits gas exchange; the
thickness of the cuticle varies among different species of plants.
2. Leaves and some other tissues have openings (stomata) that regulate gas and water exchange.
3. Apical tissue has the ability to produce complex tissues and organs.
23.2 Evolution of Bryophytes: Colonization of Land
1.
2.
3.
4.
5.
6.
7.
8.
The bryophytes (liverworts, hornworts, mosses) are the first plants to colonize land.
Bryophytes are nonvascular plants because they lack true roots, stems, and leaves, although they have
rootlike, stemlike, or leaflike structures.
The bryophytes evolved during the Ordovician period.
Molecular data suggests that these plants have individual lines of descent and they do not form a
nonophyletic group.
Mosses may be more closely related to vascular plants than hornworts and liverworts.
Bryophytes and vascular plants share traits including:
a. Alternation of generation life cycle.
b. Apical tissue that produces complex tissues.
c. Bodies are covered by a cuticle.
d. Hornwarts and mosses have stomata.
The gametophyte is the dominant generation we recognize in bryophytes.
a.
Flagellated sperm swim to the vicinity of the egg in a continuous film of water.
b.
The gametophyte produces eggs in the archegonia, flagellated sperm in the antheridia.
c.
The sperm swim to the egg in a continuous film of water.
d.
The sporophyte is attached to and derives nourishment from the photosynthetic
gametophyte.
Nonvascular plants are quite small because of lack of vascular tissue and the need for sperm to swim to
the archegonia in water.
a.
Because sexual reproduction involves flagellated sperm, they are usually found in moist
habitats.
b.
Bryophytes compete well in harsh environments because the gametophyte can reproduce
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asexually.
A. Liverworts
1. There are two groups: the thallose liverworts with flattened bodies known as a thallus, and the leafy
liverworts, which superficially resemble mosses.
2. Marchantia is an example of a liverwort.
a. It has a flat, lobed thallus about a centimeter in length.
b. The upper surface of thallus is smooth; the lower surface bears numerous rhizoids projecting into
the soil.
c. It reproduces asexually and sexually.
3. Rhizoids are the hairlike extensions that anchor it and absorb water and minerals from the soil.
4. Asexual reproduction involves gemmae in gemmae cups on the upper surface of the thallus; gemmae
can start a new plant.
5. Sexual reproduction depends on antheridia and archegonia.
a. Antheridia are on disk-headed stalks and produce flagellated sperm.
b. Archegonia are on umbrella-headed stalks and produce eggs.
c. The zygote develops into a tiny sporophyte composed of a foot, short stalk, and capsule.
d. Spores produced within the capsule of the gametophyte are disseminated by wind.
B. Hornworts
1. Hornworts are photosynthetic, but also have a symbiotic relationship with cyanobacteria, which can
fix atmospheric nitrogen.
2. The small sporophytes of a hornwort look like tiny green broom handles and are attached to a filmy
gametophyte that is less than two cm in diameter.
C. Mosses
1. Mosses are the largest phyla of nonvascular plants with over 15,000 species are known.
a. Three distinct classes exist: peat mosses, true mosses, and rock mosses.
2. They prefer damp, shaded localities but some survive in deserts; others in bogs and streams.
3. Mosses store much water; when they dry out, they become dormant; when it rains, they become green.
4. The moss life cycle begins with algalike protonema developing from the germination of a haploid
spore.
a. Three days of favorable growing conditions produce upright shoots covered with leafy structures.
1) Rhizoids (rootlike filaments) anchor the protonema, to which the shoots are attached.
2) The shoots bear antheridia and archegonia at their tips.
3) The antheridia produce flagellated sperm which need external water to reach eggs in the
archegonia.
4) The archegonium looks like a vase with a long neck; it has an outer layer of sterile cells with a
single egg at the base.
5) Fertilization results in a diploid zygote that undergoes mitotic division to develop a
sporophyte.
b. The sporophyte consists of a foot (which grows down into the gametophyte tissue starting at the
former archegonium), a stalk, and an upper capsule (sporangium) where spores are produced.
1) At first the sporophyte is green and photosynthetic.
2) At maturity it is brown and nonphotosynthetic.
5. The Uses of Bryophytes
a. Sphagnum (bog or peat moss) has tremendous ability to absorb water and is important in
gardening.
b. Sphagnum does not decay in some acidic bogs; the accumulated dried peat can be used as fuel.
23.3 Evolution of Lycophytes: Vascular Tissue


Vascular Tissue
1. Xylem is vascular tissue that conducts water and minerals upward from the roots.
2. Phloem is vascular tissue that transports sucrose and hormones throughout the plant.
3. Lignin strengthens the walls of conducting cells in xylem.
Evolutionary History
1. Rhyniophytes (phylum Rhyniophyta) were dominant from mid-Silurian Period of the Paleozoic Era to
the mid-Devonian.
2. Cooksonia may have been the first vascular plant and colonizer of land.
3. The photosynthetic stems, not true leaves or roots, have sporangia at their tips; they are attached to a
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rhizome.
4. Similar to bryophytes, these plants were homosporous, producing one type of spore.
5. Since they were not a seed plant, they were called a seedless vascular plant, like the lycophytes.
A. Lycophytes
1. The first lycophytes had the stem of early vascular plants, roots, and leaves called microphylls, so
called because they had only one strand of vascular tissue.
2. Microphylls evolved as simple side extensions of the stem; roots evolved as lower extensions of the
stem; the vascular tissue is centrally placed, as it is with today’s lycophytes.
3. Today’s lycophytes are also called club mosses and include the ground pines (Lycopodium), spike
mosses (Selaginella), and quillworts (Isoetes).
4. There are approximately 1,150 species of lycophytes.
5. Roots come off a branching, underground stem called a rhizome.
6. Sporophylls are microphylls that bear sporangia.
7. Sporophylls are grouped into club-shaped stoboli.
8. Similar to all vascular plants, the sporophyte generation is the dominant generation in lycophytes.
9. Ground pines are homosporous—the spores germinate into inconspicuous and independent
gametopytes, like in ferns.
10. The sperm are flagellated in bryophytes, lycophytes, and ferns.
23.4 Evolution of Pteridophytes: Megaphylls
1.
2.
Pteridophytes (ferns and their allies, horsetails, whiskferns) are seedless vascular plants.
Pteridophytes and seed plants both have megaphylls, which are broad leaves with several strands of
vascular tissue.
3. Megaphylls evolved about 370 MYA.
4. Their function is to allow plants to efficiently collect solar energy, which leads to the production of
more food and the possibility of producing more offspring than plants without megaphylls.
5. The pteridophytes were dominant from the late Devonian period through the Carboniferous period.
6. Club mosses (35 m), horsetails (18 m), and ferns (8 m) were larger than today’s specimens and formed
great swamps.
A. Horsetails
1. Horsetails consist of one genus, Equisetum, and 25 species.
2. A rhizome produces aerial stems that stand about 1.3 meters tall.
3. Whorls of slender side branches encircle nodes of a stem, resembling a horse’s tail.
4. Small scalelike leaves also form whorls at each node.
5. Many horsetails have strobili at the tip of all stems; others send up special buff-colored stems that bear
stroboli.
6. The spores germinate into inconspicuous and independent gametophytes.
7. The tough, rigid stems have silica in the cell walls; early Americans used them as “scouring brushes.”
B.
Whisk Ferns
1. Whisk ferns are represented by two genera: Psilotum and Tmesipteris.
2. Whisk ferns occur in the southern United States and in the tropics as epiphytes or on the ground.
3. Whisk ferns have no leaves or roots.
4. A branched rhizome with rhizoids and a mycorrhizal fungus helps gather nutrients.
5. Two or three species of the genus Tmesipteris have appendages that some maintain and reduced
megaphylls
C. Ferns
1. Ferns consist of about 11,000 species.
2. Ferns are widespread and especially abundant in warm, moist tropical regions.
3. Ferns range in size from low—growing mosslike forms to tall trees.
4. Fronds are megaphylls that are variable in size and shape.
5. The life cycle of ferns applies to the life cycle of other types of vascular seedless plants.
a. The dominant sporophyte produces windblown spores by meiosis within sporangia.
b. In ferns, the sporangia can be located within sori on the underside of the leaflets.
c. Windblown spores disperse the gametophyte.
d. Flagellated sperm are released by antheridia and swim to the archegonia in a film of water.
e. Many ferns can reproduce asexually by fragmentation of the fern rhizome.
6. The Uses of Ferns
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a.
b.
c.
d.
e.
Ostrich fern is the only edible fern to be traded as a food —eaten as fiddleheads.
The fern Azolla harbors the nitrogen fixing cyanobacteria Anabaena.
Azolla converts more atmospheric nitrogen into a form available for plant growth than all the
legumes.
Ferns and their allies can also be used as medicines to treat boils and ulcers, whooping cough, and
dysentery.
Ferns are used heavily as ornamental plants by florists and as home decorations.
23.5 Evolution of Seed Plants: Full Adaptation to Land
1.
2.
Seed plants are vascular plants that use seeds as the dispersal stage.
Seeds are mature ovules containing embryonic sporophyte and stored food enclosed in a protective
seed coat.
3.
Seeds are resistant to adverse conditions such as dryness and temperature extremes.
4.
A food reserve supports the emerging seedling until it can exist on its own.
5. The survival value of seeds contributes greatly to the success of seed plants and to their present
dominance.
6. Seed plants are heterosporous, meaning they have microspores and megaspores.
7. Microspores become male gametophytes, called pollen grains.
8. Pollination is the transfer of pollen to the vicinity of the female gametophyte.
9. Sperm is delivered to an egg through a pollen tube; no external water is required for fertilization.
10. A megaspore develops into a female gametophyte within an ovule, which becomes a seed following
fertilization.
11. In gymnosperms, the ovules are not completely enclosed by sporophyte tissue at pollination.
12. In angiosperms, the ovules are completely enclosed within diploid sporophyte tissues which become a
fruit.
A. Gymnosperms
1.
2.
3.
Gymnosperms include the conifers, cycads, ginkgo, and gnetophytes; they are classified into 780
species.
a. All have ovules exposed on the surface of sporophylls or similar structures.
b. Ancient gymnosperms were present in swamp forests of the Carboniferous Period.
Conifers
a. There are about 575 species of conifers.
b. Conifers are cone-bearing trees and shrubs such as pines, hemlocks, and spruces.
c. Conifers usually have evergreen needlelike leaves well adapted to withstand extremes in climate.
d. The oldest and largest trees in existence are conifers:
1) The coastal redwood (Sequoia semperivirens) is the tallest living vascular plant and grows to
nearly 100 meters high.
2) Bristlecone pines grow in the White Mountains of California and Nevada mountains; one
is 4,900 years old.
e. Conifer forests cover vast areas of northern temperate regions.
f. Pine needles have thick cuticle and recessed stomata.
g.
Conifers are monoecious since they produce both pollen and seed cones.
h.
The Uses of Pines
1)
Pines provide wood for construction.
2)
Pines make attractive additions to parks and gardens.
3) Pine needles are rich in vitamins A and C and can be boiled to make tea to ease cold
symptoms.
4) Inner bark of white pines can be used in wound dressing or provide medicine for colds and
coughs.
5)
Pine oil is used to scent room sprays and masculine perfumes.
6)
Resin is harvested commercially for a derived product called turpentine.
Cycads
a. Cycads include 10 genera and about 140 species.
b. The trunk is stout and unbranched; the large leaves are compound giving a palmlike appearance.
c. Cycads have pollen and seed cones on separate plants.
d. The cycad life cycle is similar to that of pine trees except they are pollinated by insects.
e. The pollen tube bursts in the vicinity of the archegonium and multiflagellated sperm swim to reach
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B.
an egg.
f. Cycads flourished during the Mesozoic Era and probably were food for herbivorous dinosaurs.
g. Today, cycads are endangered because of their very slow growth.
4. Ginkgoes
a. Only one species of ginkgo (Ginkgo biloba) survives today.
b. It is called the maidenhair trees because its forked-veined, fan-shaped leaves resemble the
maidenhair fern.
c. Ginkgoes are dioecious—some trees produce seeds.
d. Ginkgo ovules are at the end of short, paired stalks; female trees produce seeds with a fleshy
covering and foul odor.
e. Similar to cycads, the pollen tube of Gingkoes bursts to release multiflagellated sperm that swim
to the egg produced by the female gametophyte in an ovule.
5. Gnetophytes
a. Gnetophytes are represented by three living genera and 70 species.
b. Gnetum consists of trees and climbing vines with broad leaves; they live mainly in the tropics.
c. Ephedra is found in U.S. desert regions, and is a many-branched shrub with small, scalelike
leaves.
d. Welwitschia is found in deserts in southwest Africa; most of it exists underground and it has two
enormous leaves.
e. The xylem and stroboli are uniform across all three genera, and all lack archegonia.
f. Angiosperms also lack archegonia, suggesting that gnetophytes are the gymnosperms most closely
related to angiosperms.
g. Some gnetophytes produce nectar in their reproductive structures, recruiting insects in pollination.
Carboniferous Forests (Ecology Focus box)
1. Coal is a fossil fuel that our industrial society runs on.
2. Fossil fuel refers to the remains of organic materials from ancient times.
3. During the Carboniferous period, more than 300 million years ago, a great swamp forest encompassed
northern Europe, Ukraine, and the Appalacian Mountains.
4. The forest consisted of giant lycophytes, horsetails, and ferns.
5. Over time, water rose and trees fell. Sediment covered the decayed remains of the plants and
sometimes changed to sedimentary rock.
6. Since sedimentary rock applied pressure, the organic material then became coal.
7. The change in climate brought upon the extinction of these trees, and only their herbaceous relatives
survived time.
C. Angiosperms
1. There are 240,000 known species of angiosperms (flowering plants).
2. This group contains six times the number of species of all other plant groups combined.
3. Angiosperms live in all habitats from freshwater to desert and from tropics to subpolar regions.
4. Flowering plant size ranges from microscopic duckweed to Eucalyptus exceeding 100 m tall.
5. They are important in everyday human life: clothing, food, medicine, and commercial products.
6. Unlike gymnosperms, angiosperms enclose their ovules within diploid tissues.
D. Origin and Radiation of Angiosperms
1. Flowering plants became the dominant plants in the late Cretaceous and early Tertiary Periods.
2. Although the first fossils are no older than 135 million years, angiosperms probably arose much
earlier, perhaps 200 million years ago.
3. Gene sequencing data indicates Amborella trichopoda, a small shrub from New Caledonia in the South
Pacific may be the most primitive survivor.
E. Monocots and Eudicots
1. Most flowering plants belong to one of two classes: Monocotyledones (65,000 species), called
monocots, or the Eudicotyledones (175,000 species), called eudicots.
2. The term eudicots is preferred to the earlier dicots; some former dicots are now known to have split off
before the rise of these two major classes.
3. Monocots produce one cotyledon (seed leaf) at germination and have flower parts mostly in threes or
multiples of threes.
4. Dicots produce two cotyledons (seed leaves) at germination and have flower parts mostly in fours or
fives, or multiples of these numbers.
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F.
The Flower
1. The peduncle, a flower stalk, expands slightly at the tip into a receptacle.
2. The receptacle is a modified stem tip to which flower parts are attached.
3. Sepals are outer ring of modified leaves of flowers; usually green, they enclose the flower before it
opens.
4. Petals (collectively a corolla) are a ring of modified leaves inside of sepals; large and colorful, they
help attract pollinators.
5. Stamens consist of two parts: each slender filament has an anther at its tip.
6. The anther produces pollen.
7. At the center of the flower is the carpel; it consists of a stigma, style, and ovary.
a. Carpels are modified sporophylls that contain ovules in which megasporangia are located.
b. A stigma is a landing platform for pollen and the site where the pollen tube enters the style.
c. The style is a slender column that holds up the stigma to receive pollen.
d. Pollen grains develop a pollen tube that takes sperm to the female gametophyte in the ovule.
e. Glands located in the region of the ovary produce nectar, a nutrient gathered by pollinators as they
go flower to flower.
G. Flowering Plant Life Cycle
1. A megaspore located in an ovule within an ovary of a carpal develops into an egg-bearing female
gametophyte called the embryo sac.
2. Usually, the embryo sac has seven cells; one is an egg and one contains two polar nuclei.
3. Microspores produced in anthers become pollen grains which mature into sperm-bearing male
gametophytes.
4. The mature male gametophyte consists of three cells; the tube cell and two sperm cells.
5. Pollination brings the male gametophyte to the stigma where it germinates.
6. During germination, the tube cell produces a pollen tube that carries the two sperm to the micropyle
opening of an ovule.
7. In double fertilization, one sperm fertilizes an egg and one sperm unites with polar nuclei to form the
triploid endosperm.
8. The ovule becomes the seed and contains the embryo (the sporophyte of the next generation) and
stored food enclosed within a seed coat.
9. A fruit is derived from an ovary and possibly accessory parts of the flower; some fruits are fleshy and
some are dry.
H. Flowers and Diversification
1. Flower variety is related to the numerous means by which flowers are pollinated and fruits are
dispersed.
2. Inconspicuous flowers disperse pollen by wind; colorful flowers attract specific pollinators (e.g., bees,
wasps, flies, butterflies, moths, and even bats) which carry only a particular pollen.
3. Flowers promote efficient cross pollination; they also aid in dispersal through production of fruits.
4. There are fruits that utilize wind, gravity, water, and animals for dispersal.
5. Since animals live in certain habitats or have particular migration patterns, they can deliver a
fruit-enclosed seed to a suitable location for germination and development.
I. Plants: Could We Do Without Them? (Ecology Focus box)
1. Plants are the producers in most ecosystems.
2. Humans derive most of their sustenance from three flowering plants: wheat, corn, and rice.
3. Wheat
a. Wheat was first cultivated in the Near East at about 8000 B.C.
b. Wheat was brought to North America in 1520 by early settlers.
c. The United States is one of the world’s largest producers of wheat.
4. Corn (Maize)
a. Maize was first cultivated in Central America about 7,000 years ago.
b. Maize developed from a plant called teosinte, native to central Mexico.
c. We grow six major varieties of corn: sweet, pop, flour, dent, pod, and flint.
5. Rice
a. Rice originated in southeastern Asia several thousand years ago, and now is grown throughout the
tropics and subtropics, where water is abundant.
b. We are familiar with white and brown rice.
c. Brown rice results when the seeds are threshed to remove the hulls.
163
d. White rice results when the seed coat and embryo are also removed.
e. The seed coat and embryo are rich in vitamin B and fat-soluble vitamins.
6. Sugarcane and sugar beets provide almost 100% of the world’s sugar.
7. Spices were the result of many explorations in search for better and cheaper routes for spice
importation.
8. Coffee, tea, and cola also come from flowering plants.
a. Coffee originated in Ethiopia and it was first used with animal fat on long journeys.
b. Coffee as a drink was not developed until the 13 th century in Arabia and Turkey.
c. Tea is thought to have been developed in central Asia.
d. Tea was exclusively used for medicinal purposes.
e. Cola is a common ingredient in tropical drinks and was used in the original Coca-Cola.
9. Household products
a. Lumber is used as a major structural portion in buildings.
b. In certain parts of the world palm leaves are used as roof materials.
c. In the Near East, houses are made of reeds.
10. Rubber
a. Rubber had its origin in Brazil from the latex of the rubber tree.
b. Stronger rubber, such as the rubber in tires, was made by adding sulfur and heating in a process
called vulcanization.
c. Today much of the rubber is produced synthetically.
11. Cotton and natural fibers
a. Cotton fibers come from filaments that grow on the cotton seed.
b. Over 30 species of native cotton now grow around the world.
12. Medicinal plants
a. Currently about 50% of all pharmaceutical drugs have their origins from plants.
b. The cinchona tree contains the chemical quinine, which is used to treat malaria.
13. Hallucinogenic effects.
a. Coca is used for cocaine and crack.
b. Opium poppy is used for morphine.
c. Yarn is used for steroids.
14. Other uses.
a. Plants are used in landscaping, providing shade or wind break.
b. Plants also produce oxygen, which is necessary for all plants and animals.
Lecture Enrichment Ideas
Experience Base: There has been a steady decrease in the number of flowers an American
student can name, reflecting far less direct exposure to plants in gardening, farming, etc.
Therefore it is likely that an instructor will overestimate the students’ experience with plants;
asking for feedback can permit a rapid appraisal of the field-knowledge of students and engaging
in class discussions can allow some students to relate experiences that other students will more
readily comprehend. Few assumptions can now be made concerning students’ comprehension of
wheat, corn, etc., before it becomes a milled product. The heavy load of technical terms and
plant anatomy will also make use of media particularly critical. Modern terminology evolves and
it is important to be consistent in the use of the correct word such as “male gametophyte,” etc.—
note the changes that have occurred since the last edition. There will also be an uphill battle
against the common vernacular, since society calls tomatoes and peppers “vegetables” rather
than “fruits.”
1. Diagram the evolution of the different kinds of land plants, pointing out how each step is an
adaptation to the land and how the plants might have moved in stages from green algae to
plants living on dry land.
2. Many regions n the U.S. have some samples of horsetails or ground pine in local parks or
164
rural areas. Use specimens or slides in class to acquaint students with organisms they may
have seen but not recognized as a surviving example of a primitive plant group.
3. Discuss why the seed plants have a reproductive advantage over plants that reproduce by
releasing spores, with emphasis on the multicellular embryo, stored food, and seed coat as
opposed to the single cell of the spore. On the other hand, if this is always an advantage, why
do the plants that use spores still survive?
4. Describe the effects of the development of pollen on the evolutionary success of the seed
plants, with emphasis on the advantage gained in having animals disperse pollen rather than
the wind. Discuss the allergies many people have to pollen and how much energy must go
into producing this pollen.
5. Have students stop to examine the products they are using in the classroom that are made
from angiosperms: wooden desks, paper, etc. They will soon go to lunch or dinner and ingest
more angiosperms.
6. Much has been made recently of singing to plants, etc. Point out the lack of nerves and
muscles, and the inability of plants to respond to the environment except by growth, changes
in turgor pressure, etc.
Critical Thinking
Question 1.
What evolutionary trend do we see in plants concerning the sporophyte and gametophyte stages
and what has probably selected for this trend?
Answer: There has been a reduction in the size of the gametophyte and an increase in the size of the sporophyte.
The gametophytes are dependent upon the diploid sporophyte and the sporophyte can evolve into many diverse
forms without any corresponding changes in the gametophyte.
Question 2.
While woody and herbaceous, and annual and perennial are used as descriptive terms, they are not
used to classify organisms in the manner that dicots and eudicots and flowering and naked seed characteristics are
used. Why?
Answer: The ability to generate a woody stem or shift from annual to perennial appears in many groups, but the
features of monocot and eudicot leaves or flowers hold together the plant groups with their relatives and their
associated features. In more technical terms, woody and herbaceous, and annual and perennial, appear many times
as independently derived features.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
165
CHAPTER
24
FLOWRING PLANTS: STRUCTURE AND
ORGANIZATION
This chapter studies the structure of plants. After a discussion of plant organs, monocot and
eudicot plants are compared and contrasted. This is followed by a description of plant tissues,
and the organization of roots, stems, and leaves. An Ecology Focus box “Paper Comes from
Plants” is presented, as is a Science Focus box “Defense Strategies of Trees.” An abundance of
terminology permeates the chapter.
Chapter Outline
24.1 Organs of Flowering Plants
1.
2.
3.
A. Roots
1.
2.
3.
4.
B. Stems
1.
2.
3.
4.
5.
6.
7.
8.
9.
C. Leaves
1.
2.
3.
4.
5.
6.
Structures of flowering plants are well adapted to varied environments, including water.
Flowering plants usually have a root system (the roots) and a shoot system (the stems and leaves).
Roots, stems, and leaves are the vegetative organs of plants; flowers, seeds, and fruits are
reproductive structures.
A plant’s root system is underground.
The root system is the primary root plus the branch roots.
It is generally equal in size to the shoot system, the part above ground.
Root systems have the following functions:
a. Roots anchor a plant in soil and give support.
b. Roots absorb water and minerals from soil; root hairs are central to this process.
1) Root hair cells are in a zone near the root tip.
2) Root hairs are numerous to increase the absorptive surface of a root.
3) Transplanting plants damages a plant when the root hairs are torn off.
4) Water and nutrients absorbed are distributed to the rest of the plant.
5) Roots produce hormones that must be distributed to the plant
c. Perennials “die back” to regrow the next season; roots of herbaceous perennials store food (e.g.,
carrots, sweet potatoes).
The shoot system of a plant consists of the stem, the branches, and the leaves.
The stem forms the main axis of the plant, along with lateral branches.
Upright stems produce leaves and array them to be exposed to as much sun as possible.
A node occurs where a leaf attaches to the stem and an internode is the region between nodes; nodes
and internodes identify a stem even if it is underground.
Axillary buds are located at a node in the upper angle between the leaf and the stem and can produce
new branches of the stem or flower.
The stem has vascular tissue to transport water and minerals from roots and sugar from leaves.
Nonliving cells form a continuous pipeline through vascular tissue.
A cylindrical stem expands in girth and length; trees use woody tissue to strengthen stems.
Stems may function in storage: cactus stems store water and tubers are horizontal stems that store
nutrients.
A leaf is the major organ of photosynthesis in most plants.
Leaves receive water from roots by way of the stem.
Broad, thin leaves have a maximum surface area to absorb CO 2 and collect solar energy.
A blade is the wide portion of a leaf with most photosynthetic tissue.
The petiole is a stalk that attaches a leaf blade to the stem.
The leaf axil is the upper acute angle between petiole and stem where an axillary (lateral) bud
originates.
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7.
Some leaves protect buds, attach to objects (tendrils), store food (bulbs), or capture insects.
D. Monocot Versus Eudicot Plants

Criteria for Monocots and Eudicots
1. Cotyledons are embryonic seed leaves providing nutrition from the endosperm before the mature
leaves begin photosynthesis.
2. Flowering plants are divided into monocots and eudicots based on these traits.
Monocots
Eudicots
a.
b.
3.
4.
Number of cotyledons in seed
one
two
Distribution of root xylem and
root xylem and phloem in root phloem between phloem
a ring of xylem
c. Distribution of vascular bundles
scattered in stem arranged in a distinct ring
d. Pattern of leaf veins
form a parallel pattern
form a net pattern
e. Number of flower parts in threes and multiples of in fours and fives and
three
multiples of four or five
f. Number of apertures in pollen
usually one
usually three
grains
Representative members:
grasses, lilies, orchids,
dandelions to oak
rice, wheat, corn trees and palm trees
The distinction between monocots and eudicots represents an important evolutionary division that
relates to many structures.
24.2 Tissues of Flowering Plants
1.
Flowering plants continually grow due to meristematic (embryonic) tissue in the stem and root tips
(apexes).
2. Apical meristems are located near the tips of stems and roots, where they increase the size of these
structures; this is called primary growth.
3. Monocots also have intercalary meristem, which allows them to regrow lost parts.
4. Apical meristem produces three types of meristem, which develop into the three types of specialized
primary tissues in the body of the plant.
a. Protoderm is the outermost primary meristem giving rise to epidermis.
b. Ground meristem is the inner meristem producing ground tissue.
c. Procambium produces vascular tissue.
5. Three specialized tissues include:
a. Epidermal tissue forms the outer protective covering.
b. Ground tissue fills the interior of the plant.
c. Vascular tissue transports water and nutrients and provides support.
A. Epidermal Tissue
1. Epidermis is an outer protective covering tissue of plant roots, leaves, and stems of nonwoody plants.
2. It contains closely packed epidermal cells.
3. Waxy cuticle covers the walls of epidermal cells, minimizing water loss and protecting against
bacteria.
4. In roots, certain epidermal cells are modified into root hairs that increase the surface area of the root
for absorption of water and minerals and help to anchor plants in the soil.
5. Protective hairs called trichomes are produced by epidermal cells of stems, leaves, and reproductive
organs.
6. Trichomes may help protect a plant from herbivores by producing a toxic substance.
7. On the lower epidermis of eudicot leaves, and both surfaces of monocot leaves, special guard cells
form microscopic pores (stomata) and regulate gas exchange and water loss.
8. In older woody plants, the epidermis of the stem is replaced by periderm, the majority component of
which is cork cells.
a. At maturity, dead cork cells may be sloughed off.
b. Cork cambium is meristem that produces new cork cells.
c. As cork cells mature, they encrust with the lipid suberin that renders them waterproof and inert.
d. Cork protects a plant and makes it resistant to attack by fungi, bacteria, and animals.
e. When the cork cambium overproduces cork in certain areas of the stem surface, ridges and cracks,
called lenticels, appear; lenticels are important in gas exchange between the interior of the stem
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and the air.
B. Ground Tissue
1. Ground tissue forms the bulk of the plant; it contains parenchyma, collenchyma and sclerenchyma
cells.
2. Parenchyma is the least specialized of all plant cell types.
a. Cells of this type contain plastids (e.g., chloroplasts or colorless storage plastids).
b. They are found in all organs of a plant.
c. They divide to form more specialized cells (e.g., roots develop from stem cuttings in water).
3. Collenchyma resemble parenchyma but has thicker primary cell walls.
a. Collenchyma cells are uneven in the corners.
b. They usually occur as bundles of cells just beneath the epidermis.
c. They give flexible support to immature regions of plants (e.g., a celery stalk is mostly
collenchyma).
4. Sclerenchyma cells have thick secondary cell walls.
a. They are impregnated with lignin that makes the walls tough and hard.
b. They provide strong support to mature regions of plants.
c. Most cells of this type are nonliving.
d. Sclerenchyma cells form fibers (used in linen and rope) and shorter sclereids (found in seed coats,
nut shells, and gritty pears).
C. Vascular Tissue
1. Xylem conducts water and mineral solutes upward through a plant from roots to leaves.
a. Xylem contains tracheids and vessel elements.
b. Tracheids
1) Tracheids are smaller, hollow, thin, long nonliving cells with tapered overlapping ends.
2) Water moves across end and sidewalls because of pits or depressions in the secondary cell
wall.
c. Vessel Elements
1) Vessel elements are hollow non-living cells lacking tapered ends.
2) They are larger than tracheids.
3) They lack transverse end walls.
4) They form a continuous pipeline for water and mineral transport.
d. Xylem also contains sclerenchyma cells to add support.
e. Vascular rays are flat ribbons of parenchyma cells between rows of tracheids; they conduct water
and minerals across the width of the plant.
2. Phloem is vascular tissue that conducts the organic solutes in plants, from the leaves to the roots; it
contains sieve-tube members and companion cells.
a. Sieve-tube Members
1) Sieve-tube cells contain cytoplasm but no nucleus.
2) They are arranged end to end.
3) They have channels in their end walls (thus, the name “sieve-tube”), through which
plasmodesmata extend from one cell to another.
b. Companion Cells
1) Companion cells are closely connected to sieve-tube cells by numerous plasmodesmata.
2) They are smaller and more generalized than sieve-tube cells.
3) They have a nucleus which may control and maintain the function of both cells.
4) They are also thought to be involved in the transport function of phloem.
3. Vascular tissue extends from root to leaves as vascular cylinder (roots), vascular bundles (stem) and
leaf veins.
24.3 Organization and Diversity of Roots
1. The eudicot root has various zones where cells are in various stages of differentiation
and where primary growth occurs.
2. The root apical meristem is the region protected by the root cap, a protective cover;
its cells are replaced constantly because they are soon ground off.
3. The primary meristems are in the zone of cell division, which continuously provides
cells to the zone of elongation by mitosis.
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4.
The zone of elongation is above the zone of cell division where cells become longer and more
specialized.
5. The zone of cell division contains meristematic tissue and adds cells to the root tip
and the zone of elongation.
6. The zone of maturation is above the zone of elongation; cells are mature and
differentiated and it has root hairs.
A. Tissues of a Eudicot Root
1. Epidermis is a single layer of thin-walled, rectangular cells.
a. The epidermis forms the protective outer layer of the root.
b. In the region of maturation, there are many root hairs.
c. Root hairs project as far as 5–8 mm into the soil.
2. Cortex is a layer of large, thin-walled, irregularly shaped parenchyma cells.
a. These cells contain starch granules; the cortex functions in food storage.
b. The cells are loosely packed; water and minerals can diffuse through the cortex without entering
cells.
3. Endodermis is single layer of rectangular cells that forms the boundary between the cortex and inner
vascular cylinder.
a. Its cells fit closely together and are bordered on four sides by the Casparian strip.
b. It regulates the entrance of minerals into the vascular cylinder.
c. The Casparian strip is an impermeable lignin and suberin layer that excludes water and mineral
ions.
d. The only access to the vascular bundle is through endodermal cells.
4. Vascular tissue
a. The pericycle is the first layer of cells within the vascular cylinder.
1) Its cells have retained the capacity to divide.
2) It can start the development of branch or secondary roots.
b. The main portion of the vascular cylinder is composed of
1) xylem, whose cells are arranged in a star-shaped pattern; and
2) phloem, whose cells are located in regions between arms of xylem.
B. Organization of Monocot Roots
1. Monocot roots have the same zones as a eudicot root but do not undergo secondary growth.
2. The monocot root has a ring of vascular tissue where alternating bundles of xylem and phloem
surround pith.
3. Monocot roots also have pericycle, endodermis, cortex, and epidermis.
C. Root Diversity
1. Roots have adaptations to help anchor plants, absorb water and minerals, and store carbohydrates.
2. There are three general root types.
a. A taproot is common in eudicots; this first or primary root grows straight down and remains the
dominant root of a plant; it is often fleshy and adapted to store food (e.g., carrots, beets).
b. The fibrous root system of monocots is a mass of slender roots and lateral branches that hold the
plant secure in the soil.
3. Adventitious roots develop from underground stems or from the base of above
ground stems.
4. A prop root’s main function is to anchor a plant (e.g., corn and mangrove plants).
5.
Pneumatophores of mangrove plants project above the water from roots to acquire oxygen.
6. Ivy has holdfast roots to anchor aerial shoots.
7.
Haustoria are rootlike projections from stems on parasitic plants (e.g., dodders and broomrapes).
a. Haustoria grow into the host plant.
b. They contact vascular tissue from which they extract water and nutrients.
8. Mycorrhizae are an association between fungus and roots.
a. In this mutualism, the fungus receives sugars and amino acids from the plant.
b. The plant receives water and minerals from the fungus.
9. Legumes (e.g., peas and beans) have root nodules containing nitrogen-fixing
bacteria.
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a.
Bacteria extract nitrogen from air and reduce it to a form that can be used by plant
tissues.
b. Legumes are often planted to bolster the nitrogen supply in the soil.
D. Paper Comes from Plants (Ecology Focus box)
1. Egyptians made the first form of paper
2. The paper making process involves grinding plant material to form a pulp that
contains “fibers,” or vascular tissue.
3. The fibers form a sheet when they are screened from the pulp.
4. Pulp must be chemically treated to remove lignin.
5. If a small amount of lignin is removed, then the paper is brown, like paper bags.
6. If more lignin is removed, the paper is white, but not very durable.
7. More durable paper is made from cotton or linen because the fibers are lignin-free.
8. Eucalyptus trees are being used to make paper, especially in Brazil.
9. Temperate hardwood trees are cultivated in Canada and tropical hardwoods are
cultivated in Southeast Asia.
10. Softwood trees in the U.S. have been genetically improved to have a higher wood
density and to be harvestable five years earlier than ordinary pines.
11. Bamboo harvesting does not destroy the roots, and the growing cycle is favorable so
it is expected to be a significant source of paper pulp despite high processing costs to
remove impurities.
12. Flax and cotton rags are used to make legal documents, high-grade bond paper, and
high-grade stationary.
13. Each person in the U.S. uses about 318 kg of paper products per year, compared to
2.3 kg per person in India.
14. If Sunday newspapers were recycled, it would save approximately 500,000 trees per
week.
24.4 Organization and Diversity of Stems
1. The terminal bud contains the shoot tip protected by bud scales, which are modified
leaves.
2. Dormant auxillary buds that can give rise to branches or flowers are here also.
3. Bud scales are scalelike coverings protecting terminal buds during winters when bud
growth stops.
4.
5.
6.
7.
The stem tip is the site of primary growth where cell division extends the length of stems or roots.
The apical meristem produces new cells that elongate and increase the height of the stem.
The shoot apical meristem is protected within a terminal bud of leaf primordia (immature leaves).
Three specialized types of primary meristem develop from shoot apical meristem.
a. Protoderm is the outermost primary meristem that gives rise to epidermis.
b. Ground meristem produces two tissues composed of parenchyma cells: the pith and the cortex.
c. Procambium is the inner meristem that produces primary xylem and primary phloem.
8. Differentiation continues; cells become the first tracheids or vessel elements within the vascular
bundle.
9. First sieve-tube cells are short-lived and do not have companion cells.
10. Mature phloem develops later after all surrounding cells have stopped expanding and a lateral
meristem, called vascular cambium, has developed.
A. Herbaceous Stems
1. Herbaceous stems are mature nonwoody stems that exhibit only primary growth.
2. The outermost tissue of herbaceous stems is epidermis covered by a waxy cuticle to prevent water loss.
3. Xylem and phloem are in distinctive vascular bundles.
a. In each bundle, xylem is found to the inside of the stem; phloem is found to the outside.
b. In the eudicot herbaceous stem, vascular bundles are arranged in a ring towards the outside of the
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stem and separating the cortex from the central pith.
In a monocot stem, vascular bundles are scattered throughout the stem; there is no well-defined
cortex or pith.
4. The cortex sometimes carries on photosynthesis; pith may function as a storage site.
B. Woody Stems
1. Woody plants have both primary and secondary tissues.
2. Primary tissues are new and form each year from primary meristem right behind the apical meristem.
3. Secondary tissues develop from second year onward from growth of lateral meristem.
4. Primary growth increases the length of a plant; secondary growth increases its girth.
5. As secondary growth continues, it is not possible to distinguish individual vascular bundles.
6. The woody eudicot stem has a different organization with three distinct areas: bark, wood, and pith.
7. Bark
a. The bark of a tree contains cork, cork cambium, and phloem.
b. Secondary phloem is produced each year by vascular cambium but does not build up.
c. This phloem tissue is soft; therefore it is easy to remove the bark of a tree.
d. Cork cambium is meristem beneath the epidermis that produces new cork cells when needed.
e. Cork cambium begins to divide, producing cork that disrupts epidermis replacing it with cork
cells.
f. Cork cells become impregnated with suberin, causing them to die but making them waterproof.
g. Consequently, cork forms an impervious barrier, even to gas exchange, except at lenticels.
8. Wood
a. Wood is a secondary xylem which builds up each year; the vascular cambium is dormant during
the winter.
b. Spring wood is composed of wide xylem vessel elements with thin walls, necessary to conduct
sufficient water and nutrients to supply abundant growth that occurs during spring.
c. Summer wood forms when moisture is scarce; composed of a lower proportion of vessels, it
contains thick-walled tracheids and numerous fibers.
d. An annual ring is one ring of spring wood followed by a ring of summer wood; this equals one
year’s growth.
e. Sapwood is the outer annual rings where transport occurs.
f. Heartwood is the inner annual rings of older trees.
1) Vessels no longer function in transport; they become plugged with resins and gums that
inhibit growth of bacteria and fungi.
2) Heartwood may help to support a tree.
9. Woody Plants
a. It is advantageous to be woody when there is adequate rainfall; woody plants can grow taller and
have adequate tissue to support and service leaves.
b. It takes energy to support secondary growth and prepare the plant for winter in temperate zones.
c. Long-lasting plants need more defense mechanisms against attack by herbivores and parasites.
d. Trees need years to mature before reproducing; they are more vulnerable to accident or disease.
C. Stem Diversity
1. Stolons are stems that grow along the ground; new plants grow where the nodes contact the soil.
2. The succulent stems of cacti are modified for water storage.
3. Tendrils of grapes and morning glories are stems adapted for wrapping around support structures.
4. Rhizomes are underground horizontal stems.
a. Rhizomes are long and thin in grasses and thick and fleshy in irises.
b. Rhizomes survive winter and contribute to asexual reproduction because each node bears a bud.
c. Some rhizomes have tubers that function in food storage (e.g., potatoes).
5. Corms are bulbous underground stems that lie dormant during winter, like rhizomes.
6. Humans use stems: sugarcane is primary source of table sugar, cinnamon and quinine are from bark,
paper is from wood, etc.
D. Defence Strategies of Trees (Science Focus box)
1. Defence strategy one
a. When a tree is injured, the tracheids and vessel elements of the xylem plug up with chemicals that
block them off above and below the site of injury.
b. If the tree cannot close off the vessel elements, then long columns of rot run up and down the
trunk and into branches, which eventually become hollow.
c.
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2.
3.
4.
Defence strategy two
a. The construction of annual rings makes it difficult for disease organisms to move towards the pith
of the tree.
Defence strategy three
a. The presence of rays, which divide the trunk of the tree, also make it difficult for disease
organisms to move completely around the trunk.
Defence strategy four
a. The reaction zone develops in the region of tree injury along the inner portion of the cambium
next to the youngest annual ring.
b. The reaction zone walls off any annual rings that were present before the injury occurred.
c. A disadvantage to this strategy is that radial cracks can occur to and through the bark, which
weaken a tree and make it more susceptible to breaking.
24.5 Organization and Diversity of Leaves
1.
2.
3.
4.
5.
6.
Leaves are organs of photosynthesis in plants; they are made of a flattened blade and a petiole.
The leaf veins reveal the presence of vascular tissue within the leaves.
The vascular tissues of leaves transport water and nutrients.
Leaf veins have a net pattern in eudicot leaves and a parallel pattern in monocot leaves.
A petiole is a stalk that attaches a leaf blade to the plant stem.
Epidermis is the layer of cells that covers the top and bottom sides of a leaf.
a. The epidermis often bears protective hairs or glands; epidermal glands produce irritating
substances.
b. The epidermis is covered by a waxy cuticle that keeps the leaf from drying out.
c. The epidermis, particularly lower epidermis, contains stomata that allow gases to move into and
out of the leaf.
7. Mesophyll is the body of a leaf and the site of most photosynthesis.
a. Palisade mesophyll is the layer of mesophyll containing elongated parenchyma cells with many
chloroplasts.
b. Spongy mesophyll contains loosely packed parenchyma cells that increase the surface area for gas
exchange.
A. Leaf Diversity
1. Simple leaves have margins not deeply lobed or divided into smaller leaflets.
2 Compound leaves are divided into smaller leaflets, and each leaflet may have its own stalk.
3. Leaves are variously modified.
a.
b.
c.
d.
e.
f.
g.
Pinnately compound leaves have the leaflets occurring in pairs.
Palmately compound leaves have all of the leaflets attached to a single point.
Leaves can be arranged in one of three ways: alternate, opposite, or whorled.
Cactus spines are modified leaves and are attached to a succulent stem.
Onion bulbs have leaves surrounding a short stem.
The tendrils of peas and cucumbers are leaves.
The Venus flytrap has leaves to trap and digest insects.
Lecture Enrichment Ideas
Experience Base: Experience with plants is rapidly becoming limited and teachers are reporting
a noticeable drop in the number of flowers a student can name, etc. An instructor can expect
fewer students to have direct experiences with cutting firewood, using raw lumber, or observing
tree stumps and rings. Actual demonstration plants and wood samples, as well as visuals, are
more necessary than before.
1. Less than 2% of students are now from rural farms; therefore visual illustrations of most crop
plants and vegetable structures will greatly increase understanding of most of the plant
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structures.
2. Detail why the root or the shoot of some plants can be used as the source of all other tissues
in the plant, as in taking a cutting of a stem and rooting it or taking a root cutting and
growing a complete plant. Describe the various differences between monocots and eudicots,
with emphasis on roots, stems, and leaves, but also mentioning seeds and flowers. In grasses,
the meristem is at the base which allows us to mow lawns and burn prairies without killing
the grass. This is also the mechanism that allows fire to be used to keep a grassland from
undergoing succession to shrubland—the fire destroys the meristem in the tips of shrub
shoots.
3. Explain how a monocot can form a tree, such as a palm tree, if there is no true secondary
tissue, by enlargement and stiffening of the cortex of the stem.
4. Discuss the difference in growth between herbaceous and woody plants, with emphasis on
the meristematic tissues and their locations.
5. It is intuitive, but wrong, for students to assume that small herbaceous plants evolved before
woody plants, or that monocots evolved before eudicots. The text clarifies the woody-beforeherbaceous scenario; it is also likely that the parallel-veined monocots evolved from stems of
eudicot ancestors.
Critical Thinking
Question 1. A farmer is building a fence between his field and a road. There are some young
trees in the fence line, so he spaces the expensive fence posts so that tree trunks also serve as
fence posts. He staples the top line of the fence to the tree at five feet above the ground. The tree
grows in height five feet each year. Where will the top strand of the fence be in ten years?
Answer: The top strand of the fence will still be five feet above the ground since the cambium grows outward, not
upward. The height of the tree is determined by apical meristem in the tips of stems, unrelated to the outwardly
expanding cambium where the wire was stapled.
Question 2.
Which season of the year should lumber be cut so the wood will be driest? Why do the cut off
trunks then “boil over” with sugary sap as spring approaches?
Answer: Timber should be cut during the winter when the sap has moved down into the roots; the wood in the stem
will then be dry and will not warp as badly. If trees are cut when sap is in the roots, the sap moves upward in the
spring and oozes out of the stump, since the rest of the tree is missing.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
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CHAPTER
25
FLOWERING PLANTS: NUTRITION AND TRANSPORT
This chapter studies the nutritional components essential for plants. Soil and its importance to
plants is discussed as are the mechanisms involved in water and mineral uptake and transport. A
Science Focus box examines “The Concept of Water Potential,”and an Ecology Focus box
discusses “Plants Can Clean up Toxic Messes.”
Chapter Outline
25.1 Plant Nutrition and Soil
1. The ancient Greeks considered plants “soil-eaters” that converted soil into plant tissue.
2. The 17th Century Dutchman Jean-Baptiste Van Helmont conducted an experiment.
a. He planted a five pound young willow tree in a pot with 200 pounds of soil.
b. After five years of watering, the tree weighed 170 pounds but only a few ounces of soil were
missing.
c. He concluded the increase in tree weight came from water; he was unaware of substances in air.
A. Essential Inorganic Nutrients
1. Approximately 95% of a plant’s dry weight is carbon, hydrogen, and oxygen.
a.
Carbon dioxide is the source of carbon for a plant.
b.
Water is the source of hydrogen.
c. Oxygen can come from either atmospheric oxygen, carbon dioxide, or water.
2.
A mineral is an organic substance usually containing two or more elements.
3. To be classified as an essential nutrient, the following criteria must be fulfilled.
a. It must have an identifiable nutritional role.
b. No other element can substitute and fulfill the same role.
c. A deficiency of the element causes the plant to die.
4. These elements are divided into macronutrients and micronutrients, according to their relative
concentrations in plant tissue.
5. Beneficial nutrients are elements required for or to improve growth of a particular plant.
a. Horsetails require silicon as a mineral nutrient.
b. Sugar beets show better growth in the presence of sodium.
c. Soybeans use nickel when root nodules are present.
B. Determination of Essential Nutrients
1. When a plant is burned, most mineral elements (except for nitrogen) remain in the ash.
2. Hydroponics (water culture) is the preferred method for determining plant mineral requirements.
a. Hydroponics is cultivation of plants in water.
b. Nutrient requirements of plants are determined by omitting a mineral and observing the effects.
c. If plant growth suffers, it can be concluded that the omitted mineral is a required nutrient.
d. This works for macronutrients but impurities make micronutrient measurement difficult.
C. Hydroponics
1.
Some of the benefits of hydroponics are the elimation of plant pests, diseases, and weeds.
2. Since water is used through a pipline system, there is little water lost through runoff, compared with
traditional irrigation methods.
D. Soil
1. Soil Formation
a. Soil formation begins with weathering of rock by freezing, glacier flow, stream flow, and
chemicals.
b. Lichens and mosses grow on barren rock and trap particles and leave decaying tissues.
c. Decayed organic matter (humus) takes time to accumulate; its acidity leaches minerals from
rocks.
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2.
d. Depending on parent material and weathering, a centimeter of soil may develop within 15 years.
The Nutritional Function of Soil
a. Soil consists of mineral particles, decaying organic matter, living organisms, air, and water.
b. The best soil includes particles of different sizes; this provides critical air spaces.
c. Mineral Particles
1) Mineral particles vary in size.
a) Sand particles are largest: 0.05–2.0 mm in diameter.
b) Sand particles are medium sized: 0.002–0.05 mm in diameter.
c) Clay particles are smallest: below 0.002 mm in diameter.
3.
4.
2) Sandy soils lose water too readily; clay packs tightly to hold water.
3) Clay particles are negatively charged and attract positively charged ions (e.g., calcium [Ca 2+]
and potassium [K+]).
4) In acidic soils, hydrogen ions replace positively charged nutrients and the nutrient ions float
free and are leached; this is why acid rain kills trees.
5) Clay cannot retain negatively charged NO3, and the nitrogen content of clay soil is low.
6) Loam (a mixture of the three mineral particles) retains water and nutrients; roots take up
oxygen in the air spaces.
d. Humus
1) A mixture of 10–20% humus mixed with a top layer of soil particles is best for plants.
2) Humus keeps soil loose and crumbly, decreases runoff and aerates soil.
3) Humus is acidic and retains positively charged minerals for plants to use later.
4) Bacteria and fungi break down organic matter in humus and return inorganic nutrients to
plants.
e. Living Organisms
1) Small plants play a major role in the formation of soil from rock and in succession.
2) Roots of larger plants penetrate the soil and weather rocks.
3) Larger moles and badgers and smaller earthworms help turn over the soil.
4) Soil animals, from mites to millipedes, help break down leaves and other plant remains.
5) Fungi, protozoa, algae and bacteria complete decomposition.
6) Soil bacteria make nitrate available to plants.
7) Some soil organisms (i.e. some roundworms) are crop pests that feed on roots.
Soil Profiles
a. A soil profile is a vertical section from the ground surface to the unaltered rock below; usually, a
soil profile has parallel layers called horizons.
1) The A horizon is the uppermost topsoil layer that contains litter and humus.
2) The B horizon lacks organic matter but contains inorganic nutrients leached from the A
horizon.
3) The C horizon is weathered and shattered bedrock.
4) Soil profiles vary by parent material, climate, and ecosystem.
5) Grassland soils have deep A horizons from turnover of decaying grasses and lack of leaching.
6) Forest soils have thinner A horizons but enough inorganic nutrients for tree root growth.
7) Tropical rain forest A horizons are shallow due to rapid decomposition; the B horizon is
deeper due to extensive leaching.
Soil Erosion
a. Soil erosion is caused by water or wind carrying away soil.
b. Erosion removes 25 billion tons of topsoil worldwide annually.
c. Deforestation and desertification contribute to erosion.
d. U.S. farmlands lose soil faster than it is formed on one-third of cropland.
e. The coastal wetlands are also losing soil at a high rate.
25.2 Water and Mineral Uptake
1.
Minerals follow the path of water uptake.
a. Some mineral ions diffuse in between the cells; this is called apoplastic transport.
b. Because of the impermeable Casparian strip (a band of suberin and lignin bordering four sides of
root endodermal cells), water must eventually enter the cytoplasm of endodermal cells.
c. Water can enter epidermal cells at their root hairs, and progress across the cortex and endodermis
of a root.
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A. Mineral Uptake
1.
In contrast to water, minerals are actively taken up by plant cells.
a. Mineral nutrient concentration in roots may be 10,000 times more than in surrounding soil.
b. During transport throughout a plant, minerals can exit xylem and enter cells that require them.
2. Mineral ions cross plasma membranes by a chemiosmotic mechanism.
a. Plants absorb minerals in ionic form: nitrate (NO3 ), phosphate (HPO4= ), and potassium ions (K+);
all have difficulty crossing a charged plasma membrane.
b. It has long been known plants expend energy to actively take up and concentrate mineral ions.
c. A plasma membrane pump, called a proton pump, hydrolyzes ATP to transport H + ions out of the
cell; this sets up an electrochemical gradient that causes positive ions to flow into cells.
d. Negative ions are carried across the plasma membrane in conjunction with H + ions as H+ ions
diffuse down their concentration gradient.
B. Adaptations of Roots for Mineral Uptake
1. Two symbiotic relationships are known to assist roots in acquiring nutrients.
2. Legumes have roots colonized by the bacterium Rhizobium.
a. Plants cannot use atmospheric nitrogen because they lack enzymes to break the N  N bond.
b. Rhizobium makes nitrogen compounds available to plants in exchange for carbohydrates.
c. Bacteria live in root nodules—structures on plant roots that contain nitrogen-fixing bacteria.
d. Rhizobial bacteria reduce atmospheric nitrogen (N2) to ammonium (NH4+) (nitrogen fixation).
e. Other plants have a relationship with free-living, nitrogen-fixing microorganisms in soil.
3. Most plants have mycorrhizae; those lacking mycorrhizae are limited in where they can grow.
a. Mycorrhizae are a mutualistic symbiotic relationship between soil fungi and plant roots.
b. The fungal hyphae may enter the cortex of roots but do not enter plant cells.
c. Ectomycorrhizae form a mantle exterior to the root, and they grow between cell walls.
d. Fungus increases the surface area for mineral and water uptake and breaks down organic matter.
e. In return the root furnishes the fungus with sugars and amino acids.
f. Orchid seeds are small with limited nutrients; they germinate only when invaded by mycorrhizae.
g. Nonphotosynthetic plants (e.g., Indian pipe) use mycorrhizae to extract nutrients from nearby
trees.
4. Some plants have poorly developed roots or no roots; other mechanisms supply minerals and water.
a. Parasitic plants (e.g., dodders, broomrapes, pinedrops) send out haustoria (rootlike projections)
that grow into the host and tap into the xylem and phloem of the host.
b. Venus flytrap and sundew obtain nitrogen and minerals as leaves capture and digest insects.
25.3 Transport Mechanisms in Plants
1. Flowering plants have transport tissues as an adaptation to living on land: xylem and phloem.
A. Reviewing Xylem and Phloem Structure
2. Xylem transport tissue conducts water and mineral solutes from roots to leaves; it contains two types
of conducting cells: tracheids and vessel elements.
a. Tracheids:
1) are hollow, nonliving cells with tapered overlapping ends;
2) are thinner and longer than vessel elements; and
3) water crosses the end and sidewalls because of pits in secondary cell wall.
b. Vessel elements:
1) are hollow, nonliving cells that lack tapered ends;
2) are wider and shorter than tracheids;
3) lack transverse end walls; and
4) form a continuous pipeline for water and mineral transport.
3. Phloem is vascular tissue that conducts organic nutrients to all parts of the plant; it contains sieve-tube
members and companion cells.
a. Sieve-tube members lack a nucleus, are arranged end to end and have channels in end walls (thus,
the name “sieve-tube”) through which plasmodesmata extend from one cell to another.
b. Companion cells connect to sieve-tube cells by numerous plasmodesmata, and are smaller and
more generalized than sieve-tube cells; they have a nucleus.
B. Determining Xylem and Phloem Function
1. The transport systems of xylem and phloem rely on the mechanical properties of water.
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a. Diffusion moves molecules from higher to lower concentrations.
b. Water potential considers both water pressure and osmotic pressure.
2. They also rely on the chemical properties of water: polarity of water and hydrogen bonding.
C. The Concept of Water Potential (Science Focus box)
1. Water potential is the potential energy of water.
a. For example, water at the top of a waterfall has a higher water potential than water at the bottom
of the waterfall.
2. In cells, two things determine water potential:
a. Water pressure across a membrane.
1) Within water pressure, pressure potential is the effect that pressure has on water potential.
2) Water moves from an area of higher pressure to an area of lower pressure.
3) The higher the water pressure, the higher the water potential.
4) Pressure potential is the concept that best explains the movement of sap in xylem and phloem
b. Solute concentration across a membrane.
1) Osmotic potential takes into account the effects of solutes on the movement of water.
2) Water tends to move across a membrane from the area of lower solute concentration to the
area of higher solute concentration.
3) The lower the concentration of solutes (osmotic potential), the higher the water potential.
3. Turgor pressure is the pressure potential that increases due to the process of osmosis.
a. Water stops entering a cell when the pressure potential inside the cell increases and balances the
osmotic potential outside the cell.
b. Plants depend on turgor pressure to maintain the turgidity of their bodies.
c. When plants do not have sufficient turgor pressure, the plant droops as a result.
D. Water Transport
1. Movement of water and minerals in a plant involves entry into roots, xylem, and leaves.
2. Water and minerals enter root cells before they reach xylem by the two routes already described.
3. Water entering root cells creates a positive pressure called root pressure.
a. Root pressure (which occurs primarily at night) tends to push xylem sap upward in a plant.
b. Guttation is the appearance of drops of water along the edges of leaves, as a result of water being
forced out of leaf vein endings; it is the result of root pressure.
c. Root pressure is not a sufficient mechanism for water to rise to the tops of trees.
4. Cohesion-Tension Model of Xylem Transport
a. Water and dissolved minerals must be transported upward from roots to xylem, perhaps as high as
90 meters.
b. The cohesion-tension model states that transpiration creates a tension (i.e., a negative pressure)
that pulls water upward in xylem without expending any energy of the plant.
c. Water molecules are cohesive with one another, adhesive with xylem walls.
d. Water molecules interact with one another and form a continuous water column in the xylem,
from the leaves to the roots that is difficult to break.
e. Transpiration is a plant’s loss of water to the atmosphere through evaporation at leaf stomata.
f. Cohesion is the tendency of water molecules to cling together due to their forming hydrogen
bonds.
g. Adhesion is the ability of water (a polar molecule) to interact with molecules comprising the walls
of xylem vessels; adhesion gives a water column extra strength and prevents it from slipping back
down.
h. In daytime, the negative water potential created by transpiration extends from leaves to roots; the
water column must be continuous.
i. If a water column within xylem is broken by cutting a stem, the water column will drop back
down the xylem vessel away from the site of breakage, making it more difficult for conduction to
occur.
j. At least 90% of the water taken up by roots is lost through stomata by transpiration.
k. With plenty of water, stomata will remain open, allowing CO 2 to enter the leaf and photosynthesis
to occur.
l. Transpiration exerts a pulling force or tension that draws the water column up in vessels.
m. Under water stress, more water is lost through a leaf than can be brought up and the stomata close;
the leaves are then protected from water loss by the waxy cuticle of the upper and lower
epidermis.
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n.
Photosynthesis requires CO2 to enter the leaf; there must be sufficient water so stomata can remain
open and allow CO2 to enter.
o. In the root, water enters the xylem passively by osmosis.
p. The water column in the xylem extends from leaves to the root.
q. Water is pulled upward from the roots due to the tension in the xylem created by the evaporation
of water at the leaves.
E. Opening and Closing of Stomata
1. Each stoma, a small pore in the leaf epidermis, has two guard cells.
2. Stomata open from turgor pressure when guard cells take up water; when they lose water, turgor
pressure decreases and stomata close.
3. Guard cells are attached to each other at their ends; the inner walls are thicker than outer walls.
4. Radial expansion is prevented by cellulose microfibrils in the walls but outer walls can expand
lengthwise.
5. As they take up water, they buckle out, thereby creating an opening between cells.
6. Since 1968, it has been known that when stomata open, there is accumulation of K + ions in guard cells.
7. A proton pump run by breakdown of ATP to ADP and P transports H+ outside the cell; this establishes
an electrochemical gradient allowing K+ to enter by way of a channel protein.
8. The blue-light component of sunlight is a signal that can cause stomata to open.
a. There is evidence that flavin pigments absorb blue light.
b. This pigment sets in motion a cytoplasmic response activating the proton pump that causes K +
ions to accumulate in guard cells.
9. Evidence suggests a receptor in the plasma membrane of guard cells brings about inactivation of the
proton pump when CO2 concentration rises, as happens when photosynthesis ceases.
10. Abscisic acid (ABA) produced by cells in wilting leaves, also causes stomata to close; photosynthesis
cannot occur, but water is conserved.
11. In plants kept in the dark, stomata open and close on a 24-hour basis as if responding to sunlight in the
daytime and the absence of sunlight at night; some sort of internal biological clock must keep time.
F. Plant Can Clean Up Toxic Messes (Ecology Focus box)
1. Phytoremediation uses plants to absorb, store, degrade, or transform pollutants that would otherwise
harm the environment.
2. Plants can clean up contaminated sites in one of two ways:
a. If the substance is an organic contaminant (i.e., spilled oil), the plants break down the substances
and the remainders can either be absorbed by the plant or left in the soil or water.
b. If the substance is an inorganic contaminant (i.e., cadmium, zinc), the plants absorb the
substances, trap the contaminants, and the plants are harvested and disposed of.
3. Poplars Take Up Excessive Nitrates
a. A farmer in Amana, Iowa planted a mile long section of poplars to absorb nitrate runoff from a
fertilized cornfield.
b. Before the trees were planted, nitrate levels were as much as ten times the amount considered safe.
c. Following the tree planting, the brook’s nitrate levels dropped more than 90%, and the trees have
thrived.
4. Canola Plants Take Up Selenium
a. Canola plants were planted in farms in California’s San Joaquin Valley to absorb excess selenium
in the soil.
b. Irrigation can cause naturally occurring selenium to rise to the soil surface and the selenium would
flow off into drainage ditches and eventually in wildlife areas. Selenium can deform and kill
waterfowl.
c. By adding selenium-accumulating canola plants to farmers’ crop rotations, selenium levels in
runoff are being managed in the area.
5. Mustard Plants Take Up Uranium
a. In a Superfund site in Aberdenn, Maryland, mustard plants were used to remove uranium from the
Army’s firing range, for as little as 10% of the cost of traditional clean up methods.
6. Limitations of Phytoremediation
a. One limitation of phytoremediation is the slow pace at which trees grow and therefore, how much
pollutants the trees can absorb each growing season.
b. A second limitation is that phytoremediation is only effective at depths that plant roots can reach,
making it useless against deep-lying contamination unless the contaminated soils are excavated.
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c.
Another limitation is that phytoremediation is not successful at cleaning up lead or other metal
contaminatated areas without adding chemicals to the soil.
d. In addition, there is the risk that animals may ingest the pollutants by eating the leaves of plants
that are being used in phytoremediation.
G. Organic Nutrient Transport
1. Marcello Malpighi (1679) suggested bark transferred sugars from leaves to roots.
a. He observed the results of removing a strip of bark from a tree (girdling).
b. Bark swells just above the cut and sugar accumulates in the swollen tissue.
c. Today, we know phloem was removed but xylem remained; therefore, phloem does transport
sugars.
2. Radioactive tracer studies using 14C confirmed phloem transports organic nutrients.
a. When 14C-labeled carbon is supplied to mature leaves, radioactively labeled sugar moves to roots.
b. Similar studies confirm phloem transports amino acids, hormones, and mineral ions.
3. Aphids Used in Study
a. It is difficult to take samples of sap from just the phloem cells without injuring the phloem.
b. Aphids (small insects) drive their mouth stylets into a sieve-tube cell; then samples are easily
taken.
c. The aphid body is cut off; the stylet becomes a small needle from which phloem is collected.
d. Such research indicates that sap can move through phloem from 60–100 cm per hour or more.
4. Pressure-Flow Model of Phloem Transport
a. The pressure-flow model explains the transport of sap through sieve tubes by a positive pressure
potential.
b. The buildup of water creates a positive pressure potential within the sieve tubes that moves water
and sucrose to a sink (e.g., at the roots).
c. Pressure exists from the leaves to the roots; at the roots, sucrose is transported out and water also
flows through due to the pressure.
d. Consequently, this pressure gradient causes a flow of water from leaves to roots.
e. The conducting cells of phloem are sieve tubes lined end to end.
f. Cytoplasm extends through the sieve plates of adjoining cells to form a continuous tube system.
g. During the growing season, leaves produce sugar.
h. Sucrose is actively transported into phloem by an electrochemical gradient established by a H +
pump.
i. Water flows passively into sieve tubes by osmosis.
j. A sink can be at the roots or any other part of the plant that requires nutrients.
k. Because phloem sap flows from source to sink, sap can move any direction along phloem.
Lecture Enrichment Ideas
Experience Base: The direct rural experiences with maple syrup, crops, and wood—the central
topics of this chapter—are very limited among today’s students. Heartwood can be related to
wood panel patterns.
1.
Western science was late to study soil because “dirt” was not considered worthy of serious study; as a result
much of our basic understanding of soil (soil names, leaching processes) came from early Russian research, and
soil still receives less academic attention in the U.S. than other countries.
2. Describe the use of potting soil, which is often completely organic matter, and how this is
different from the soil of our yards. Students can now also relate this information to the
variable erosion of sand (wind eroded) and clay (water eroded), and the reason the
intermediate silt is left in houses after flooding. The term “loam” can now be seen in a
scientific context and students can begin to make the connections as to why many plants
grow better in certain soil environments.
3. Give an example of the cohesion-tension model that students know about from their own
experience such as having blood drawn at the doctor’s office, where the blood fills the
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capillary tube without suction. Discuss how the same thing is going on in a plant’s xylem, but
that there is also a pull exerted by the evaporation of water from the stomates as water moves
out from the spongy mesophyll air pockets.
4.
Note that Marcello Malpighi is also the same individual after whom Malpighian tubules are named; some early
biologists were amazingly eclectic and Malpighi was particularly observant of detail.
5. Describe the benefits to the plant and its symbiont in the relationships between legumes and
nitrogen-fixing bacteria or between many forest trees and their mycorrhizae. Mention the
relationship between the death angel mushroom or the edible truffles and oak or apple trees,
for example, and their mycorrhizae.
6. Actual specimens of wood, slides of tree cross-sections, maple syrup tapping, etc., will help
illustrate some of these concepts for students, including some who have never ventured into
the woods.
Critical Thinking
Question 1.
There are two hypotheses for what triggers stomates to open and close. One states flavin pigments
absorb blue light and this pigment then activates the proton pump. The second responds to CO 2; as its concentration
rises, a membrane receptor inactivates the proton pump. Which, if either, do the following observations support?
a) The clear guard cells of a ladyslipper orchid lack chloroplasts, yet the guard cells regularly open and close.
b) Stomates do not respond when exposed to a narrow band of green light.
c) The cells of wilting leaves produce abscisic acid; abscisic acid causes guard cells to close.
Answer: The lack of pigments in ladyslipper orchids (a) rules out support of the blue light hypothesis. Since green
light is reflected rather than absorbed, the lack of a response to green light (b) does not support any hypothesis. The
response to abscisic acid is evolutionarily logical but not connected to either hypothetical mechanism.
Question 2.
Nearly all deciduous tree leaves have all stomates on the bottom side; none on the top. On the
other hand, corn, oats, lilies, wheat, etc., have as many, if not more, stomates on the top side of the leaf. Why?
Answer: While a tree has its leaves high in the air to receive the most intense sunlight, it also has its leaves exposed
to extensive wind that can dry out a leaf if the hot upper surface is full of holes. The leaves of the grass crops are
more sheltered with stagnant air around much of their length; in addition, their leaves are upright and not oriented
with one side to the sun and the other always shaded, as with horizontal tree leaves.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
CHAPTER
26
FLOWERING PLANTS: CONTROL OF GROWTH
RESPONSES
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The mechanisms underlying plant growth and response are discussed in this chapter. The various plant hormones
and their activities are described, as is the phenomenon of photoperiodism. Plants’ defense mechanisms are also
discussed. A Science Focus box presents “Arabidopsis is a Model Organism.”
Chapter Outline
26.1 Plant Hormones
1.
2.
3.
4.
5.
6.
7.
8.
One defining characteristic of life is an ability to respond to stimuli.
Adaptive organisms respond to environmental stimuli because it leads to longevity and survival of the
species.
Plant cells utilize signal transduction when they respond to stimuli.
Signal transduction involves:
a. Receptors
1)
Plant cells have proteins that are activated by a specific signal.
b. Transduction pathway
1) There are a series of relay proteins or enzymes that transform the signal to a signal that is
understood by the cell’s machinery.
2) The stimulated receptor can either communicate immediately with the transduction pathway,
or use a second messenger to initate a response.
c. Cellular response
1)
The response is the result of the transduction pathway.
2)
The response brings about the observed microscopic response, i.e., closing stomata.
For plants to respond to stimuli, activities of plant cells and structures have to be coordinated.
Almost all plant communication is done by hormones.
Hormones are chemical messengers, produced in very low concentrations that are active in another
part of the organism.
Hormones are synthesized in one part of a plant; they travel in the phloem after a plant receives an
appropriate stimulus.
A. Auxins
1. Apically produced auxin prevents the growth of axillary buds, a phenomenon called apical
dominance.
a. When a terminal bud is removed, the nearest buds begin to grow and the plant branches.
b. Application of a weak solution of auxin causes roots to develop from the ends of cuttings.
c. Auxin production by seeds promotes growth of fruit.
d. As long as auxin is concentrated in leaves and fruits rather than stem, they do not fall off.
2. Auxin-controlled cell elongation is involved in gravitropism and phototropism.
a. When gravity is perceived, auxin moves to the lower surface of roots and stems.
b. The Darwins discovered with oat seedlings, phototropism would not occur if the tip of a seedling
is cut off or covered by a cap; they concluded the cause of curvature moved from the coleoptile
(the protective sheath for the leaves of a seedling) tip to the rest of the shoot.
3. Frits W. Went (1926) experimented with coleoptiles.
a. He cut off tips and placed them on agar.
b. When an agar block was placed to one side, the coleoptile would curve away from that side
regardless of the light.
c. He deduced a chemical caused the curved growth, and he named it auxin after the Greek word for
“promoting growth.”
B. How Auxins Cause Stems to Curve
1. In a plant exposed to unidirectional light, auxin moves from the bright side to the shady side of a stem.
2. Auxin binds to receptors and activates the ATP-driven proton (H+) pump.
3. As hydrogen ions are pumped out of the cell, the cell wall becomes acidic, breaking hydrogen bonds.
4. Cellulose fibrils are weakened and activated enzymes further degrade the cell wall.
5. The electrochemical gradient established causes of uptake of solutes and water follows by osmosis.
6. The result is elongation of the stem on the shady side so that it bends toward the light.
C. Gibberellins
1. Gibberellins are a group of some 70 plant hormones that chemically differ only slightly.
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2.
3.
4.
GA3 is the most common of the natural gibberellins.
Gibberellins are growth promoters that elongate cells.
Gibberellins were discovered in 1926 by Ewiti Kurosawa, a Japanese scientist investigating a fungal
disease of rice plants called “foolish seedling disease.”
a. His fungus-infected plants produced an excess chemical gibberellin, named after the fungus.
b. By 1956, gibberellic acid was finally isolated from a flowering plant rather than fungus.
5. The dormancy of seeds and buds can be broken by applying gibberellins.
6. After the embryo produces gibberellins, amylase appears. So it seems that gibberellins lead to
activation of the gene that codes for amylase.
D. Cytokinins
1. Cytokinins are a class of plant hormones that promote cell division.
2. Cytokinins are derivatives of the purine base adenine.
3. A natural cytokinin zeatin is found in corn kernels; kinetin is a synthetic cytokinin.
4. Researchers discovered cytokinins in work on growing plant tissues in culture.
5. Oligosaccharins, chemical fragments released from the cell wall, also direct differentiation.
6. Researchers hypothesize that auxin and cytokinins are part of a reception-transduction response
pathway that activates enzymes that release these fragments from the cell wall.
E. Senescence
1.
Aging processes are senescence; large molecules break down and are transported elsewhere in the
plant.
2. Cytokinins prevent senescence of leaves; they also initiate development of leaf growth.
3. Cytokinins initiate growth of lateral buds despite apical dominance.
F. Abscisic Acid
1. Abscisic acid (ABA) is produced by any tissue that contains chloroplasts, monocot stems, and roots.
2. ABA is sometimes called the “stress hormone”; it maintains seed and bud dormancy and causes
closure of stomata.
3. It was once thought that ABA functioned in abscission, but now the hormone is no longer believed to
function naturally in this process.
4. Dormancy occurs when a plant organ readies itself for adverse conditions by stopping growth.
a. ABA moves from leaves to vegetative buds in the fall; thereafter these buds are converted to
winter buds which are covered by thick, hardened scales.
b. Reduction in ABA and increase in gibberellins break seed and bud dormancy; seeds germinate and
buds send forth leaves.
5. Abscisic acid brings about the closing of stomata when a plant is under water stress.
a. By some unknown mechanism, ABA causes K+ ions to leave guard cells.
b. As a result, guard cells lose water and the stomata close.
G. Ethylene
1. Ethylene is a gas involved in abscission and ripening of fruits.
2. Auxin and perhaps gibberellin initiates abscission, but once abscission has begun, ethylene stimulates
production of enzymes such as cellulase that cause leaf, fruit, or flower drop.
3. It was an early practice to prepare citrus fruit for market by storage in a room with a kerosene stove.
4. Later work revealed incomplete combustion of kerosene produced ethylene which ripens fruit.
5. Ethylene stimulates production of cellulase, an enzyme that hydrolyzes cellulose in plant cell walls.
6. A barrel of ripening apples can induce ripening of a bunch of bananas some distance away.
7. Ethylene releases from the site of a physical wound; therefore one rotten apple spoils the whole bunch.
8. The presence of ethylene in air inhibits the growth of plants in general.
H. Arabidopsis Is a Model Organism (Science Focus box)
1. To study the actions of genes, including those that control growth and development, scientists have
chosen the week Arabidopsis thaliana as a study plant because:
a. The plant is small so many hundreds of plants can grow in a small amount of space.
b. The plant has a short generation time (5–6 weeks), and can product about 10,000 seeds.
c. The plant typically self pollinates, but can cross pollinate, which is helpful for gene mapping and
production of strains with multiple mutations.
d. The number of DNA base pairs is relatively small.
2. Since the Arabidopsis genome has been sequenced, genes can be clones and used as probes for the
isolation of the homologous genes from plants of economic value.
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3.
Mutant plants are created to discover what each of its genes do.
a. For example, one of the mutant genes that alters the development of flowers has been cloned and
reintroduced into tobacco plants, where it caused sepals and stamens to appear where petals would
ordinarily be.
4. Knowledge about the development of the flowers in Arabidopsis can have far-ranging applications,
including leading to more productive crops.
5. In addition, the study of the Arabidopsis genome may promote plant molecular genetics, in general.
26.2 Plant Responses
A. Tropisms
1. A tropism is plant growth toward or away from a unidirectional stimulus.
a. The stimulus comes from only one direction instead of many.
b. Growth toward a stimulus is a positive tropism; growth away from a stimulus is a negative
tropism.
c. By differential growth, one side elongates faster; the result is a curving toward or away from a
stimulus.
2. Three well-known tropisms are named for the stimulus that causes the response.
a. Gravitropism is response to Earth’s gravity; roots demonstrate positive gravitropism
and stems demonstrate negative gravitropism.
b. Phototropism is growth of plants in response to light; stems show positive phototropism.
c. Thigmotropism is unequal growth due to touch (e.g., coiling of tendrils around a pole).
3. Response to a stimulus first involves reception of the stimulus.
4. The next step is transduction of the stimulus into a form meaningful to the organism.
5. Finally, there is a response by the organism.
B. Gravitropism
1. An upright plant placed on its side displays negative gravitropism; it grows upward opposite gravity.
2. Charles Darwin and his son were the first to show that roots display positive gravitropism.
a. If the root cap is removed, roots no longer respond to gravity.
b. Later researchers showed root cap cells contain statoliths, starch grains within amyloplasts.
c. Due to gravity, amyloplasts settle to the lowest part of the cell.
3.
The connection between amyloplasts is uncertain.
a.
Amyloplasts could cause stems and roots to respond differently than auxin.
b.
The upper surface of a root elongates so that the root curves downward and the lower
surface of a stem elongates, so that the stem curves upwards.
C. Phototropism
1. Phototropism occurs because cells on shady side of stems elongate due to the presence of auxin.
2. Negative tropism occurs when a plant curves away from light.
3. It is now known that phototropism occurs because plants respond to blue light.
a. When the blue light is formed, the photopropin undergoes a conformation change.
b. The change occurs from a transfer of a phosphate group from ATP, to a protein portion of the
photoreceptor.
c. The photoreceptor then triggers a transduction pathway that leads to the entry of auxin into the
cell.
D. Thigmotropism
1. Unequal growth due to contact with solid objects is thigmotropism.
2. The coiling of morning glory or pea tendrils around posts, etc., is a common example.
3. Cells in contact with an object grow less while those on the opposite side elongate.
4. This process is quite rapid; tendrils have been observed to encircle an object in ten minutes.
5. A couple of minutes of touching can bring about a response that lasts for several days.
6. The response can be delayed; tendrils touched in the dark will respond when illuminated.
a. ATP rather than light can cause the response; the need for light is simply a need for ATP.
b. The hormones auxin and ethylene are involved; they induce curvature of tendrils in the absence of
touch.
7. Thigmomorphogenesis is a touch response involving the whole plant.
a. An entire plant responds to the presence of wind or rain.
b. A plant growing in a windy location has a shorter, thicker trunk.
c. Simple rubbing of a plant inhibits cellular elongation and produces a shorter, sturdier plant.
E. Nastic Movements
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1.
2.
F.
Turgor movements are dependent on turgor pressure changes in plant cells.
In contrast to tropisms, turgor movements do not involve growth and are not related to the source of
the stimulus.
3. Turgor movements result from touch, shaking, or thermal stimulation.
4. When a Mimosa pudica leaf is touched, the leaflets fold because the petiole droops.
a. This response takes only a second or two and is due to a loss of turgor pressure within cells.
b. A pulvinus is a thickening at the base of such leaflets where turgor pressure can rapidly drop.
c. Potassium ions move out of the cell and water follows by osmosis.
d. A single stimulus, such as a hot needle, can cause all of the leaves to respond; this requires a nerve
impulse-like stimulus for communication.
5. Venus flytrap
a. This plant has three sensitive hairs at the base of the trap.
b. When touched by an insect, an impulse-type stimulus triggers the trap to close.
c. Two ideas as to what causes the electrical charge are:
1) The cells located near the outer region of the lobes rapidly secrete hydrogen ions (H +) into
their cell walls, loosening them, and allowing the walls to swell rapidly by osmosis; or
2) The cells in the inner portion of the lobes and the midrib rapidly lose ions, leading to a loss of
water by osmosis and collapse of these cells.
3) Whatever the mechanism, turgor movements are involved.
Sleep Movements
1. Circadian rhythms are sleep movements that occur regularly in a 24-hours cycle.
2. A common example of circadian rhythms is in the prayer plant (Maranta leuconeura).
a. At night, the leaves of the prayer plant fold upward into a shape resembling hands of prayer.
b. This movement is also due to changes in the turgor pressure of motor cells in the pulvinus located
at the base of each leaf.
3. Other example of circadium rhythms are:
a. Morning glories (Ipomoea leptophylla) open its flowers during the early part of the day and close
them at night.
b. In most plants the stomata open in the morning and close at night.
c. In some plants necture is secreted at the same time of the day or night.
4. To qualify as a circadium rhythm, the activity must:
a. Occur every 24-hours.
b. Take place in the absence of external stimuli.
c. Be able to be reset if external cues are provided.
5. A biological clock is an internal mechanism maintaining biological rhythms in the absence of stimuli.
6. Biological clocks are synchronized by external stimuli to 24-hour rhythms.
7. Photoperiod is more reliable an indicator of seasonal changes than temperature change.
8. The biological clock may involve the transcription of a number of “clock genes.”
a. One model proposes that the information-transfer system from DNA to RNA to enzyme to
metabolite, with all its feedback controls, is intrinsically cyclical and could be the basis for
biological clocks.
b. These genes control sleep movements, opening and closing of stomata, the discharge of floral
fragrances, and metabolic activities associated with photosynthesis.
9. The biological clock also influences seasonal cycles that depend on day-night length.
G. Photoperiodism
1.
Many physiological changes in plants (e.g., seed germination, the breaking of bud dormancy, and the
onset of senescence) are related to a seasonal change in day length.
2. Photoperiodism is a physiological response prompted by changes in the length of a day or night in a
24-hour daily cycle.
H. Phytochrome
1. Phytochrome is a blue-green leaf pigment that is present in the cytoplasm of plant cells.
2. Pr (phytochrome red) absorbs red light (wavelength of 660 nm); it is converted to P fr.
3. Pfr is phytochrome far-red and absorbs far-red light (wavelength of 730 nm); it is converted to P r.
4. During a 24-hour period, there is a shift in ratio of these two pigments.
a. Direct sunlight contains more red than far-red light; Pfr is present in plant leaves during the day.
b. Shade and sunsets have more far-red than red light; Pfr is converted to Pr as night approaches.
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I.
J.
c. There is a slow metabolic replacement of Pfr by Pr during night.
Functions of Phytochrome
1. The Pr  Pfr conversion cycle controls other growth functions in plants.
2. In addition to being involved in flowering, P fr promotes seed germination and inhibits stem elongation.
3. Following germination, the presence of P r dominates; the stem elongates and grows toward sunlight
while the leaves remain small.
4. Once a plant is exposed to sunlight and Pr is converted to Pfr, the plant begins to grow normally—
leaves expand and the stem branches.
5. Flowering plants can be divided into three groups, based their flowering status:
a. Short-day Plants
1)
These plants flower when day length is shorter than a critical length.
2)
Examples include cocklebur, poinsettia, and chrysanthemum.
3)
In effect, they require a period of darkness that is longer than a critical length to flower.
b. Long-day Plants
1)
These plants flower when the day length is longer than a critical length.
2)
Examples include wheat, barley, clover, and spinach.
3)
In effect, they require a period of darkness that is shorter than a critical length to flower.
c. Day-neutral Plants
1)
These are plants for which flowering is not dependent on day length.
2)
Examples include tomato and cucumber.
6.
A long-day and a short-day plant can have the same critical length.
a. These plants differ in specific sequence of day lengths as a season progresses.
b. Spinach is a long-day plant that flowers in summer when day length increases to 14 hours.
c. Ragweed is a short-day plant that flowers in fall when day length shortens to 14 hours or less.
7. Scientists experimented with artificial lengths of dark and light periods.
a. Cocklebur, a short-day plant, flowers as long as the dark period lasts over 8.5 hours.
b. If the dark period is interrupted by a flash, it does not flower; but darkness amidst a day cycle has
no effect.
c. Long-day plants require a dark period shorter than a critical length regardless of the length of the
light period.
d. Therefore, length of the dark period controls flowering, not length of the light period.
Flowering Plants Respond to the Biotic Environment
1.
Plant have many defense mechanisms to protect themselves from herbivore and parasite attacks.
2.
Plants have physical defenses such as cuticle-covered epidermis and bark.
3. Despite the physical defenses, herbivores can still attack the plant, so plants need several other types of
defenses.
4. Plants produce secondary metabolites are defense mechanisms.
a.
Tannins, in or on the leaf epidermis, are defensive compounds that interfere with the
outer proteins of bacteria and fungi.
b.
Tannins also deter predators because of the astringent effect on the mouth and
interference with digestion.
c.
Alkaloids (e.g., morphine, nicotine, caffeine) are also secondary metabolites.
1) Coffee plant seedlings have a high concentration of caffeine, which can kill insects and
fungi by blocking DNA and RNA synthesis.
d. Cyanogenic glycosides break down to cyanide and inhibit cellular respiration.
1) Foxglove (Digitalis purpurea) produces deadly cardiac and steroid glycosides, which
cause nausea, hallucinations, and death in animals that ingest them.
5.
Plants also respond when they have been wounded or attacked.
a.
After a leaf has been chewed or injured, the plant produces proteinase inhibitors,
systemin, which is a chemical that destroy the digestive enzymes of a predator feeding on them.
6. Sometimes plants produce a specific gene product that binds to either a viral, bacterial, or fungal gene
product made within the cell, so the plant can “recognize” a particular pathogen.
a.
Following gene production, a transduction pathway ensues, and the final result is a
hypersensitive response (HR) that seals off the infected area and will also initiate a wound
response.
7.
Some plant defense mechanisms are indirect defense mechanisms.
a.
Since female butterflies are less likely to lay their eggs on plants that already have
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8.
butterfly eggs, the leaves of some passion flowers (genus Passiflora) display physical structures
resembling the eggs, so the butterflies do not lay eggs on those plants.
b.
Lima beans produce volatiles that attract carnivore mites only when they are being
damaged by a spider mite.
c.
Corn and cotton plants release volatiles that attract wasps, which then inject their eggs
into caterpillars eating their leaves.
Some plants have mutualistic relationships with other animals to protect them from herbivores.
a.
A species of acacia, Pseudomyrmex ferruginea, has swollen thorns with a hollow interior
and ants will live and feed (without harming) off the acacia. In turn, the ants protect the plant by
attacking and stinging herbivores.
Lecture Enrichment Ideas
Experience Base: Many students have not directly experienced farming or gardening and
therefore have not seen many of the phenomena discussed here. Substantial actual examples can
be brought to class and media will be necessary to picture the various plant “behaviors.”
1. Care should be taken not to confuse students with the pseudoscience surrounding plants
being able to hear music, etc.; only animals have nervous systems and muscles, and the
mechanisms described from plants do not involve a central nervous system and are very
distant from animal systems for behavior.
2. Hydroponics is an important regime for simplifying experimental designs and excluding the
unknown elements in soil. Explain how it is important to isolate plants from other effects in
order to ascertain that they are indeed responding to a single growth hormone or stimulus.
3.
Describe the kinds of hormones found in a rooting compound sold in gardening stores, and compare the stores’
expectations with what you find on the label of such compounds.
4. Explain the effects of auxins and cytokinins on the undifferentiated callus of plant tissue in
culture, where auxins produce root tissue development and cytokinins produce shoot
development.
5. Consider the actions of ethylene, which is produced by a ripening fruit and causes its
continued ripening. This is an example of positive feedback, not a reversible reaction.
6. Discuss how the chemical changes of Pr to Pfr in the presence of red (normal) light and of Pfr
to Pr in the presence of far-red (twilight) light or darkness are associated with the actions of
long-day and short-day flowering plants. Pfr acts to stimulate flowering in a long-day plant
and acts to inhibit flowering in a short-day plant, which explains the effects of a short burst
of light in the middle of darkness in both kinds of plants. There is an intuitive but erroneous
presumption that long- and short-day plants must, therefore, respond to the daylight period
“because the name says so.”
Critical Thinking
Question 1.
If we transferred a plant (specimen or seed) from a latitude of 40 o North in the northern
o
hemisphere to 40 South in the southern hemisphere, what would be the difficult adjustment for the plant?
Answer: The day-night cycles would be the same but the seasons would be six months off cycle. This means that
the vegetative specimen is probably not easy to transfer since the seasons will be dramatically different. The critical
factor is this: can the seeds germinate early or late? Otherwise, day length would be the same.
Question 2.
The biochemistry of photoperiodism is not simple. Why would plants not all evolutionarily
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“select” to flower in the same season by the first mechanism that evolved (all spring flowers or all fall flowers)?
Answer: The pollinators are a resource that is limited. This would reduce competition with other flower species. In
addition, there are other adaptations that would have to be made in seed durability and dormancy; they too would be
beneficial if they distanced the plant from competition with others.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
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CHAPTER
27 FLOWERING PLANTS: REPRODUCTION
Plant reproductive processes are explored in this chapter. Included are sections on life cycles,
seed development and dispersal, fertilization and subsequent development, and asexual
reproduction. A Science Focus box discusses “Plants and Their Pollinators.”
Chapter Outline
27.1 Sexual Reproductive Strategies
A. Life Cycle Overview
1. In contrast to animals with only one multicellular stage in their life cycle, all plants have two: a diploid
and a haploid generation.
2. In flowering plants, the diploid sporophyte is dominant, and is the generation that bears flowers.
3. A flower, the reproductive structure of an angiosperm, produces two types of spores, microspores and
megaspores.
a. A microspore is a plant spore that develops into a male gametophyte, which is a pollen grain.
b. A megaspore is a plant spore that develops into a female gametophyte, the embryo sac which
remains within a sporophyte plant.
4. At maturity, a pollen grain (which is either windblown or carried by an animal to the vicinity of the
female gametophyte) travels via a pollen tube to the embryo sac and fertilizes the egg.
5. The zygote becomes an embryo, which develops into a seed.
6. A seed contains the embryo and stored food surrounded by a seed coat.
7. The ovary becomes a fruit.
8. The sporophyte is the generation that contains vascular tissue and has other adaptations suitable to
living on land, including production of flowers.
B. Flowers
1. Flowers are unique to angiosperms; aside from producing the spores and protecting gametophytes,
flowers attract pollinators and produce fruits to enclose the seeds.
2. The shoot apical meristem stops forming leaves to form flowers; axillary buds can become flowers
directly.
3. Monocot flower parts are in threes or their multiples; eudicot flower parts are in fours or fives or their
multiples.
4. Sepals are leaflike, usually green; this outermost whorl protects the bud as a flower develops within.
Collectively, sepals are called the calyx.
5. Petals are interior to sepals; coloration accounts for attractiveness of many flowers. Collectively, the
petals are called the corolla.
a. The size, shape, and color of a flower are attractive to a specific pollinator.
b. Wind-pollinated flowers often have no petals at all.
6. Grouped about a pistil are stamens, stalked structures that have two parts.
a. The anther is a saclike container within which pollen grains develop.
b. A filament is a slender stalk that supports the anther.
7. The carpel is the vaselike structure located at the center of a flower; carpels usually have three parts.
a. The stigma is an enlarged sticky knob on the end of a style; stigma serves to receive pollen grains.
b. The style is a slender stalk that connects stigma with the ovary.
c. The ovary is an enlarged base of a carpel that contains a number of ovules.
8. Not all flowers have sepals, petals, stamens, and a pistil.
a. Complete flowers have sepals, petals, stamens, and a pistil; incomplete flowers do not.
b. Perfect (bisexual) flowers have both stamens and a pistil.
c. Imperfect (unisexual) flowers have only stamens or carpels.
d. Carpellate flowers have only carpels.
9. If staminate and carpellate flowers are on the same plant, the plant is monoecious.
10. If staminate and carpellate flowers are on different plants, the plant is dioecious.
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C. Life Cycle in Detail
1. In all land plants, the sporophyte produces haploid spores by meiosis; in animals, meiosis produces
gametes.
2. Flowering plants are heterosporous, producing microspores and megaspores that become spermbearing pollen grains and egg-bearing embryo sacs, respectively.
3. Development of Male Gametophyte
a. Microspores are produced in the anthers of flowers.
b. An anther has four pollen sacs; each contains many microsporocytes (microspore mother cells).
c. Microsporocytes undergo meiotic cell division to produce four haploid microspores.
d. The haploid nucleus then divides mitotically forming two cells enclosed in a finely sculptured
wall; this is a pollen grain, which is at first an immature male gametophyte, containing a tube
cell and a generative cell.
e. The larger tube cell will eventually form the pollen tube.
f. Eventually each generative cell will divide mitotically to form two sperm.
g. Once both events have occurred, the pollen grain is the mature male gametophyte.
4. Development of Female Gametophyte
a. The ovary contains one or more ovules.
b. An ovule is covered by parenchymal cells except for one small opening, the micropyle.
c. One parenchyma cell enlarges to become a megasporocyte that undergoes meiotic cell division to
produce four haploid megaspores.
d. Three megaspores are nonfunctional; one megaspore nucleus divides mitotically into eight nuclei
in a female gametophyte.
e. When cell walls form around the nuclei later, there are seven cells, one of which is binucleate.
f. The female gametophyte (or embryo sac) consists of seven cells:
1) one egg cell,
2) two synergid cells,
3) one central cell with two polar nuclei, and
4) three antipodal cells.
5. Development of New Sporophyte
a. Walls separating the pollen sacs in the anther break down when the pollen grains are to be
released.
b. Pollination is the transfer of pollen from an anther to the stigma of a carpel.
c. Self-pollination is transfer of pollen from anther to stigma of the same plant.
d. Cross pollination is transfer of pollen from anther of one plant to stigma of another plant; plants
often have mechanisms that promote cross pollination such as the carpel only maturing after
anthers have released their pollen.
e. When a pollen grain lands on a stigma, it germinates, forming a pollen tube.
f. A germinated pollen grain, containing a tube cell and two sperm, is the mature male gametophyte.
g. As a pollen tube grows, it passes between the cells of the stigma and the style to reach the
micropyle of an ovule.
h. Double fertilization occurs after the release of both sperm cells into the ovule.
i. One sperm nucleus unites with the egg nucleus, forming a 2n zygote.
j. The other sperm nucleus migrates and unites with the polar nuclei of the central cell, forming a 3n
endosperm cell.
k. The zygote divides mitotically to become the embryo; the endosperm nucleus divides mitotically
to become the endosperm.
l. The endosperm is tissue that will nourish the embryo and seedling as they undergo development.
m. A mature seed contains:
1) the embryo
2) stored food
3) the seed coat
n. Some species rely on wind pollination (grasses, grains).
o. Much of the plant’s energy goes into making pollen to ensure that some pollen
grains actually reach a stigma.
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p. The use of animal pollinators is unique to flowering plants and it helps account
for why these plants are so successful on land.
q. Coevolution means that as one species changes, the other changes too, so that in
the end, the two species are suited to one another.
6. Plants and Their Pollinators (Science Focus box)
a.
b.
c.
d.
e.
f.
Plants and their pollinators have a mutualistic relationship in which each benefits.
1)
Plants use pollinators to ensure that cross-pollination takes place.
2)
The pollinator uses the plant as a source of food.
This relationship came about through coevolution—codependency of the plant and the pollinator
is the result of suitable changes in the structure and function of each.
Bee-Pollinated Flowers
1)
Bees do not see red wavelengths but do see ultraviolet wavelengths.
2) Bee-pollinated flowers are usually brightly colored and predominatly blue or yellow. They
may also have ultraviolet shadings called nector guides.
3) In addition, the bees’ mouthparts are adapted to sucking the plant’s nectar.
4) When bees travel to one flower, the pollen attaches to their body. When they travel to the next
flower, they deposit the pollen, participating in cross pollination.
5) Bee pollinated flowers are also sturdy, often with a landing pad, able to support the weight of
the bee.
Moth- and Butterfly-Pollinated Flowers
1)
Moths feed at night and have a well-developed sense of smell.
2) Moth pollinated flowers are usually light colored and have a strong, sweet perfume to attract
the moths.
3) Moth pollinated flowers have deep tubes with open margins to allow the moths to reach the
nectar with their long proboscis.
4) In contrast, butterflies feed during the day, have good vision, but weak sense of smell.
5) Butterfly pollinated flowers have bright colors and are odorless.
Bird- and Bat-Pollinated Flowers
1)
Hummingbirds have good eyesight but have a weak sense of smell, and hover when they
feed.
2) Hummingbird pollinated flowers are red, with a slender floral tube and margins that are
curved back and out of the way, and have little odor.
3) Bats are nocturnal and have an acute sense of smell.
4) Bat pollinated flowers open only at night and are light-colored or white. They also have a
similar odor that bats produce to attract each other.
Coevolution
1) The use of animal motility to achieve cross-fertilization resulted in the
evolution of flowers, which have features to attract pollinators.
2) As cross-fertilization continued, more and more flower variations likely
developed, and pollinators became increasingly adapted to specific
angiosperm species.
3) The success of angiosperms has probably contributed to the success of insects,
and visa versa.
27.2 Seed Development
1.
2.
Development of the seed is the next event in the life cycle of the angiosperm.
Plant growth and development involves cell division, cell elongation, and differentiation of cells into
tissues and then organs.
A. Stages of Eudicot Development
1. Immediately after double fertilization, the endosperm nucleus divides to produce a mass of endosperm
surrounding the embryo.
2. The single-celled zygote also divides, but asymmetrically, forming two parts: embryo and suspensor,
which anchors the embryo and transfers nutrients to it from the sporophyte plant.
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3.
Globular Stage
a. During this stage, the proembryo is a ball of cells.
b. The root-shoot axis is established; cells near the suspensor will become a root, those at the
opposite end will become a shoot.
c. The outermost cells become dermal tissue; by dividing with the cell plate perpendicular to the
surface, they produce one outer cell layer.
d. Dermal tissue prevents dessication and also has stomata that regulate gas exchange.
4. The Heart Stage and Torpedo Stage Embryos
a. The embryo has a heart-shape when the cotyledons appear; it then grows to a torpedo shape.
b. With elongation, the root and shoot apical meristems are distinguishable.
c. Ground meristem responsible for most of the interior of the embryo is also present now.
5. The Mature Embryo
a. After differentiation into embryo and suspensor, one or two cotyledons develop.
b. The embryo continues to differentiate into three parts.
c. The epicotyl is between the cotyledons and first leaves; it contributes to shoot development.
d. The hypocotyl is below the cotyledon and contributes to stem development.
e. The radicle is below the hypocotyl and contributes to root development.
f. The cotyledons are quite noticeable in a eudicot embryo, and may fold over.
B. Monocots Versus Eudicot Seeds
1. Monocots have only one cotyledon.
a.
In monocots, the cotyledon rarely stores food.
b.
It absorbs food molecules from the endosperm and passes them to the embryo.
2. Eudicots have two cotyledons
a.
During development of a eudicot embryo, cotyledons usually store the nutrients the
embryo uses.
b. The endosperm seemingly disappears as the nutrients are consumed.
27.3 Fruit Types and Seed Dispersal
1.
2.
3.
A fruit is a mature ovary that encloses seeds; sometimes they retain other flower parts.
Fruits serve to protect and disperse offspring.
The fruit protects the peach seed well but makes germination difficult; the peas escape easily but are
lightly protected.
A. Kinds of Fruits
1. Fruits can be simple or compound.
a. Simple fruit develops from a single carpel or several fused carpels of a compound ovary.
b. A compound fruit develops from several individual ovaries.
c. An aggregate fruit develops from ovaries from a single flower (e.g., blackberry).
d.
A multiple fruit develops from ovaries from separate flowers fused together (e.g.,
pineapple).
2. As fruit develops, the ovary wall thickens to become the pericarp, which can have as many three
layers:
a. The exocarp forms the outermost skin of a fruit.
b. The mesocarop is often the fleshy tissue between the exocarp and endocarp of the fruit.
c. The endocarp serves as the boundary between the seed(s). It may be hard (peach pits), or papery
(apples).
3. Legumes and cereal grains are examples of dry fruits; such fruits are mistaken for seeds because a dry
pericarp adheres to the seed within.
a. Legumes are dehiscent because they split open when ripe.
b. Grains are indehiscents because they do not split open when ripe.
B. Dispersal of Fruits
1. Many dry fruits are wind dispersed.
a. Woolly hairs, plumes, and wings are adaptations for wind dispersal.
2. Other fruits are animal dispersed.
a. Hooks and spines of clover, bur, and cocklebur attach to the fur of animals.
b. Fleshy fruit attract animals and provide them with food, and they defecate the seeds at a distance.
c. Squirrels and other animals gather seeds and fruits and bury them some distance away.
C. Seed Germination
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1.
2.
3.
4.
Seed germination occurs when growth and metabolic activity resume.
Some seeds do not germinate until they have been dormant for a period of time.
a. Seed dormancy is a time during which no growth occurs even though conditions are favorable.
b. In temperate zones, seeds may have to be exposed to cold weather before dormancy is broken.
c. Fleshy fruits contain inhibitors so that germination does not occur while the fruit is still on the
plant.
d. For some seeds to take up water, bacterial action and even fire is required (e.g. Pinus banksiana).
e. Once water enters, the seed coat bursts and the seed germinates.
Prior to germination, a eudicot embryo consists of the following:
1) two cotyledons that supply nutrients to the embryo and seedling, but soon shrivel and
disappear;
2) a plumule—a rudimentary plant that consists of an epicotyl bearing young leaves;
3) the hypocotyl, which becomes the stem; and
4)
the radicle, which develops into roots.
In monocots, the endosperm is the food-storage tissue and the cotyledon does not have a storage
role.
a.
b.
c.
A monocot “seed” such as a corn kernel is actually the fruit and the outer covering is the pericarp.
The plumule and radicle are enclosed in protective sheaths, the coleoptile and the coleorhiza,
respectively.
The plumule and radicle burst through these coverings when germination occurs.
27.4 Asexual Reproduction Strategies
1.
In asexual reproduction, offspring arise from a single parent and inherit genes of that parent only.
a. Asexual reproduction is often advantageous when the parent is already adapted to a particular
environment and the production of genetic variations is not a necessity.
2. Types of asexual reproduction include:
a. Plant reproduction via stolons—aboveground horizontal stems.
1)
The asexual offspring grows from the stolons off of the parent plant.
2)
The characteristics of the offspring are identical to those of the parent plant.
b. Plant reproduction via rhizomes—underground stems.
1) Irises have no aboveground stem because their main stem is a rhizome that grows horizontally
underground.
2)
New plants grow at the nodes of a rhizome.
3)
Each eye of a potato plant tuber is a bud that produces a new plant.
c.
Many trees can be started from small “suckers.”
1) Stem cuttings have long been used to propagate a wide array of plants (e.g., sugarcane,
pineapple).
2) In these plants, a cut stem will automatically produce roots.
3) The discovery that auxin will cause roots to develop has expanded our ability to use stem
cuttings.
A. Tissue Culture of Plants
1. Tissue culture is the process of growing tissue artificially in a liquid or solid culture medium.
2. Many plant cells are totipotent; each cell has full genetic potential of the organism.
3. Three methods of cloning plants due to the ability of plants to grow from single cells are:
a. Somatic embryogenesis
1) In this method, hormones are added to the medium and they cause leaf or other tissue cells to
generate small masses of cells, which can be genetically engineer before being allowed to
become many new identical plants.
2) Tomato, rice, celery, asparagus, lilies, begonias, and African violets have been produced
using somatic embryogenesis.
3) Not all plants are genetically identical clones because mutations, called somaclonal
variations, can airse spontaneously during the production process.
b. Meristem tissue culture
1) Meristem culture micropropagates many new shoots from a single shoot apex culture in a
medium with correct proportions of hormones.
2) When the shoots are removed, more shoots form.
3) Meristem culture generates meristem that is virus-free; the plants produced are also virus-free.
192
c.
4.
Anther tissue culture
1)
Haploid cells within pollen grains are cultured in order to produce haploid plantlets.
2)
A diploid (2n) plantlet can be produced if chemical agents are added to the culture.
3) Similarly to meristem tissue cultures, the offspring are genetically identical to the parent
plant.
Cell suspension culture allows scientists to extract chemicals from plant cells in high concentrations
without having to overcollect wild-type plants growing in their natural environment.
a. Single cells or small clumps form a suspension of cells; all produce the same chemicals as the
entire plant.
Lecture Enrichment Ideas
Experience Base: Student experience with recognizing distinct flower parts will vary widely
with high school science experiences and farming background, but in most cases is declining.
Flower materials are available, safe, inexpensive, and convenient to provide and visuals are
attention-getting.
1. Provide specimens of flowers that have only “male parts” and others that have only “female
parts.” Describe how these may be found on the same plant or on completely different plants.
Examine the corn flowers, for example, with the male tassel on top producing pollen and the
female flowers with the long silks that lead to the ovaries; each corn kernel is a single fruit,
and the cob is a stem on which the individual flowers and fruits develop.
2. Provide visuals showing how some plants have characteristics that attract animals as
pollinators. For example, the shape of some orchid flowers resembles the reproductive organs
of female insects, stimulating male insects to visit different flowers and attempt copulation,
thus transferring pollen. The production of scent and nectar, and the bullseye markings
visible to insects but not to humans, could also be discussed.
3. Describe the flowers of oak or other trees that do not have insect pollinators. Spring semester
classes have an advantage when wind-pollinated trees are ubiquitous. Try to bring specimens
or photographs, and compare how these flowers are different from what we normally think of
as a flower.
4.
Query what happens when different kinds of pollen (from different species of plants) land on a flower’s stigma.
There must be antigenic (protein) recognition between the pollen grain and the tissue of the stigma for proper
interaction to occur; and genetic information for building a pollen tube of the proper length must be present if
pollen germination does occur.
5. Current newspaper articles on newly engineered plants provide a relevant application of the
techniques for genetic engineering explained in this chapter. Genetically-engineered crops
are the center of a major political dispute between the U.S. which has high acceptance, and
Europe where many are being banned.
Critical Thinking
Question 1. A human zygote at the 64-cell stage has 61 cells devoted to becoming the placenta to support the
developing embryo that develops from the other three cells. In what way is this similar and different compared to the
plant embryo and endosperm?
Answer: Both the placenta and endosperm function to provide food to the embryonic organism. However, the
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placenta develops from the cells derived from a single-fertilization event that also produced the embryo, while the
endosperm developed from a separate fertilization event that only provided for this support unit.
Question 2. Descriptions of the various techniques for inserting genes into plant cells indicate that the cell wall is a
fairly formidable barrier. What are the practical consequences for introducing new genes, such as herbicide
resistance into crop plants, if this barrier was not present?
Answer: While it would be considerably easier to develop a resistant strain of crop plant, there would also be a danger of
the gene moving into undesirable weeds and providing them with resistance.
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PART VI ANIMAL EVOLUTION AND DIVERSITY
This part has been reorganized to be consistent with molecular data regarding the relationship of groups of animals.
Classification is based on molecular data. To accommodate the new hypotheses, Chapter 28 now includes molluscs,
annelids, arthropods, and echinoderms. Chapter 29 is devoted exclusively to the vertebrates.
28 Invertebrate Evolution
29 Vertebrate Evolution
30 Human Evolution
CHAPTER
28
INVERTEBRATE EVOLUTION
This chapter begins a three-chapter section on animal evolution. In this chapter, many different invertebrates are
discussed in detail, including protostomes and deuterostomes, and human parsitic diseases involving certain
invertebrates are described.
Chapter Outline
28.1 Evolution of Animals
1. Members of the kingdom Animalia are multicellular heterotrophs that ingest their
food.
2. Animals have the diploid life cycle, and usually reproduce sexually.
3. Muscle and nerve tissues characterize animals.
a. The evolution of these tissues enabled many types of animals search actively for
their food and prey on other organisms.
4. Animals are monophyletic—meaning both invertebrates and vertebrates can trace
their ancestry to the same ancestor.
5. Adult vertebrates have a spinal cord (or backbone), while invertebrates do not have
a backbone.
A. Ancestry of Animals
1. The colonial flagellate hypothesis states that animals are descended from an ancestor
that resembled a hollow spherical colony of flagellated cells.
2. The colonial flagellate hypothesis implies that radial symmetry preceded bilateral
symmetry in the history of animals.
a. In a radially symmetrical animal, any longitudinal cut produces two identical
halves.
b. In a bilaterally symmetrical animal, only one longitudinal cut yields two identical
halves.
3. The choanoflagellates (collared flagellates) most likely resemble the last unicellular
ancestor of living animals, and molecular data illustrates that they are the closest
living relatives of animals.
4. A choanoflagellate is a single cell, 3–10 μm in diameter, with a flagellum surrounded
by a collar of 30–40 microvilli.
5. As the water moves through the microvilli, they engulf bacteria and debris from the
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water.
B. Evolution of Body Plans
1. As an animal develops, there are many possibilities regarding the number, position,
size, and patterns of its body parts.
2. Slight shifts in genes called Hox (homeotic) genes are responsible for the major
differences between animals that arise during development.
a. Maybe the changes in the expression of Hox developmental genes explan why all
the animal groups of today had representations of the Cambrian seas
C. The Phylogenetic Tree of Animals
1. The phylogenetic tree of animals is based on molecular and morphological data.
2. It is assumed that the more closely related two organisms, the more rRNA nucleotide
sequences they will have in common.
3. Molecular data have resulted in a phylogenetic tree that is quite different from the one
based only on morphological characteristics.
D. Morphological Data
1. Types of Symmetry
a. Asymmetry means there is no particular body shape (e.g., sponge).
b. Radial symmetry describes body parts arranged around an axis, like spokes of a
wheel (e.g., starfish).
1) Radially symmetrical animals may be sessile (i.e., attached to a substrate or
less motile).
2) This symmetry enables an animal to reach out in all directions from one
center.
c. Bilateral symmetry describes a body having a right and left, or complementary
halves.
1) Only one longitudinal cut down the center produces mirror halves.
2) Bilaterally symmetrical animals tend to be active and to move forward at an
anterior end.
3) The development of a head to localize the brain and sensory organs at the
anterior end is called cephalization.
2. Embryonic Development
a. The first three tissue layers are called germ layers because they give rise to the
organs and organ systems of complex animals.
b. Animals (e.g., cnidarians) that have two tissues layers (ectoderm and endoderm)
as embryos are diploblastic with the tissue level of organization.
c. Animals that develop further and have all three tissue layers (ectoderm,
mesoderm, and endoderm) as embryos are triploblastic and have the organ level
of organization.
1)
Animals that have three tissue layers are either protostomes or
deuterostomes.
d. Protostomes exhibit the following events during embryological development.
1)
Spiral cleavage, in which the cells divide without an increase in
size.
2)
The fate of the cells is fixed—each contributes to development in
only one particular way.
3) The blastopore is associated with the mouth.
4) A coelom forms by splitting of the mesoderm, which has arisen from cells
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near the blastopore.
e. Deuterostomes exhibit the following events during embryological development.
1) Radial cleavage, where the new daughter cells sit atop the previous cells; the
fate of these cells is indeterminate.
2) The blastopore is associated with the anus; the mouth appears later.
3) A coelom forms by the fusion of mesodermal pouches from the primitive gut.
f.
Protostomes are divided into two groups:
1)
Ecdysozoans include roundworms and arthropods.
a.
Both of these animals molt; they shed their outer covering
as they grow.
2)
Lophotrochozoa contains two groups
a.
Lophophores have the same type of feeding apparatus.
b.
Trochophores either have presently or their ancestors had
a trochophore larva.
28.2 Introducing the Invertebrates
A. Sponges (phylum Porifera)
1. Sponges have no symmetry and no tissues and remain at a cellular level of evolution.
2. Their saclike bodies are perforated by many pores.
3. Sponges are aquatic, largely marine animals that vary greatly in size, shape, and
color.
4. They have a canal system that allows water to move through their bodies.
5. Beating collar cells produce currents through pores in the wall into a central cavity
and out through osculum.
6. Simple sponges 10 cm tall can filter as much as 100 liters of water a day.
7. Sponges are sessile filter feeders; they stay in one place and filter food from the
water.
a. Collar cells engulf and digest food particles in food vacuoles.
8. Sponge skeleton prevents the body from collapsing.
a. All sponges have fibers of spongin, a modified form of collagen, which gives a
sponge its flexibility.
b. The endoskeleton of sponges also contains spicules, which are tiny needle-shaped
structures with one to six rays, depending on chemical structure.
c. Sponges have few predators, due to spicules, and the production of foul smelling
and toxic substances that discourage predators.
9. Sponges reproduce asexually by budding, which can produce quite large colonies.
10. Fragmentation occurs when sponges are chopped up; each piece can start a complete
sponge.
11. Sponges reproduce sexually when eggs and sperm are released into a central cavity;
the zygote develops into a ciliated larva.
B. Comb Jellies and Cnidarians
1. Comb jellies (phylum Ctenophora) represent the largest animals propelled by beating
cilia and range in size from a few centimeters to 1.5 m in length.
a. Most of their body is a jellylike packing material called mesoglea.
b. Long tentacles covered with sticky filaments—or an entire body covered by
sticky mucus—captures prey.
2. Cnidarians (phylum Cnidaria) are tubular or bell-shaped.
a. They mostly live in coastal waters but there are oceanic, freshwater, and brackish
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forms.
b. Cnidaria have cnidocytes, a specialized cell that contains a nematocyst.
1)
The nematocyst is a fluid-filled capsule, which contains a long, spirally
coiled hollow thread.
2)
When the trigger of the cnidocyte is touched, the nematocyst is
discharged.
3) Some threads merely trap prey or predators; others have spines that penetrate
and inject paralyzing toxins.
c. A cnidarian body is a two-layered sac with the epidermis derived from ectoderm.
d. The inner tissue layer derived from the endoderm secretes digestive juices into the
gastrovascular cavity.
e. The gastrovascular cavity digests food, circulates nutrients, and serves as a
supportive hydroskatic skeleton.
f. Cnidaria have two basic body plans.
1)
A polyp is vase-shaped and the mouth is directed upward.
2)
A medusa is bell-shaped and the mouth is directed downward.
3) A medusa has more mesoglea than a polyp; tentacles are concentrated on the
margin of the bell.
4)
Both body forms may have been a part of life cycle of early cnidaria.
5) When both stages are present, the animal is dimorphic and the polyp stage is
sessile and produces the medusae.
6)
The medusa stage is motile and produces the egg and sperm, dispersing
the species.
3. Cnidarian Diversity
a. Sea anemones are solitary polyps 0.5–20 cm in length and 0.5–10 cm in diameter
or larger.
1) Their upward turned oral disk that contains the mouth is surrounded by a large
number of hollow tentacles containing nematocytes.
b. Corals resemble sea anemones encased in a calcium carbonate house.
1)
Some may be solitary; most are colonial.
2) Corals are responsible for building coral reefs by the slow accumulation of
limestone resulting in massive reefs.
c. The hydrozoans have a dominant polyp.
1)
Two examples of hydrozoans are Hydra and Portuguese man-of-war.
2) The Portuguese man-of-war is a colony of polyps; the original polyp becomes
a gas-filled float.
3)
Other polyps bud to specialize for feeding or reproduction.
4) It can cause serious injury to swimmers with a tentacle having numerous
nematocysts; each tentacle arises from the base of each feeding polyp.
d. In the true jellyfishes (e.g., Aurelia) the medusa stage is dominant in jellyfish; the
polyp remains small.
1)
Jellyfishes are an important part of the zooplankton, the food for larger
marine animals.
e. Hydra are solitary, freshwater hydrozoan polyps.
1)
The hydra body is a small tube about one-quarter inch in length.
2)
Four to six tentacles containing nematocysts surround the mouth, the only
opening at one end.
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3)
In the epidermis are nematocyst-containing cnidocytes and sensory cells
that make contact
with the nerve cells within a nerve net, which allow
transmission of impulses in several directions at once
4)
Hydra reproduces asexually by budding.
5) They can also reproduce sexually: sperm from a testis swim to an egg within
an ovary; after early development within an ovary, a protective shell allows
the egg to survive until conditions are optimum for it to emerge.
6)
Hydras can grow as entire organisms from a small piece (like sponges).
28.3 Variety Among the Lophotrochozoans
1. Lophotrochozoans are bilaterally symmetrical at least in some stage of development.
2. As embryos, they have three germ layers; as adults they have the organ level of
organization.
3. The two groups of lophotrochozoans are:
a. The lophophores
1)
Examples include bryozoans and brachiopods.
2)
They are not closely related but share a similar feeding apparatus called
the lophophore.
b. The trochophores
1)
Examples include flatworms, rotifers, molluscs, and annelids.
2)
They either have a trochophore larva today, or their ancestor had
trochophore larva.
A. Flatworms
1. Flatworms (phylum Platyhelminthes) are trochozoans.
2. Flatworms have a sac body plan and thus an incomplete digestive tract.
3. When an animal has two openings, they have a complete digestive tract.
4. Flatworms have no body cavity.
5. In addition to endoderm and ectoderm, a mesoderm layer fills the space between
their organs.
B. Free-living Flatworms
1. The planaria Dugesia lives in lakes, ponds, and streams and feed on small living or
dead organisms.
2. Digestion
a. Planaria capture food by wrapping around prey, entangling it in slime, and
pinning it down.
b. The pharynx is a muscular tube that extends through the mouth and through
which food is ingested.
c. In a three-branched gastrovascular cavity, digestion is both extracellular and
intracellular.
d. Digestive system delivers nutrients and oxygen to the cells and there is not
circulatory or respiratory system.
e. Waste exits through the mouth.
3. Excretion
a. The flame-cell system consists of a series of interconnecting canals that run the
length of the body on either side of the longitudinal axis and side branches of the
canals.
b. A flame cell is a bulb-shaped cell containing a tuft of cilia inside the hollow
interior of the bulb; cilia move back and forth, bringing water into canals that
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empty through pores at the surface.
c. It functions in both water excretion and osmotic regulation.
4. Planaria can reproduce both sexually and asexually.
a. They constrict beneath the pharynx; each half will grow into a whole animal—
regeneration.
b. Planaria are hermaphroditic, possessing both male and female sex organs.
c. Planaria cross-fertilize each other.
d. Fertilized eggs are enclosed in a cocoon and hatch in two to three weeks into tiny
worms.
5.
Nervous system
a.
Planarias have a ladder-type nervous system.
b.
Paired ganglia function as a brain in the head region.
c.
The head is bluntly arrow-shaped; side extensions (auricles) are sensory
organs to detect food and enemies.
d. The two light-sensitive eyespots have pigmentation that makes them look crosseyed.
6. Three muscle layers—an outer circular, an inner longitudinal, and a diagonal—allow
for varied movement.
7. In larger forms, locomotion is accomplished by movement of cilia on ventral and
lateral surfaces.
8. Numerous gland cells secrete a mucous material upon which the animal moves.
C. Parasitic Flatworms
1. As parasites, flukes and tapeworms have characteristic modifications.
a. Loss of predation allows a lack of cephalization; the head carries hooks and
suckers to attach to a host.
b. There is extensive development of the reproductive system with loss of other
systems.
c. Well-developed nervous and gastrovascular systems are not needed; it does not
seek out or digest prey.
d. Flukes and tapeworms are covered by a tegument that protects them from host
digestive juices.
2. Two hosts
a. Flukes and tapeworms use a secondary (intermediate) host to travel from primary
host to primary host.
b. A primary host is infected with sexually mature adults; the secondary host
contains the larval stage(s).
3. Flukes
a. Blood, liver, and lung flukes inhabit those organs.
b. Fluke bodies are generally oval and elongate.
c. At the “head,” an oral sucker is surrounded by sensory papillae; another sucker
also helps attach.
d. Flukes have reduced digestive, nervous, and excretory systems.
e. The reproductive system is well developed and they are usually hermaphroditic.
f. The blood fluke causes schistosomiasis.
1) Schistosomiasis disease is found predominantly in the Middle East, Africa,
and Asia where about 200,000 infected persons die each year.
2) Blood flukes are male or female; the female fluke deposits eggs in blood
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vessels around the intestines.
3) The eggs migrate to the intestine and are passed out with feces.
4) Tiny larvae hatch in water and swim until they detect and enter a particular
species of snail.
5) The larvae reproduce asexually inside the snail and eventually leave the snail.
6) If the larvae penetrate the skin of the human body, they begin to mature in the
liver and implant in the small intestine blood vessels.
7) A weakened person is more likely to die from secondary diseases.
g. The Chinese liver fluke (Clonorchis sinensis) requires two intermediate hosts (a
snail and a fish).
1) Humans become infected when they eat uncooked fish.
2) Adults live in the liver and deposit eggs in the bile duct, which carries eggs to
the intestine.
3) Larval flukes must then pass through two intermediate hosts, a snail and a
fish.
4. Tapeworms
a. A tapeworm head or scolex contains hooks and suckers for attachment to the
intestinal wall of the host.
b. Behind this head is a short neck and then a long string of proglottids.
c. Each proglottid segment contains a full set of both male and female sex organs
and little else.
d. There are excretory canals but no digestive system and only rudiments of nerves.
e. After fertilization, proglottids become a bag of eggs; mature proglottids break off
and pass out with feces.
f. If eggs of tapeworms are ingested by pigs or cattle, larvae become encysted in the
muscle of hosts.
g. The covering of ingested eggs is digested away and larvae burrow through the
intestinal wall and travel by bloodstream to lodge and encyst in muscle.
h. A cyst is a hard-walled structure sheltering a larval worm.
i. If humans eat the meat of infected pigs or cattle and fail to cook the meat
properly, they too become infected.
D. Rotifers
About 2,000 species of rotifers belong to phylum Rotifera.
Rotifers are abundant in freshwater.
A crown of cilia (corona) causes a rotating motion; this organ of locomotion also directs
food to the mouth.
They can desiccate for during harsh conditions and remain dormant for lengthy periods of
time and thus are called “resurrection animacules.”
E. Molluscs
1. Molluscs (phylum Mollusca) live in marine, freshwater, and terrestrial environments.
2. Phylum Mollusca includes chitons, limpets, slugs, snails, abalones, conchs,
nudibranchs, clams, scallops, squid, and octopuses.
3. Molluscs have a three-part body plan: a visceral mass, a mantle, and a foot.
4. The visceral mass contains internal organs: digestive tract, paired kidneys, and
reproductive organs.
5. A mantle covering partly surrounds the visceral mass; it may secrete a shell and help
develop the gills or lungs.
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6. The foot is muscular and adapted for locomotion, attachment, food capture, or a
combination of functions.
7. The radula in the mouth bears many rows of teeth and is used for feeding; a radula is
not present in all molluscs.
8. In molluscs, the true coelom is reduced and limited to the region around the heart.
9. Nutrients and oxygen diffuse into the tissues from the sinuses instead of being carried
into the tissues by capillaries.
10. Most molluscs have an open circulatory system: a heart pumps hemolymph through
vessels into a hemocoel.
11. Blue hemocyanin, not red hemoglobin, is the respiratory pigment found in molluscs.
12. Some are slow moving with no head; others are active predators with a head and
sense organs.
13. The nervous system consists of several ganglia connected by nerve cords.
14. Chitons are in the class Polyplacophora.
a. Chiton shells consist of a row of eight overlapping plates.
b. The flat chiton foot is muscular and creeps along or clings to rocks.
c. They scrape algae and other plant food from rocks with a well-developed radula.
F. Bivalves
1. Class Bivalvia contains the bivalves (clams, oysters, mussels, scallops).
2. Bivalves are two-part shells that are hinged and closed by powerful muscles.
3. They have no head, no radula, and little cephalization.
4. Clams burrow with a hatchet-shaped foot; mussels use it to produce threads to attach
to objects.
5. Scallops both burrow and swim; rapid clapping of their valves releases water in
spurts.
6. The shell is secreted by the mantle.
a. The shell is composed of protein and calcium carbonate with an inner layer of
pearl.
b. The shell deposits around a foreign particle inserted between the mantle and the
shell to form a pearl.
7. A compressed muscular foot projects down from the shell; by expanding the tip, it
pulls in the body.
8. The beating cilia of the gills cause water to enter the mantle cavity by way of an
incurrent siphon and exit by way of an excurrent siphon.
9. The cilia of gills move water through the mantle cavity.
10. Gills capture particles and move them toward the mouth; the mouth leads to the
stomach, which leads to the intestine, which passes through a heart and ends at the
anus.
11. The circulatory system is open; the heart pumps hemolymph into vessels that open
into the hemocoel.
12. The nervous system consists of three pairs of ganglia that connect the front, back, and
foot.
13. Two excretory kidneys below the heart remove ammonia waste from the pericardial
cavity.
14. The sexes are separate; the gonad is located around coils of intestine.
15. Some clams and annelids have the same type of larva, indicating an evolutionary
relationship between molluscs and annelids.
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G. Gastropods
1. Gastropods are the largest class of molluscs and include snails, land slugs, whelks,
conchs, periwinkles, sea hares, and sea slugs.
2. Most are marine but some are freshwater or terrestrial.
3. They have an elongated, flattened foot and most have a one-piece coiled shell that
protects the visceral mass.
4. The anterior end of gastropods has a well-developed head region with a cerebral
ganglion and eyes on the end of tentacles.
5. Land snails are hermaphroditic.
a. In pre-mating behavior, they meet and shoot calcareous darts into each other’s
body wall.
b. Each inserts a penis into the other’s vagina; this provides sperm for future
fertilization of eggs.
c. Eggs are deposited in soil and development proceeds without the formation of a
larvae.
H. Cephalopods
1. Cephalopods include squids, cuttlefish, octopuses, and nautiluses.
2. Squids and octopuses squeeze water out of the mantle cavity; the water forced out
through a funnel propels them by jet propulsion.
3. Around the head are tentacles with suckers or adhesive secretions adapted for
grasping prey.
4. A head equipped with a powerful beak can tear prey apart.
5. Well-developed sense organs include focusing camera-type eyes.
6. Cephalopods, particularly octopuses, have well-developed brains with a capacity for
learning.
I. Annelids
1. Annelids are the only trochozoan with segmentation and a well-developed coelom.
2. Segmentation is the repetition of body parts along the length of the body.
3. A well-developed, fluid-filled coelom and tough integument act as a hydrostatic
skeleton.
4. Using a hydrostatic skeleton, partitioning the coelom allows for independent
movement of the segments so it can not only burrow but crawl on the surface.
5. Setae are bristles that protrude from the body wall, can anchor the worm, and help it
move: oligochaetes have few setae, polychaetes have many setae.
6. The common earthworm, Lumbricus terrestris is an oligochaete.
a. Locomotion requires coordinated movement of body muscles and the help of
setae.
1)
As longitudinal muscles contract, segments bulge, and setae protrude to
anchor into soil.
2) When circular muscles contract, a worm lengthens, setae are withdrawn, and
the segment can be pulled forward.
b. Earthworms live in moist soil; a moist body wall allows for gas exchange.
c. Earthworms are scavengers that extract organic remains from soil they eat.
d. A muscular pharynx draws food into the mouth.
e. Food is stored in a crop and ground up in a thick, muscular gizzard.
f. The dorsal surface of the intestine is expanded into a typhlosole for more surface
area for digestion.
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g. Each external segment corresponds to an internal septum; a wall that separates
each body segment.
h. A long ventral nerve cord leads from the brain to ganglionic swellings and lateral
nerves in each segment.
i. The ventral nerve cord leading from the brain has ganglionic swellings and lateral
nerves in each segment.
j. The coiled nephridia tubules in most segments have two openings: one is a
ciliated funnel that collects coelomic fluid, and the other is an exit through the
body wall.
k. Between the two openings, a coiled nephridia tubule removes waste from blood
vessels.
l. The dorsal blood vessel moves red blood anteriorly; five pairs of hearts pump
blood to a ventral vessel.
m. The extensive closed circulatory system has blood vessels running the length of
the body and to every segment; muscular aortic arches (“hearts”) propel blood
through the vessels.
n. Earthworms are hermaphroditic; the male organs are the testes, sperm ducts, and
seminal vesicles; the female organs are the ovaries, oviducts, and seminal
receptacles.
o. Mating involves aligning parallel to each other facing opposite directions to
exchange sperm.
p. Each possesses a clitellum that secretes mucus that slides off, forming a slime
tube that protects the sperm and eggs from drying out.
q. The slime tube forms a cocoon around the fertilized eggs as they develop.
r. Embryonic development lacks a larval stage.
7. Approximately two-thirds of all annelids are marine polychaetes.
a. Polychaetes possess parapodia and setae.
1)
Parapodia are paddle-like appendages used in swimming and for
respiration.
2)
Setae are bristles, attached to parapodia, that help anchor polychaetes or
help them move.
b. Clam worms such as Nereis are active predators.
c. Many have well-developed cephalization; the head has well-developed jaws, eyes,
and other sense organs.
d. Other sedentary filter feeders possess tentacles with cilia to create water currents
and sort out food particles.
e. Only during breeding seasons do polychaetes have reproductive organs.
f. In Nereis, many worms coordinate the shedding of a portion of their bodies that
contain either eggs or sperm; these segments float to the surface where
fertilization takes place.
g. Marine worm zygotes develop larva similar to those of marine clams; again this
shows a relatedness between annelids and molluscs.
8. Leeches are annelids that mostly live in freshwater habitats.
a. They lack setae and each body ring has several transverse grooves.
b. Leeches possess a small anterior sucker around the mouth and a larger posterior
sucker.
c. Although some are free-living predators, most feed on body fluids.
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d. Leeches keep blood from coagulating by hirudin, an anticoagulant in their saliva.
28.4
Quantity Among the Ecdysozoans
A. Roundworms
1. Roundworms (phylum Nematoda) are nonsegmented worms that are prevalent in
almost any environment.
2. The internal organs lie within a pseudocoelom—a body cavity that is incompletely
lined with mesoderm.
3. Parasitic roundworms
a. In Ascaris lumbricoides, males are smaller than females and their posterior end
curves to a point.
b. These worms move by whiplike motions because only longitudinal muscles lie
next to the body wall.
c. Mating produces eggs that mature in the soil; therefore, most are limited to
warmer climates.
d. When eggs are swallowed, larvae burrow through the intestinal wall to the liver,
heart, and/or lungs.
e. In the lungs, the larvae molt; after 10 days they migrate up the windpipe to the
throat and are swallowed.
f. Back in the intestine, mature worms mate and the female deposits eggs that are
lost with feces.
g. Feces must reach the mouth of the next host to complete a life cycle; therefore,
proper sanitation easily prevents infection.
4. Trichinosis is a serious infection caused by Trichinella spirallis.
a. Humans contract Trichinella spirallis by eating raw pork with encysted larvae.
b. After maturation, the adult female burrows into the wall of the small intestine and
produces living offspring that are carried by the bloodstream to skeletal muscles
where they encyst.
5. Filarial worms cause various diseases.
a. In the U.S., a filarial worm is a parasite of dogs.
b. It lives in the heart and the arteries that serve the lungs and thus is called
heartworm disease.
6.
Elephantiasis occurs in tropical Africa.
a. It is caused by a filarial worm (Wuchereria bancroftji) that utilizes the mosquito
as a secondary host.
b. Adult worms reside in and block the lymphatic vessels; ultimately this results in
the limbs of an infected individual swelling to monstrous size.
c. It is treatable only in the early stages but not after scar tissue has blocked the
lymphatic vessels.
7. Pinworms are the most common nematode in the United States.
a. Adult parasites live in the cecum and large intestines.
b. Females migrate to anal region at night to lay their eggs.
c. Scratching an infected area can contaminate the area with eggs. The eggs are
swallowed and the life cycle begins again.
B. Arthropods
1. Arthropods (phylum Arthropoda) are considered highly successful because:
a. Arthropods have a rigid exoskeleton with freely movable jointed appendages.
1)
The exoskeleton is a strong but flexible outer covering composed mainly
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of chitin.
2)
Chitin is a strong, flexible, nitrogenous polysaccharide.
3) The exoskeleton serves for protection, attachment for muscles, locomotion,
and prevention of desiccation.
4) Because the exoskeleton is hard and nonexpandable, arthropods must molt
(shed) the exoskeleton to grow larger.
a) Before molting, the body secretes a larger soft and wrinkled exoskeleton
underneath.
b) Enzymes partially dissolve and weaken the old exoskeleton.
c) The arthropod breaks the old exoskeleton open and wriggles out.
d) The new exoskeleton then quickly expands and hardens.
b. Segmentation can be observed because each segment has a pair of jointed
appendages; some segments of arthropods have fused into regions (e.g., a head, a
thorax, and an abdomen).
1)
Jointed appendages of arthropods are hollow tubes moved by muscle.
2)
The appendages are modified for food gathering, reproduction,
locomotion.
c. Arthropods have a well-developed nervous system.
1)
A brain is connected to a ventral solid nerve cord.
2)
The head bears various sensory organs.
3)
Compound eyes have many complete visual units; each collects light
independently.
4) The lens of each visual unit focuses the image on the light-sensitive
membranes of a few photoreceptors.
5) In simple eyes, a single lens brings the image to focus into many receptors,
each of which receives only a portion of the image.
6) Many arthropods have well-developed touch, smell, taste, balance, and
hearing, and display complex behaviors and methods of communication
d. Arthropods use a variety of respiratory organs.
a)
Marine forms use gills with vascularized, thin-walled tissue specialized for
gas exchange.
b)
Terrestrial forms have book lungs (e.g., spiders) or air tubes called
tracheae (e.g., insects).
e. Metamorphosis is a drastic change in form and physiology that occurs as a larva
becomes an adult.
a)
Metamorphosis contributes to the success of arthropods.
b)
A larva eats foods and lives in environments different from the adult.
c)
Competition between the immatures and adults of a species is reduced.
d)
This reduction in competition allows more members of the species to exist
at one time.
C. Crustaceans
1. Crustaceans have a hard, crusty exoskeleton, which contains calcium carbonate in
addition to chitin.
2. The head of crustaceans usually bears compound eyes and five pairs of appendages.
a. The first two appendages are antennae and antennules; in front of the mouth, they
have sensory functions.
b. The next three pairs (mandibles, first and second maxillae) lie behind the mouth
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D.
and are used in feeding.
3. Biramous appendages have two branches; one branch is a gill and the other is the leg
branch.
4. Copepods and krill feed on algae; numerous, they are an important link in marine
food chains.
5. Barnacles have a thick, heavy shell and are sessile.
a. Stalked barnacles attach by their stalk; stalkless barnacles attach directly to the
shell.
b. Barnacles begin as free-swimming larvae and become sessile on wharf pilings,
rocks, etc.
c. They extend feathery structures (cirri) to filter feed.
6. The head and thorax of a crayfish are fused into a cephalothorax, which is covered on
the top and sides by a nonsegmented carapace.
a. Abdominal segments have a pair of swimmerets, small paddlelike structures.
b. The first two pairs of swimmerets in the male (claspers) stronger to pass sperm to
the female.
c. The last tail segments are the uropod and the telson, which together make a fanshaped tail.
d. A crayfish awaits prey and uses its claws to carry it to the mouth.
e. Crayfish have gills that lie above the walking legs and are protected by the
carapace.
f. The crayfish stomach has two main regions.
1) The anterior gastric mill with chitinous teeth grinds food.
2) A posterior region filters coarse particles before absorption in the digestive
glands.
g. Green glands in the head area excrete metabolic wastes through a duct at the base
of the antennae.
h. The coelom is reduced in arthropods and forms the space about the reproductive
system.
i. The heart pumps hemolymph containing bluish hemocyanin into a hemocoel
where it washes around the organs.
j. A brain is connected to a ventral nerve cord; periodic ganglia give off lateral
nerves.
k. The sexes are separate in crayfish.
1) The male has a coiled sperm duct that opens to the outside at the base of its
fifth walking leg.
2) The female’s ovaries open at the base of the third walking legs.
3) The fold between the bases of the fourth and fifth pair of legs serves as a
seminal receptacle.
4) Following fertilization, the eggs are attached to the swimmerets of the female.
Centipedes and Millipedes
1. There are about 3,000 species of centipedes (“hundred-leggers”).
a. The body is composed of a head and trunk with many segments; each segment has
one pair of legs.
b. Centipedes are carnivorous; the head bears antennae and mouthparts with poison
fangs.
2. The millipedes (“thousand-leggers”) have a cylindrical segmented body.
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a. Some body segments are fused with two pair of legs on each resulting segment.
b. They possess more legs than centipedes, although not one thousand as the name
states.
c. Millipedes dwell in the soil, feeding on dead organic matter.
E. Insects
1. Most insects live on land; some are secondarily aquatic.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
The insect body is divided into a head, thorax, and abdomen.
The head bears sense organs and mouthparts.
The thorax bears three pairs of legs and one or two pairs of wings; the wings provide advantages in
escaping enemies, finding food, and mating.
In grasshoppers, the third pair of legs is suited to jumping.
The front wings are protective and leathery; the thin hind pair of wings foldup.
Each side of the first abdominal segment has a tympanum for sound wave reception.
Two paired projections form an ovipositor in females used to dig a hole for laying eggs.
The grasshopper digestive system is complex.
The mouth mechanically breaks apart food, and salivary secretions begin digestion.
The crop temporarily stores food.
A gizzard finely grinds the food.
Digestion is completed in the stomach; gastric ceca cavities assist absorption of nutrients.
a. The excretory system consists of Malpighian tubules.
1) The tubules extend into the hemocoel.
2) Nitrogenous wastes are collected and excreted into the digestive system.
3) Formation of solid nitrogenous wastes (uric acid) conserves water.
b. The respiratory system begins with openings in the exoskeleton called spiracles.
1) Air enters into small tubules called tracheae.
2) The tracheae branch many times until they reach moist areas of gas
absorption.
3) Air movement through this tracheal system is assisted by air sacs.
4) Air enters the anterior four spiracles and exits by the posterior six.
5) Breathing by tracheae is a factor that limits the size of insects.
c. The circulatory system contains a slender, tubular heart.
1) This heart lies against the dorsal wall of the abdominal exoskeleton.
2) The heart pumps hemolymph into a hemocoel where it circulates before
returning to the heart.
3) Insect hemolymph is colorless—it lacks any respiratory pigment since the
tracheal system transports gases.
d. Grasshoppers undergo incomplete metamorphosis; the immature nymph
resembles the adult.
e. Other insects undergo complete metamorphosis; the wormlike larvae reorganize
into different adults.
f. Some species (e.g., bees and ants) exhibit colonial social behavior.
F. Chelicerates
1. Chelicerates in the subphylum Chelicerata include spiders, scorpions, and horseshoe
crabs.
2. The first pair of appendages are chelicerae, the second pair are pedipalps, and the next
four pairs are walking legs.
a. Chelicerae are appendages that function as feeding organs.
b. Pedipalps are feeding or sensory structures.
3. All of the appendages attach to a cephalothorax, a fusion of head and thoracic
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regions.
4. The marine horseshoe crab (genus Limulus) is common along the east coast of North
America.
a. Their body is covered by exoskeletal shields.
b. The anterior shield is a horseshoe-shaped carapace with two compound eyes.
5. Arachnids include ticks, mites, scorpions, spiders, and harvestmen.
a. Over 25,000 species of mites and ticks have been classified.
b. Ticks are ectoparasites of various vertebrates and are carriers for Rocky Mountain
spotted fever and Lyme Disease.
c. When not on a host, ticks hide on plants and in the soil
6. Scorpions
a. They are nocturnal and spend the day hidden under a log or rock.
b Pedipalps are large pincerlike appendages; the abdomen ends in a stinger
containing venom.
7. Spiders (over 35,000 species) have a narrow waist separating the cephalothorax from
the abdomen.
a. Spiders have numerous simple eyes rather than compound eyes.
b. Spider chelicera are modified as fangs with ducts leading from poison glands.
c. The abdomen has silk glands; they may spin a web to trap prey.
28.5 Deuterostomes
A. Echinoderms
1.
Echinoderms are primarily bottom-dwelling marine animals.
2. Echinoderms have 5-pointed radial symmetry as adults; as larvae, they are freeswimming filter feeders with bilateral symmetry.
3. They have an endoskeleton of spiny calcium-rich plates (ossicles).
4. Echinoderms have a water vascular system that consists of canals and appendages
that function in locomotion, feeding, gas exchange, and sensory reception.
5. Asteroidea contains sea stars.
6. Holothurians include sea cucumbers.
a. Sea cucumbers have long leathery bodies and resemble a cucumber.
7. Echinoidea includes the sea urchin and sand dollar.
a. Both these animals use their spines for locomotion, defense, and burrowing.
8. Ophiuroidea includes brittle stars.
a. They have a dentral disk surrounded by radially flexible arms.
9. Crinoidea is the oldest group of echinoderms and includes stalked feather stars and
motile feather stars.
B. Sea Stars
1. There are 1,600 species of sea stars.
2. They are found along rocky coasts, where they feed on clams, oysters, and other
bivalves.
3. Out of their body wall, they have:
a. spines from the endoskeletal plates that serve as protection.
b. pincerlike structures around the bases of spines to keep the surface free of small
particles.
c. skin gills that are used for respiration.
d. Each arm has a groove lined by tube feet.
4. Digestion
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a. Sea stars pull mollusk shells open using their tube feet.
c.
Once open, the sea star inverts its cardiac stomach and pushes through the
crack and secretes enzymes.
d.
Digestion begins externally; digestion continues in the pyloric stomach
using enzymes from the digestive glands found in each arm.
e. The short insteine opens at the anus on the aboral side.
5. Each arm contains a well-developed coelom that has a pair of digestive glands and
gonads.
6. The nervous system is a central nerve ring that gives off radial nerves in each arm.
7. A light-sensitive eyespot is at the tip of each arm.
8. Locomotion consists of expansion and contraction of the tube feet.
9. Echinoderms do not have a respiratory, excretory, or circulatory system; the water
vascular system carries out these functions.
10. Sea stars reproduce asexually by fragmentation; each fragment can regenerate a
whole animal as long as a portion of the central disk is present.
11. Spawning is the sexual reproduction method in echinoderms.
Lecture Enrichment Ideas
Experience Base: Because the organisms discussed in this chapter are small size or remote from
inland temperate populations, few students will be directly familiar with them aside from specific
targeted biology labwork. International students from tropical countries may be willing to
discuss their experiences with some of these parasitic organisms.
1.
Provide specimens, slides, or media to compare size, structure, habitats, and relative complexity of the different
invertebrates.
2.
Discuss possible reasons why, based on fossils, there appeared to be a sudden explosion of development of
animal phyla during the early Cambrian Period. Recent molecular work tracing backward from modern animals
suggests that the animal lineage goes back over twice as far in time to a common animal ancestor, leaving twothirds of the time of animal evolution before the Cambrian fossil record begins.
3.
Contrast the inverted structures of the medusa and polyp forms of cnidaria, and examine how the body structure
is uniquely different from that of a sponge. Many students have the misconception that evolution occurs only to
add more complexity. Evolution often involves loss of features, as is readily seen in parasites. This lesson can
be started with the cnidarians specializing in the medusa or polyp stages only, or the loss of body systems by
tapeworms that adapted to life in a host gut.
4. Compare the body structure of the flatworm and the roundworm, with emphasis on body
layers, digestive tract, and reproduction.
5. Most of these parasitic diseases are tropical; utilize slides or other audiovisuals to illustrate
the ravages of elephantiasis, fluke infections, etc. Many cultural habits have evolved to
circumvent ingestion of these parasites; describe or solicit the benefits of hygiene, plumbing,
etc., noting that some depend upon a minimal level of affluence that is not available in poor
regions of the world. Many students do not realize the extent they avoid suffering due to U.S.
social customs, civil engineering, relative affluence, and our temperate climate.
6. Describe the similarities between the protostomes and deuterostomes—presence of a coelom,
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tube-within-a-tube body plan, mesoderm, etc.; audiovisuals are of great help in illustrating
embryological development.
7. Examine the different adaptations of the mollusc body plan, and compare the body parts that
differ within the groups. Describe how these body parts differ from those of an annelid or an
arthropod. If it is a problem in your region, spend some time to illustrate the case of the
introduced (exotic) zebra mussel.
8. Discuss the presence of sensory receptors in the head in some annelids, their absence in the
earthworm, and what this implies about the environments in which these worms live.
9. Query why insects are so successful on land and why there are so many different species of
insects.
Critical Thinking
Question 1. The integument of flukes and roundworms allows them to live intimately inside
our bloodstream and ducts without being rejected by our immune system, or easily affected by
drugs. If customs are not easily changed, how can the chain of transmission be broken?
Answer: Intermediate hosts are involved. Molluscicides kill snails and without snails, many of
these cannot complete their life cycle to re-enter human hosts. In the case of filariasis,
mosquitoes carry the microscopic worms and mosquito control interrupts the cycle.
Question 2. Both sponges and sea anemones are sessile. What is the difference, if any, in
being inside the central cavity of these two organisms?
Answer: A sponge only soaks up food particles as they pass through the pores in its wall, and
any organism in the open central cavity simply flows out to the top osculum. However, the
central cavity of a sea anemone is a gastrovascular cavity with digestive fluids that break down
food. An organism in this cavity would soon be digested.
Question 3.
Why are giant ants and spiders the size of humans or elephants not possible?
Answer: The ability of air to diffuse through tracheae limits the depth of tissue that can receive
sufficient oxygen and unload carbon dioxide. In addition, the hard exoskeleton must be
continually molted. It also takes a closed circulatory system to better distribute nutrients to a
larger system, and it takes an internal system of support to prevent herniating. If you add lungs, a
closed circulatory system, a skeleton, etc., you no longer have an arthropod but a vertebrate.
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CHAPTER
29
VERTEBRATE EVOLUTION
This chapter begins a study of the vertebrates with a discussion of the anatomy and physiology of
the chordates. Extensive discussion of the taxonomic classes of vertebrates precedes a detailed
description of the mammals. The chapter concludes with the taxonomic orders of mammals and
the animals found within. A Science Focus box “Vertebrates and Human Medicine,” describes
the use of certain animals as sources of pharmacological agents for humans.
Chapter Outline
29.1 The Chordates
1. Chordates (phylum Chordata) are deuterostomes, and have an internal skeleton, with muscles attached
to the outer surface.
2. At some time during their life, all chordates have four basic characteristics.
a. Notochord
1) This supporting rod is located dorsally just below the nerve cord.
2) It provides support and is replaced by the vertebral column in vertebrates.
b. Dorsal Tubular Nerve Cord
1) This cord contains a fluid-filled canal.
2) In vertebrates, this is the spinal cord and it is protected by vertebrae.
c. Pharyngeal Pouches
1) These openings function in feeding, gas exchange, or both.
2) They are seen only during embryonic development in most vertebrates.
3) In invertebrate chordates, fish, and amphibian larvae, they become functioning gills.
4) In terrestrial vertebrates, the pouches are modified for various purposes.
5) In humans, the first pair of pouches becomes the auditory tubes, the second become tonsils,
and the third and fourth pairs become the thymus and parathyroid glands.
d. A postanal tail extends beyond the anus; in some this only appear in embryos.
A. Nonvertebrate Chordates
1. Lancelets
a. Lancelets are in the group cephalochords.
b. Lancelets are named for their resemblance to a lancet—a two-edged surgical
knife.
c.
d.
2.
They inhabit shallow coastal waters; they filter feed partly buried in sandy substrates.
They feed on microscopic particles filtered from a constant stream of water that enters the mouth
and exits through gill slits into an atrium that opens at the atriopore.
e. Lancelets retain the four chordate characteristics as adults.
f. The notochord extends from head to tail, accounting for their group name cephalochordates.
g. They possess segmented muscles and the dorsal hollow nerve cord has periodic branches.
Sea Squirts
a. Sea squirts belong in the group urochordates.
b. Adults have a body composed of an outer tunic; an excurrent siphon squirts out water when it is
disturbed.
c. The larvae are bilaterally symmetrical and have the four chordate characteristics.
d. The larvae undergo metamorphosis to develop into sessile adults.
e. Water passes into a pharynx and out numerous gill slits, the only chordate characteristic that
remains in adults.
f. It is hypothesized that sea squirts are directly related to vertebrates. A larva with the four chordate
characteristic may have become sexually mature without developing the other sea squirt
characteristic, and then evolved into a fishlike vertebrate.
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29.2 Vertebrates
1.
2.
As embryos, vertebrates have the four chordate characteristics.
Vertebrates also have these features:
a. Vertebral column
1)
The embryonic notochord is replaced by a vertebral column.
2)
Remnants of the notochord are seen in the intervertebral disks.
3) The vertebral column is part of a flexible, strong endoskeleton that is also evidence of
segmentation.
b. Skull
1) A skull is an anterior component of the main axis of vertebrate endoskeleton; it encases the
brain.
2)
The high degree of cephalization in vertebrates is accompanied by complex sense organs.
3)
The eyes developed as outgrowths of the brain.
4) The ears—equilibrium devices in water—function as sound-wave receivers in land
vertebrates.
c. Endoskeleton
1)
The endoskeleton and muscles together permit rapid and efficient movement.
2) The pectoral and pelvic fins of fish evolved into jointed appendages allowing vertebrates to
move onto land.
d. Internal organs
1)
Vertebrates possess a complete digestive system and a large coelom.
2)
The circulatory system is closed and the blood is contained within blood vessels.
3)
Gills or lungs provide efficient gas exchange.
4)
The kidneys efficiently excrete nitrogenous waste and regulate water.
5)
Reproduction is usually sexual with separate sexes.
A. Vertebrate Evolution
1. Chordates (including vertebrates) appeared at the start of the Cambrian period.
2. The earliest vertebrates were fishes; most of which have jaws.
3.
Jawed fish and other vertebrates are gnathostomes—animals with jaws.
4.
5.
6.
Fish also had a bony skeleton, lungs, and fleshy fins, which were preadaptive for a land existence.
Amphibians are the first vertebrates to live on land and to have four limbs (tetrapods).
Amphibians are not fully adapted to living on land because they still have to reproduce in an aquatic
environment.
7. Reptiles are fully adapted to living on land because they produce an amniotic egg.
8. Amniotes develop within an aquatic environment but of their own making.
9. In placental mammals, the fertilized egg develops inside the female, where it is surrounded by an
amniotic membrane.
10. Another feature for living on land includes watertight skin, and can be seen in reptiles and mammals.
29.3 The Fishes
A. Jawless Fishes
1. Small, jawless, and finless ostracoderms are the earliest vertebrate fossils.
2. Today’s jawless fishes, or agnathans have a cartilaginous skeleton and persistent notochord.
3. They have smooth nonscaly skin.
4. They have cylindrical bodies and are up to a meter long.
5. Many lampreys are filter feeders similar to their ancestors.
6. Parasitic lampreys have a round muscular mouth equipped with teeth; they attach themselves to fish
and suck nutrients from the host’s circulatory system.
7. Marine parasitic lampreys entered the Great Lakes and devastated the trout population in the 1950s.
B. Fishes with Jaws
1. Fishes with jaws have:
a. Ectothermy—they depend on the environment to regulate their body
temperature.
b.
Gills are used for gas exchange.
1) Jawed fish have a single-looped, closed circulatory pathway with a heart that pumps the blood
first to the gills (for oxygen exchange) and then to the rest of the body.
213
c. Cartilaginous or bony skeleton—the endoskeleton of jawed fishes include
vertebral column, skull with jaws, and paired pectoral or pelvic fins.
1) Jaws evolved from the first pair of gill arches of agnathans; the second pair of arches became
support structures for the jaws.
d. Scales cover and protect the fish skin.
2. Placoderms are extinct jawed fishes of the Devonian Period.
a. They were armored with heavy plates and had strong jaws.
b. Like extant fishes, they had paired pectoral and pelvic fins.
c. Paired fins allow a fish to balance and maneuver well in water; this helps predation.
C. Cartilaginous Fishes
1. Sharks, rays, skates, and chimaeras are marine cartilaginous fishs (chondrichthyes).
2. They have a cartilaginous skeleton rather than bone.
3. Five to seven gill slits are on both sides of the pharynx; they lack the gill covers found on bony fish.
4. They have many openings to the gill chamber located behind the eyes called spiracles.
5. Their body is covered with dermal denticles.
6. The teeth of sharks are enlarged scales; there are many rows of replacement teeth growing behind the
front teeth.
7. They have three well developed senses to detect electric currents in water, pressure (a lateral line
system), and smell.
8. The largest sharks are filter feeders, not predators; the basking and whale sharks eat tons of crustacea.
9. Most sharks are fast, open-sea predators; a great white shark eats dolphins, sea lions and seals.
10. Rays and skates live on the ocean floor; their pectoral fins are enlarged into winglike fins and they
swim slowly.
11. Stingrays have a venomous spine.
12. Electric rays feed on fish that have been stunned with an electric shock that may reach over 300 volts.
13. Sawfish rays have a large anterior “saw” that they use to slash through schools of fish.
14. Chimaeras (ratfishes) live in cold marine waters and are known for their unusual shape and iridescent
colors.
D. Bony Fishes
1. There are about 25,000 species of bony fishes (osteichthyes).
2. Bony fishes have a skeleton of bone; most are ray-finned with thin, bony rays supporting the fins.
3. The gills of bony fishes do not open separately but instead are covered by an operculum.
4. The swim bladder is a gas-filled sac whose pressure can be altered to regulate buoyancy and depth.
5. Bony fish have a single-loop, closed cardiovascular system.
6. They have a well-developed nervous system.
7.
Fish sperm and eggs are usually shed into water.
8. For most fish, the fertilization and embryonic development occur outside the female’s body.
9. The lobe-finned fishes include six species of lungfishes and two species of coelacanth.
a. Their fleshy fins are supported by central bones.
b. Lungfishes have lungs and gills for gas exchange.
c. Lungfishes and lobe-finned fishes are grouped together as sarcopterygii.
d. Lungfishes live in stagnant fresh water or in ponds that dry up annually; they are found in Africa,
South America, and Australia.
e. Coelacanths live in deep oceans; once considered extinct, more than 200 have been captured since
1938.
29.4 The Amphibians
1. Today’s amphibians are tetrapods; they have four limbs.
a. The skeleton is well-developed for locomotion.
2. Amphibians have smooth and nonscaly skin.
a. The moist skin plays an active role in water balance, respiration, and temperature regulation.
3. Amphibians usually have small lungs supplemented by gas exchange across porous skin.
4. The single-loop circulatory path of fish is replaced by a closed double-loop circulatory system;
however oxygen-rich blood mixes with some oxygen-poor blood.
a. A three-chambered heart with a single ventricle pumps mixed blood before and after it has gone to
the lungs.
5.
Amphibians have sense organs that are adapted to life on land.
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a.
b.
c.
d.
The brain is larger than that of fishes; their cerebral cortex is more developed.
A specialized tongue is used for catching prey.
The eyelids keep their eyes moist.
Amphibian ears are adapted for detecting sound waves; in turn, the larynx produces
sounds.
6.
Amphibians are ectothermic, depending upon the environment to regulate body temperature.
a. If winter temperature drops too low, temperate ectotherms become inactive and enter torpor.
7. Amphibians return to the water to reproduce.
a.
They shed eggs into the water for external fertilization.
b.
Generally, amphibian eggs are protected by a coat of jelly but not by a shell.
c.
The young hatch into aquatic larvae with gills (tadpoles).
d.
The aquatic larvae usually undergo metamorphosis to develop into a terrestrial
adult.
e. Some amphibians evolved mechanisms that allow them to bypass the aquatic larval stage and
produce on land.The lobe-finned fishes of Devonian are ancestral to amphibians.
A. Evolution of Amphibians
1. Amphibians evolved from the lobe-finned fishes with lungs by way of transitional forms.
2. Two hypotheses describe evolution of amphibians from lobe-finned fishes.
a. Lobe-finned fishes that could move from pond-to-pond had an advantage over those that could
not.
b. The supply of food on land and the absence of predators promoted adaptation to land.
3. Paleontologists found a fossil, Tiktallik roseae, from the late Devonian period in Arctic Canada that
represents an intermediate between lobe-finned fishes and tetrapods with limbs.
B. Diversity of Living Amphibians
1.
Modern amphibians include three groups: salamanders and newts, frogs and toads, and caecilians.
2.
Salamanders and newts have a long body and tail, and two pairs of legs
3.
Their S-shaped locomotion is similar to fish movements.
4.
Salamanders and newts are carnivorous, feeding on insects, snails, etc.
5. Salamanders practice internal fertilization; the males produce a spermatophore that females pick up
with the cloaca (the common receptacle for the urinary, genital, and digestive canals).
6. Frogs and toads are tailless as adults; the hind limbs are specialized for jumping.
7. Some skin glands secrete poisons; those tropical species often have brilliant warning coloration.
8. Frogs and toads have the head and trunk fused; frogs live near or in fresh water while toads live in
damp places away from water.
9. Caecilians are legless; most burrow in soil and feed on worms, etc.
29.5 The Reptiles
1. Reptiles (class Reptilia) are a successful group of terrestrial animals.
2. Reptiles have many characteristics showing that they are fully adapted to living on land.
a. Reptiles have paired limbs and are adapted for climbing, running, paddling, or flying.
b.
Reptiles have a thick, scaly skin that is impermeable to water.
1)
Reptile’s protective skin prevents water loss but it also requires several molts a year.
c. Reptile have efficient breathing - their lungs are more developed than in amphibians; air
rhythmically moves in and out of the lungs due to an expandable rib cage.
d. Reptiles have efficient circulation. The heart prevents mixing of blood. Oxygen-rich blood is
more fully separated from oxygen-poor blood.
e. Reptiles are ectothermic.
1) They require a fraction of the food per body weight of birds and mammals.
2) They are behaviorally adapted to warm their body temperature by sunbathing.
f. Reptiles have well-adapted reproduction.
1)
The sexes are separate and fertilization is internal.
2)
The amniotic egg contains extraembryonic membranes.
3) Extraembryonic membranes are not a part of the embryo and are disposed of after
development.
4)
They protect the embryo, remove nitrogenous wastes, and provide oxygen, food, and
water.
5) The amnion is one extraembryonic membrane; it fills with fluid to provide a “pond” for
embryo to develop.
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A. Evolution of Amniotes
1.
The amniotes consist of three lineages:
a.
The turtles, in which the skull has no openings behind the orbit—eye socket.
b.
All the other reptiles, in which the skull has two openings behind the orbit.
c.
The mammals, in which the skull has one opening behind the orbit.
2.
The reptiles have no common ancestor; they are a paraphyletic group and not a
monophyletic group.
3.
Other reptiles are diapsids have a skull with two openings behind the eyes.
4. Thecodonts are diapsids that gave rise to the ichthyosaurs, which returned to the aquatic
environment, and the pterosaurs, which were terrestrial.
a.
The pterosaurs of the Jurassic period had a keel for attachment of flight muscles
and air spaces in bones to reduce weight.
b.
The thecodonts gave rise to the crocodiles and dinosaurs.
5. Dinosaurs varied in size and behavior; some had a bipedal stance and gave rise to birds.
6. Dinosaurs dominated earth for about 170 million years; then most died out at the end of the Cretaceous
period.
7. One theory of mass extinction:
a. A large meteorite or comet at the end of the Cretaceous Period could have set off earthquakes and
fires, raising enough dust and smoke to block out the sun.
b. An iridium layer, a mineral common in meteorites, occurs in rocks at the end of this period.
8. Diversity of Living Reptiles
a. Most reptiles today live in the tropics or subtropics; lizards and snakes live on soil; turtles,
crocodiles, and alligators live in water.
b. Turtles have a heavy shell fused to the ribs and to the thoracic vertebrae.
1) Turtles lack teeth but use a sharp beak.
2)
Sea turtles must return to lay eggs onshore.
c. Lizards have four clawed legs and are carnivorous.
1) Marine iguanas on the Galápagos Islands are adapted to spend long times in the sea.
2) Chameleons live in trees, have a long sticky tongue to catch insects, and change color.
3) Geckos are nocturnal and have adhesive pads on their toes.
4) Skinks have reduced limbs and shiny scales.
d. Snakes evolved from lizards and lost legs as an adaptation to burrowing.
1) Their jaws can readily dislocate to engulf large food.
2) A tongue collects airborne molecules to transfer them to Jacobson’s organ for tasting.
3)
Some snakes are poisonous and have special fangs to inject venom.
4)
Snakes have internal ears that can detect low-frequency sounds and vibrations.
e. Tuataras are lizardlike animals found in New Zealand.
1) They possess a well-developed “third eye,” which is light sensitive and buried beneath the
skin in the upper part of the head.
2) They are the only member of an ancient group of reptiles that incuded the common ancestor
of modern lizards and snakes.
f. Crocodiles and alligators are largely aquatic, feeding on fishes and other animals.
1) Their powerful jaws have numerous teeth; a muscular tail is both a paddle to swim and a
weapon.
2) Male crocodiles bellow to attract mates; males of some species protect the eggs and young.
B. Birds
1. Birds share a common ancestor with crocodilians and have scales (feathers are modified scales), a tail
with vertebrae, and clawed feed.
2. Feathers keep birds warm, and help birds fly and steer.
a. Feathers are modified scales.
b. Birds molt and replace their feathers annually.
3. Birds have a modified skeleton.
a. The collarbone fused and sternum has a keel.
b. Other bones are fused to make the skeleton more rigid than the reptilian skeleton.
c. The breast muscles are attached to the keel.
4. Birds have modified respiration.
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a.
Bird respiratory air sacs are extensive, even extending into some larger bones.
1) Using a one-way flow of air, air sacs maximize gas exchange and oxygenation of blood.
2) Efficient supply of oxygen to muscles is vital for the level of muscle activity needed for flight.
5. Brids are endothermic; they have the ability to maintain a constant, relatively high body temperature.
6. Birds have well-developed sense organs and nervous system.
a. Birds have very acute vision.
b. Birds muscle reflexes are excellent.
c. Bird migration allows use of widespread food sources; an enlarged portion of the brain is
responsible for instinctive behaviors.
7. Diversity of Living Birds
a. Most birds can fly; some, however, are flightless.
b. Bird classification is based on beak and foot types, and some on habitats and behaviors.
1) Birds of prey have notched beaks and sharp talons.
2) Shorebirds have long slender bills and long legs.
3) Waterfowl have webbed toes and broad bills.
C. Vertebrates and Human Medicine (Science Focus box)
1. There are many pharmaceutical products that come from vertebrates.
a. The venom of the Thailand cobra is the source of Immunokine, which has been used for ten years
in multiple sclerosis patients.
b. ABT-594 from the poison dart frog is about 50 times more powerful than morphine without the
addictive properties.
2. Animal pharming uses genetically altered vertebrates (mice, sheep, goats, cows, chickens, pigs) to
produce medically useful pharmaceutical products.
a. The human gene for some useful product is inserted into the embryo of the vertebrate.
b. That embryo is implanted into a foster mother, which gives birth to the transgenic animal.
3. Xenotransplantation is the transplantation of nonhuman vertebrate tissues and organs into humans.
a. The use of transgenic vertebrates for medical purposes raises many health and ethical concerns.
29.6 The Mammals
1. The following characteristics distinguish mammals:
a. Hair
1)
Hair provides insulation against heat loss.
2)
Hair color can provide camouflage to blend into its surroundings.
3)
Hair can serve sensory functions.
b. Mammary glands
1)
Mammary glands enable females to feed young without deserting them to obtain food.
2) Nursing creates a bond between mother and offspring to ensure parental care while the young
are helpless.
c. Skeleton
1) The mammal skull is bigger than reptiles’, their teeth are differentiated into molars and
premolars, and the vertebral column provides more movement.
d. Internal organs
1)
Gas exchange is efficiently accomplished by lungs.
2) Mammals possess a four-chambered heart and a double-loop circulatory system.
3) Kidneys are adapted to conserving water.
4) The nervous system and sensory organs are highly developed.
e. Internal development
1)
In most mammals, the young are born alive after a period of development in uterus.
A. Evolution of Mammals
1. Mammals evolved during the Mesozoic Era from mammal-like synapsids.
2. True mammals appeared during the Triassic period, about the same time as the first dinosaurs.
a. The first mammals were small, about the size of mice.
b. Some of the earliest mammalian groups were monotremes and marsupials.
c. Placental mammals evolved later to occupy habitats vacated by dinosaurs.
3. Monotremes
a. Monotremes are mammals that have a cloaca and lay hard-shelled amniote eggs.
b. They are represented by the duckbill platypus and two species of the spiny anteaters.
c. A female duckbill platypus lays her eggs in a burrow in the ground where she incubates them.
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d.
e.
4.
5.
6.
After hatching, the young lick milk seeping from modified sweat glands on the abdomen.
The spiny anteater has a pouch formed by swollen mammary glands and muscle; the egg moves
from cloaca to pouch and hatches; the young remain for 53 days and live in the burrow where the
mother feeds them.
Marsupials
a. Marsupials begin development inside the mother’s body but are then born in a very immature
state.
b. The newborns crawl up into a pouch on their mother’s abdomen.
c. Inside a pouch they attach to the nipples of the mother’s mammary glands and continue to
develop.
d. Today, most marsupials are found in Australia where they underwent adaptive radiation for
several million years without competition from the placental mammals, only introduced recently.
Placental Mammals
a. Developing placental mammals are dependent on a placenta, an organ of exchange between
maternal and fetal blood.
b. The placenta supplies nutrients to and removes wastes from the blood of developing offspring.
c. A placenta also allows a mother to move about while the offspring develop.
d. The placenta enables young to be born in a relatively advanced stage of development.
e. Placental mammals are very active animals; they possess acute senses and a relatively large brain.
f. The brains of placental animals have cerebral hemispheres proportionately larger than other
animals.
g. The young go through a long period of dependency on their parents after birth.
h. Most are terrestrial, but some are aquatic, and bats can fly.
The main types of placental animals are:
a. Ungulates are hoofed animals, which make up about 1/3 of all living an extinct mammal groups.
1) They have a reduced number of toes and divided according to whether an odd or even number
of toes remain.
2)
They have elongated limbs adapted for running across open grassland.
3)
They are herbivorous and have large grinding teeth.
b. Carnivores are meat eaters with large and conical-shaped canine teeth.
1) Aquatic carnivores such as seals and sea lions must return to land to reproduce.
c. Primates tree-dwelling fruit eaters.
1) Their digits have nails, not claws; the thumb is more opposable.
2) Primates, particularly humans, have well-developed brains.
d. Cetaceas are marine whales and dolphins.
1)
They lack substantial hair or fur.
2)
Baleen whales that strain plankton from the water.
3)
Toothed whales feed on fish and squid.
e. Chiroptera are nocturnal flying bats.
1) Wings are layers of skin and connective tissue stretched between the elongated bones of all
fingers but the first.
2) Many species use echolocation to locate their usual insect prey.
3) Some bats also eat birds, fish, frogs, and plant tissues.
f. Rodents are most often small plant eaters.
1)
Rodents have incisors that grow continuously.
g.
Probiscideans are the herbivorous elephants.
1)
They are the largest living land mammals.
2)
The upper lip and nose have become elongated and muscularized to form a
trunk.
i.
h.
Lagomorphans are rabbits, hares, and pikas.
1) They resemble rodents but have two pairs of continuously growing incisors.
2) Their hind legs are longer than their front legs and they are herbivores.
Insectivores include the shrews and moles, mammals with short snouts that live underground.
1)
Insectivores were thought to be most like the original placentas. However, recent analysis
suggests that the edentates (anteaters) and the pangolins (scaly anteaters) are the more primitive
groups of placentals.
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Lecture Enrichment Ideas
Experience Base: Many students will be familiar with the basic biology of those mammals that
are common domestic pets or, in rural areas, farm animals. Being a mammal, we can understand
a large part of mammalian physiology, at least on the superficial level, based on our own
physiology.
1. Contrast features shared by vertebrates and arthropods that make them so successful. If harsh
environmental conditions arose, which would likely survive longest?
2. Consider that the smallest terrestrial vertebrates (shrews) begin in size where the largest
terrestrial arthropods stop. Query students why these scales to not overlap. On the other hand,
aquatic crustacea do grow much bigger than the large grasshopper size limit that terrestrial
arthropods face. Again, ask why.
3. Compare the “new” evolutionary characteristics of jawed fishes, amphibians, and reptiles
that make them more successful than their predecessors at competing to fill a habitat.
4. Most of these organisms are far more interesting to students if you have good visual images
of them, particularly the more exotic animals.
5. Discuss the evolutionary significance of continental drift in the development of monotremes,
marsupials, and placental mammals. Discuss the fact that South America was inhabited by
marsupials until relatively recently (within the last 20,000 years or so), when the land bridge
formed between North and South America, which allowed placental mammals to reach South
America and led to the extinction of many marsupials. Consider whether the same thing is
happening in Australia now, since humans have introduced dogs, rats, rabbits, etc.
Critical Thinking
Question 1.
Aristotle would have classified birds, bats, and insects together as animals of the air. He would
have placed earthworms and burrowing snakes together. Why don’t we?
Answer: The evolution of wings in insects and birds do not involve the same structures. Nor do the superficial
similarities in earthworms and snakes have the importance of the many other features that place earthworms with
invertebrates and snakes with vertebrates. The major embryological and structural groupings based on coelom,
blastopore, skeleton, etc., correlate with a large number of other similarities—structural, physiological and
molecular—as well as match the fossil record.
Question 2.
Of the chordates that are not vertebrates, which is most likely closest related to the vertebrates?
Answer: The tunicates only display chordate characteristics in the larval stage; the lancelets display all four
chordate traits as adults and they display a fishlike body.
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219
CHAPTER
30
HUMAN EVOLUTION
This chapter describes the evolution of primates, beginning with physical characteristics unique
to primates, and moving through the evolutionary process ultimately leading to Homo sapiens.
Evidence of evolution through fossilized remains is presented. A Science Focus box describes
the origins of the genus Homo and an Ecology Focus box discusses biocultural evolution that
began with Homo.
Chapter Outline
30.1 Evolution of Primates
A. Primate Characteristics
1. Primates differ from other mammals by being adapted for arboreal life (living in
trees).
2. Mobile Forelimbs and Hindlimbs
a. In primates, the limbs are mobile and the hands and feet have five digits each.
b. In most primates, flat nails replace claws and sensitive pads develop on the
underside of fingers and toes.
c. Many primate hands have an opposable thumb (i.e., can touch each of the other
digits); some also have an opposable big toe.
d. These features allow the free grasping of tree limbs and easy harvesting of fruit.
3. Stereoscopic Vision
a. Primates have a reduced snout and the face is relatively flat.
b. The sense of smell is generally reduced.
c. The eyes are moved to the front of the face for overlapping views that provide
stereoscopic vision.
d. Cone cells provide greater visual acuity and color vision but require bright light.
4. Large, Complex Brain
a. Better senses requires both sense organs and a more complex brain to process the
input.
b. More of the brain becomes devoted to processing information received from the
hands and thumb, less to smell.
5. Reduced Reproductive Rate
a. Primates have more single births, which reduces the need for care for several
offspring.
b. The period of parental care is extended with an emphasis on learned behavior and
complex social interactions.
B. Sequence of Primate Evolution
1. Hominins are humans and species that closely resemble humans.
a. They first evolved about 6 MYA.
b. Hominins, chimpanzees, and gorillas shared a common ancestor during the
Miocene period.
2. Hominines include hominins, chimpanzees, and gorillas.
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3. Hominids include hominines and orangutan.
4. Hominoids include hominids and gibbon.
a. The hominoid common ancestor first evolved about 23 MYA.
5. Anthropoids include hominoids and the Old World monkeys and New World
monkeys.
a. Old World monkeys don’t have tails and have protruding noses.
1)
Examples of Old World monkeys are the baboon and the rhesus monkey.
b. New World monkeys have long prehensile (grasping) tails and flat noses.
1)
Examples of New World monkeys are the spider monkey and the
capuchin.
6. Primate fossils similar to monkeys are first found in Africa, about 45 MYA.
7. Prosimians (lemurs and tarsiers) were the first type of primate to diverge from the
common ancestor for all primates.
8. Primates share one common mammalian ancestor, which lived about 55 MYA.
9. Hominoid Evolution
a. About 15 MYA, dozens of hominoid species arose.
b. Proconsul was one species that lived at this time, and it is believed to be ancestral
to the dryopithecines, from which the hominoids arose.
c. About 10 MYA, Africarabia joined with Asia, and the hominoids migrated into
Europe.
d. During this period, two ancestral groups of primates were the dryomorphs and the
ramamorphs, the latter now believed to be the ancestral orangutan.
e. Dryopithecus was a tree-dweller that moved similar to orangutans but did not
walk along tree limbs as did Proconsul.
30.2 Evolution of Early Hominins
1. Molecular data is used to determine the beginning of hominin evolution.
2. When two lines of descent first diverge from a common ancestor the genes of the two
lineages are nearly identical.
3. However, as time passes, each lineage accumulated genetic changes.
4. Genetic changes suggest that hominin evolution began about 7 MYA.
A. Derived Characters of Hominins
1. Humans and chimpanzees have many traits in common.
2. Several distinct differences exist. In humans:
a. The skull is in the midline of the body.
b. The longer curved spine places the center of gravity over the feet.
c. The broader pelvis and hip joints prevent swaying when walking.
d. A longer neck on the femur in humans causes the femur to angle inward at the
knees.
e. The human knee joint is modified to support the body’s weight.
f. The human toe is not opposable but the foot has an arch for long distance
walking.
B. The Early Hominins
1. Until recently, science thought that the climate changed forests into savannas; there is
little evidence of a shift in vegetation at 7 MYA.
2. Additional advantages of bipedalism include reduction of heat stroke and carrying
food back to females.
3. While still living in trees, the first hominids may have walked upright on two feet
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(bipedalism) to collect overhead fruit.
4. Early Hominid Fossils
a. The braincase of Sahelanthropus tchadensis has been dated at 7 MYA.
b. Skull fragments from Ardipithecus ramidus have been found and dated at 4.5
MYA. It was likely bipedal.
c. Fossils dated 4 MYA show a direct link between A. ramidus and australopiths.
30.3 Evolution of Later Hominins
1. Australopithecines (called australopiths for short) evolved in Africa 4 MYA.
2. Expanding fossil records show it is not an orderly sequence between forms.
3. Australopithecines evolved and diversified in Africa with gracile and robust forms
with varied diets; they show adaptations to different ways of life.
4. They were apelike above the waist and humanlike below the waist; human
characteristics probably did not evolve all together at the same time. This is an
example of mosaic evolution.
A. The Fossils
1. Australopithecus africanus, described by Raymond Dart in the 1920s, is a gracile
type from southern Africa.
2. Abundant fossils of A. africanus date about 2.8 MYA.
3. Paranthropus robustus was a robust type; it had a brain size of 500 cc similar to A.
africanus and dated from 2 to 1.5 MYA.
4. Both had forelimbs longer than hindlimbs but probably walked upright.
5. Africanus had a larger brain and is the best candidate as ancestor to early Homo.
6. Australopithecus afarensis is based on many skeletal fragments (Lucy) dated at 3.18
MYA.
a. Its brain was small at 400 cc.
b. This may have been ancestral to the robust types, P. aethiopicus and P. boisei,
that later died out.
c. They may be the species that left the Laetoli footprints in volcanic ash about 3.7
MYA.
d. This species is thought to have stood upright and walked bipedally.
e. It is possible that Australopithecus afarensis is ancestral to early Homo.
7. A 3.3-million-year-old juvenile A. afarensis was found in 2000, just 4 km away from
where Lucy had
been discovered.
a. This skeleton is often called “Lucy’s baby,” even though she is tens of thousands
of years older than Lucy.
b. This skeleton also represents the most complete A. afarensis fossil to date.
8. A fossil, A. garhi, may be the transitional link between the australopiths and the early
Homo species.
a. A. garhi is an australopith, but it made tools.
B. Origins of the Genus Homo (Science Focus box)
1. The genus Homo evolved from the genus Australopithecus.
2. The brain size of the two are very different: Homo has the brain about the size of a
grapefruit, while Australopithecus has a brain the size of an orange.
3. In Homo, the newborn has a high rate of fetal brain growth through the first year after
birth, while Australopithecus does not.
4. However, some negatives that go along with a larger brain size include: weak and
uncoordinated newborns, unable to cling to mother.
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5. This evolutionary compromise must have outweighted the negative aspects of infant
helplessness or else Homo would have never evolved.
6. As genetic information is increasing, we may discover a particular gene or gene
combinations that cause Homo to have a large brain.
30.4 Evolution of Early Homo
1. Fossils are assigned to the genus Homo based on the following traits:
a. brain size 600 cc or greater;
b. jaw and teeth are human-like; and
c. tool use seems evident.
2. Homo ergaster and Homo erectus
a. Homo ergaster evolved in Africa perhaps from H. rudolfensis.
b. Homo erectus fossils were found in Asia different than those of H. ergaster and
H. rudolfensis.
c. Fossils found in Africa, Asia, and Europe date between 1.9 and 0.3 million years
ago.
d. H. erectus had a brain capacity of 1000 cc, a fatter face, and a projecting nose.
e. H. erectus fossils found in Java and the Republic of Georgia at 1.9 MYA and 1.6
MYA indicate an early migration from Africa, followed by H. erectus evolving in
Asia and spreading to other areas.
f. These are the first hominids to use fire, fashion more advanced tools, to be
systematic game hunters, and possibly to use home bases.
g. Fossil remains of Homo floresiensis were discovered in 2004 on the island of
Flores in the South Pacific; it was the size of a three-year-old human being but
with a braincase only one-third the size.
1) A study in 2007 supports that H. floresiensis evolved from normal-sized,
Homo erectus populations that reached Flores about 840,000 years ago
2) H. floresiensis used tools and fire.
A. Biocultural Evolution Began with Homo (Ecology Focus box)
1. Homo habilis, from Africa, made simple stone tools (Oldowan tools) for hammering,
chopping, and digging.
a. Their diet most likely consisted of eating plants and the occasional animal.
2. Homo erectus, from Eurasia, made sharper stone tools (Acheulian tools).
a. They made a wider variety of tools using many types of sources compared to H.
habilis.
b. They ate plants and ate more meat than H. habilis.
c. H. erectus also made fire for cooking, protection, warmth.
d. H. erectus are considered hunter-gatherers—they hunted animals and gathered
plants.
3. Hunting most likely encourages the development and spread of culture between
individuals and generations.
4. The cultural achievements of H. erectus began a new phase of human evolution,
called bio-cultural evolution—in which natural selection is influenced by cultural
achievements rather than by anatomic phenotype.
30.5 Evolution of Later Homo
 Two contradicting hypotheses are suggested about the origin of modern humans:
1. The replacement model or out-of-Africa hypothesis states that modern humans
originated only in Africa and after migrating into Europe and Asia, they replaced the
223
archaic Homo species found there; current evidence leans toward this hypothesis.
a. All extant humans are descended from a few individuals from about 100,000
years ago.
b. Mitochondrial DNA analyses indicate a close genetic relationship among all
Europeans; although the first analysis was flawed, the data tend to support the
out-of-Africa hypothesis.
2. The multiregional continuity hypothesis proposes that modern humans originated
separately in Asia, Europe, and Africa.
a. If valid, then a distinctive continuity in anatomy and genetic variation is expected
in each location.
b. Evolution of modern humans would be essentially similar in several different
places.
A. Neandertals
1. Neandertals were named for Neander Valley in Germany where skeletons were dated
as early as 200,000 years ago.
2. Neandertals are classified as Homo neandertalensis.
3. Classic Neandertal anatomy includes massive brow ridges; a nose, jaws, and teeth
that protruded forward; a low sloping forehead; a lower jaw sloping back without a
chin; a longer pubic bone; a slightly larger brain than that of modern humans; shorter
and thicker limb bones; and heavier muscles in the shoulder and neck.
4. It is speculated that a larger brain than that of modern humans was required to control
the extra musculature.
5. The sturdy build of Neandertals was likely an adaptation to cold climate; they lived in
Eurasia during the last Ice Age.
6. The Neandertals give evidence of being culturally advanced.
a. Most lived in caves, but those who lived in the open may have built houses.
b. They manufactured a variety of stone tools, including spear points, scrapers, and
knives.
c. They used and could control fire, which probably helped in cooking frozen meat
and in keeping warm.
d. They buried their dead with flowers and tools and may have had a religion.
B. Cro-Magnons
1. Cro-Magnons are the oldest fossils to be designated H. sapiens; they were found in
Eurasia 100,000 years ago.
2. Cro-Magnons are named for a fossil location in France and had a thoroughly modern
appearance.
3. They were hunter-gatherers like H. erectus.
4. They had advanced stone tools and may have been the first to throw spears.
5. Cro-Magnons hunted cooperatively, and perhaps were the first to have had a
language.
6. They may have been responsible for the extinction of large mammals during the late
Pleistocene.
7. Cro-Magnon culture included figurines carved out of bone and antler, and cave
paintings.
C. Human Variation
1. Some human variation evolved as adaptation to local environmental conditions:
darker skin to protect from UV light, lighter skin for vitamin D production, etc.
224
2. A bulkier body also benefits in colder regions while hot climates favor a slight build
and longer limbs.
3. Hair texture, eyelid fold, and other traits are not explained as adaptations.
4. Variation among modern populations is considerably less than among archaic human
populations of 250,000 years ago.
5. Comparative studies of mDNA indicate that human populations had a common
ancestor no more than a million years ago.
6. The great majority of genetic variation, about 85%, occurs within ethnic groups, not
among them.
7. Genetic Evidence for a Common Ancestry
a. The replacement model suggests that modern humans have a relatively recent
common ancestor who evolved in Africa then spread to other regions.
b. Studies with mitochondrial DNA show that differences among human populations
are consistent with their having a common ancestor no more than a million years
ago.
Lecture Enrichment Ideas
Experience Base: New discoveries requiring reinterpretation of our phylogeny will cause this
chapter to have new or different concepts from what students may have learned in high school
biology. While the changes may seem casual and arbitrary, there is a consistency and complex
rationale for the evolutionary schema for recent humans that students can understand if it is not
presented in a cynical or cavalier way. Visuals of the growing number of fossils will help
students see the distinctions described in this chapter. There may be a small portion of the
students who may have some concerns with this chapter based on religious background.
Forthright and clear explication of the current status of our science knowledge may help.
1. Discuss why it is incorrect to say that humans descended from apes, since humans and apes
are both descended from a common ancestor that was not a modern ape or human. Point out
that if you go back far enough, we also share a common ancestor with a dog, a lizard, a frog,
a fish, an earthworm, a plant, or a bacterium. However, it makes no sense to say that one
modern living animal species is descended from another modern living animal species.
2. Discuss how the development of language and culture makes the human so different from our
closest relative, the chimpanzee.
3. Query students for what makes humans unique. Tool use? Language? Hand grip? The ability
to know what we know (mental compartmentalization)?
4. Should evolution be taught in high school, and, indeed, college without appropriate time
being given to creationistic ideas? This subject is certain to elicit robust discussion.
Critical Thinking
Question 1. Why is it difficult to pinpoint exactly when humans evolved from nonhuman
ancestors?
Answer: 1) It is difficult to define exactly what constitutes human uniqueness. 2) The
225
evolutionary process was gradual. 3) The fossil record is not continuous. 4) Not all body parts
evolved at the same rate (mosaic evolution).
Question 2. Author Elaine Morgan has championed the aquatic ape theory that provides an
alternative scenario for human evolution: there was an aquatic wading stage that accounts for our
hair pattern, diving reflex, upright stance, etc. What evidence would be required for scientists to
shift from the savannah hunter-scavenger scenario to the aquatic ape theory?
Answer: Fossil evidence would be desirable but there may be little fossil evidence or tools left in
such erodible shoreline environments. Evidence of other apes with aquatic habits might help.
Most deductions for this new theory are extrapolated from current anatomical and physiological
comparisons.
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226
PART
VII
COMPARATIVE ANIMAL BIOLOGY
Animal organization and homeostasis provide a context for describing animal systems detailed in the following
chapters. This comparative approach concludes with reproduction and the development of various selected animals.
31
32
33
34
35
36
37
38
39
40
41
42
Animal Organization and Homeostasis
Circulation and Cardiovascular Systems
Lymph Transport and Immunity
Digestive Systems and Nutrition
Respiratory Systems
Body Fluid Regulation and Excretory Systems
Neurons and Nervous Systems
Sense Organs
Locomotion and Support Systems
Hormones and Endocrine Systems
Reproductive Systems
Animal Development
CHAPTER
31
ANIMAL ORGANIZATION AND HOMEOSTASIS
This chapter presents a detailed study of the various types of tissues found in animals. The section Organs and
Organ Systems focuses primarily on the structure and function of the skin. The chapter concludes with a description
of the homeostasis and homeostatic control mechanisms (negative and positive feedback); examples are given of
each. A Health Focus box discusses state-of-the-art knowledge on nerve regeneration.
Chapter Outline
31.1 Types of Tissues
A tissue is composed of specialized cells of the same type that perform a common function in the body.
A. Four Major Types of Tissue
1. Epithelial tissue covers body surfaces and lines body cavities.
2. Connective tissue binds and supports body parts.
3. Muscular tissue causes body parts to move.
4. Nervous tissue responds to stimuli and transmits impulses.
B. Epithelial Tissues
1. Epithelial tissue forms a continuous layer over the body surfaces including inner cavities.
2. Epithelial tissue cells are packed tightly; they join to one another in one of three ways:
a. Tight junctions have plasma proteins extending between neighboring cells to bind cells tightly.
b. Adhesion junctions have cytoskeletal elements joining internal plaques in neighboring cells.
c. Gap junctions form when two identical plasma membrane channels of neighboring cells join so
that ions and small molecules pass between cells.
3. Epithelial cells are exposed to the environment on one side; the other side is basement membrane, a
thin layer of various types of proteins that anchors the epithelium to underlying connective tissue.
4. Simple Epithelia
a. Squamous epithelium is composed of flat cells (e.g., air sac linings of lungs, walls of capillaries).
b. Cuboidal epithelium has cube-shaped cells.
c. Columnar epithelium has elongated cells that resemble pillars or columns (e.g., small intestine).
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d. Simple epithelium has one cell layer; all cells contact a basement membrane.
e. Pseudostratified epithelium appears layered, but actually all cells contact the basement membrane.
5. Stratified epithelia are composed of more than one layer of cells.
a. The outer layer of skin is stratified squamous epithelium, but the cells have been reinforced by
keratin.
6. Glandular epithelia
a.
A gland can be a single epithelial cell or a group of cells that secrete products into the lumen of or
onto the lining of a tube or cavity, into blood, or to outside of the body; they are classified in two
types:
1) Exocrine glands secrete their products into ducts or directly into a tube or cavity.
2) Endocrine glands secrete their product directly into the bloodstream.
C. Connective Tissue
1. Connective tissue binds structures together, provides support and protection, fills spaces, stores fat,
and forms blood cells.
2. Connective tissue provides source cells for muscle and skeletal cells in animals that regenerate parts.
3. Connective tissue cells are separated widely by a matrix, a noncellular material between cells.
4. Fibrous Connective Tissue
a. Cells of loose and fibrous connective tissues are fibroblasts.
b. Fibroblasts are spaced apart and are separated by a jelly matrix of white collagen fibers and yellow
elastic fibers.
c. Collagen fibers provide flexibility and strength; elastic fibers provide elasticity.
d. Loose fibrous connective tissue supports epithelium and provides support, flexibility, and
protective covering encasing many internal organs.
e. Adipose Tissue
1) This is loose connective tissue that insulates the body, provides protective padding, and stores
fat.
2)
In mammals, adipose tissue is beneath the skin, around the kidneys, and on surface of the
heart.
f. Dense fibrous connective tissue contains closely packed collagenous fibers; it is found in
tendons, which attach muscles to bone, and ligaments, which bind bones to other bones at joints.
5. Supportive Connective Tissue
a. Cartilage and bone are rigid connective tissues.
b. Structural proteins (cartilage) or calcium salts (bone) are deposited in an intercellular matrix.
c. Cartilage cells or chondrocytes lie in small chambers or lacunae embedded in a strong, flexible
matrix.
1) Hyaline cartilage is the most common type of cartilage; it is found in the nose, the ends of
long bones and ribs, the rings of the respiratory passages, and the human fetal skeleton.
2) Elastic cartilage is more elastic than hyaline cartilage; it is found in the outer ear.
3) Fibrocartilage has a matrix of strong collagen fibers; it is found in areas that must withstand
tension and pressure (e.g., pads between vertebrae, knee joint).
6.
Bone
1) In bone, a matrix of calcium salts is deposited around protein fibers.
2) Calcium salts give bone rigidity while protein fibers provide elasticity and strength.
3) Compact bone has cells called osteocytes that lie within lacunae arranged in concentric
circles within osteons (Haversian systems) around tiny tubes called central canals.
4) These canals contain nerve fibers and blood vessels.
5) Nutrients brought by the blood reach all of the cells via minute canals (canaliculi) containing
thin processes of osteocytes that connect them with one another and with the central canals.
6) Spongy bone at end of long bones is designed for strength, and has many long bony bars and
plates.
7. Fluid Connective Tissue
a. Blood transports nutrients and oxygen to cells and removes CO 2 and wastes; blood also has a role
in fluid, ion and pH balance and distributes heat.
b. Blood is a connective tissue with cells separated by liquid plasma.
c. In vertebrates, the blood cells are mainly of two types.
1) Red blood cells (erythrocytes) carry oxygen.
2) White blood cells (leukocytes) aid in fighting infection.
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d.
Platelets present in plasma are fragments of giant cells found in bone marrow and play a role in
blood clotting.
e. Lymph is a fluid connective tissue located in lymphatic vessels.
1) Lymphatic vessels absorb excess tissue fluid and transport it to vessels of the cardiovascular
system.
2) Lacteals, special lymphatic capillaries, absorb fat molecules from the small intestine.
3) White blood cells congregate in the lymph nodes; lymph is cleansed as it passes through.
D. Muscular Tissue
1. Muscular (contractile) tissue is composed of cells called muscle fibers.
2. Muscle fibers contain actin and myosin filaments; interactions result in animal movement.
3. The three types of vertebrate muscle tissue are skeletal, cardiac, and smooth muscle.
4. Skeletal muscle attaches by tendons to the bones of the skeleton.
a. Skeletal muscle moves body parts, is under voluntary control, and contracts faster than other
types.
b. Skeletal muscle fibers are long, cylindrical, multinucleate cells arising from the fusion of several
cells.
c. Skeletal fibers are striated due to the light and dark bands of overlapping actin and myosin
filaments.
5. Smooth (visceral) muscle is not striated.
a. Spindle-shaped fibers form layers with the thick middle portion of one fiber opposite the thin ends
of adjacent fibers.
b. The nuclei form an irregular pattern in the tissue.
c. Smooth muscle is not under voluntary control; it is therefore involuntary.
d. Smooth muscle is found in the walls of viscera (e.g., intestine, stomach, etc.) and blood vessels.
e. Smooth muscles drive the intestinal contractions and blood vessel constrictions.
6. Cardiac muscle is found only in the heart wall and powers the heartbeat that pumps blood.
a. Cardiac muscle combines the features of both smooth and skeletal muscle.
b. Unlike skeletal muscles with many nuclei, cardiac muscles have one centrally placed nucleus.
c. Although it appears to be one mass of muscle fibers, the cardiac muscle fibers are individual cells.
d. Cardiac muscle cells are bound end-to-end at intercalated disks where the folded membranes
between two fibers contain desmosomes and gap junctions
e. Impulses move from cell to cell so the heartbeat is coordinated.
E. Nervous Tissue
1.
2.
F.
Nervous tissue contains neurons in the brain, spinal cord, and nerves.
A neuron has three parts.
a. Dendrites receive a stimulus and conduct signals to the cell body.
b. The cell body contains most of the cytoplasm and the nucleus of the neuron.
c. The axon conducts nerve impulses away from the cell body; long axons are covered by myelin.
d. Long axons and dendrites form neuron fibers; bound together by connective tissue, they form
nerves.
e. Outside the brain and spinal cord, fibers bound by connective tissue form nerves.
3. Neuroglia
a. There are several types of neuroglia in the central nervous system.
b. Neuroglia outnumber neurons 50 to 1, and were once thought to only support or nourish neurons.
c. Microglial cells support neurons and also phagocytize bacterial and cellular debris.
d. Astrocytes provide nutrients and produce a growth factor known as glia-derived growth factor that
someday may be used to cure diseases of neural degeneration.
e. Oligodendrocytes form the myelin around an axon.
f.
Neuroglia lack long processes but communicate among themselves and with neurons.
Nerve Regeneration (Health Focus box)
1. In humans, axons cannot regenerate inside the brain and spinal cord.
2. This is especially important for central nervous system (CNS) degeneration victims, who may have
permanent loss of nervous function.
3. Currently, one effective method for paralysis victims is exercise.
4. For example, Christopher Reeves, who fell off his horse and became paralyzed from the neck down,
exercised for at least five hours a day, and after time, began to have sensation throughout his body.
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It is possible that Reeve’s advances were the result of improved strength and bone density, which lead
to stronger nerve signals.
6. Another method to regain axon regeneration may from stem cells.
7. Research has shown that both embryonic and bone marrow stem cells can differentiate into neurons in
the laboratory.
31.2 Organs and Organ Systems
Organs are combinations of two or more different tissues performing common functions.
1. An organ system contains many different organs that cooperate to carry out a process (e.g., digestion).
2. The integumentary system is composed of the skin and accessory organs (i.e., nails, hair, glands, and
sensory receptors).
A. Skin as an Organ
1. Human skin protects the underlying tissues from trauma, desiccation, radiation damage, and microbial
invasion.
2. The skin produces a precursor molecule that is converted to vitamin D after exposure to UV light.
3. The skin also helps regulate body temperature.
4. Laden with sensory receptors, the skin collects information about the external environment.
B. Regions of Skin
1. The skin has both an outer epidermal layer (epidermis) and a deeper layer (dermis); a subcutaneous
layer (hypodermis) is found between the skin and underlying structures.
2. The epidermis is the outer, thinner layer of skin.
a. The epidermis is composed of stratified squamous epithelium.
b. Epidermal cells are derived from the basal layer of stem cells that undergo continuous cell division
underneath.
c. The newly formed cells push to the surface away from their blood supply; they flatten and harden
as they accumulate keratin, a hard, waterproof protein.
d. Eventually, the keratinized cells die and are sloughed off.
e. Melanocytes located in the basal layer produce a melanin pigment that absorbs UV light,
protecting deeper cells from radiation damage; certain cells in the epidermis convert a steroid
related to cholesterol into vitamin D, a chemical required for proper bone growth.
f. Skin Cancer
1) Too much ultraviolet radiation is dangerous and can lead to skin cancer.
2) Excessive exposure to UV radiation can convert cells in the basal layer of the epidermis into
cancer cells (basal cell carcinoma); melanoma is skin cancer derived from melanocytes.
3. The dermis is fibrous connective tissue that forms a thicker and deeper layer of skin.
a. The dermis contains both elastic fibers and collagen fibers; these run parallel with the skin surface.
b. The dermis contains blood vessels that can constrict and dilate.
c. Many small sensory receptors are present in the dermis.
1) There are separate receptors for pressure, touch, temperature, and pain.
4. The subcutaneous layer is not technically a part of the skin; it is composed of loose connective tissue
and adipose cells, which store fat.
a. The subcutaneous layer lies below dermis.
b. This is composed of loose connective tissue, including adipose tissue.
c. Adipose tissue helps insulate the body by minimizing both heat gain and heat loss.
d. This layer of adipose gives a rounded appearance to the body.
e. The excessive development of adipose tissue occurs with obesity.
C. Accessory Organs of the Skin
1. Nails grow from special epidermal cells at the base of the nail in a region called the nail root.
a. The visible portion of a nail is the nail body.
b. Cells become keratinized as they grow out over the nail bed.
c. The vascular dermal tissue under the nail provides the pink color; the white half-moon area is the
thicker germinal area.
2. A hair follicle contains a nonliving hair shaft and the living hair root that produced it.
a. The hair shaft is formed of dead, keratinized epidermal cells that protect the surface of the skin.
b. The arrector pili muscle is a smooth muscle attached to the hair follicle; contracting it causes the
hair to erect.
c. Follicles have oil (sebaceous) glands producing sebum, an oil secreted to lubricate both the hair
and the skin.
5.
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3.
The sweat (sudoriferous) glands are coiled tubules present in most of the regions of skin that secrete
a fluid (sweat) onto the surface of the skin.
D. Organ Systems
 In most animals, individual organs function as part of organ systems.
 The organ systems carry out life processes common to organisms.
1. Body Cavities
a. The human body has two main cavities: the dorsal cavity holds the brain and spinal cord, and the
larger ventral cavity.
b. The ventral cavity located on the front side of the body develops from the coelom and is divided
by a muscular diaphragm in humans and other mammals.
c. The upper (thoracic or chest) cavity is located in the upper part of the ventral cavity, above the
diaphragm, and contains the heart and lungs.
d. The lower (abdominal) cavity is located in the lower part of the ventral cavity; it contains the
major portions of the digestive and excretory systems, and much of the reproductive system.
31.3 Homeostasis
Homeostasis is the maintenance of internal conditions in a cell or organism by means of self-regulating mechanisms
that curtail fluctuations above and below a normal range.
1. The organ systems of the human body contribute to homeostasis.
a. The respiratory system adds oxygen and removes carbon dioxide; the amounts are altered to meet
needs.
b. The liver removes and stores glucose as glycogen and then replaces the blood glucose levels when
they lower.
c. The hormone insulin is secreted by the pancreas to regulate glucose levels.
d. The kidneys are under hormonal control to excrete wastes and salts and to maintain blood pH.
2. Although homeostasis is controlled by hormones, it is ultimately controlled by the nervous system.
3. The brain contains centers that regulate temperature and blood pressure.
4. Regulation requires a receptor that detects unacceptable levels and signals a regulator center that can
direct an adaptive response; once normalcy is obtained, the receptor is no longer stimulated.
A. Negative Feedback
1. A negative feedback mechanism involves a response in which a variable is kept close to a particular
set point.
a. The process involves a sensor and a control center.
b. The sensor detects a change in the internal environment.
c. The control center brings about an effect to bring conditions back to normal.
d. Example: When blood pressure rises, sensory receptors signal a control center in the brain. This
center stops sending nerve impulses to the arterial walls and they relax. Once the blood pressure
drops, signals no longer go to the control center.
e. A home heating system is a mechanical example of a negative feedback mechanism.
f. Human Example: Regulation of Body Temperature
1) The sensor and control center are located in the hypothalamus.
2) When body temperature is above normal, the control center directs blood vessels in the skin to
dilate—heat is lost to the environment.
3) When body temperature is below normal, the control center directs blood vessels in the skin
to constrict—heat is conserved in the body.
B. Positive Feedback
1. A positive feedback mechanism involves output that intensifies and increases the input, thereby
increasing the process; an ever-greater change in the same direction occurs.
2. Once childbirth begins, each event amplifies; the process continues until birth occurs.
3. Positive feedback mechanisms can be harmful, e.g., when a fever causes metobolic changes that push
the fever even higher.
Lecture Enrichment Ideas
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Experience Base: This overview of systems may seem abstract but will gain more relevance as
each system is detailed in later chapters. However, the skin is covered here in detail and is
generally unappreciated as a dynamic organ and often overlooked in biology texts.
1. Show slides of various tissue types, and point out the characteristics of each specific type,
particularly the different kinds of connective tissue.
2. Emphasize that organ systems or body regions also often define medical specialties: thoracic
surgeon, brain surgeon, etc.
3. Emphasize the presence of different kinds of tissues in nearly every organ, such as
epithelium lining the stomach’s interior and exterior surfaces, muscle that allows stomach
contractions to occur, nerves that supply the impulses for those contractions, and connective
tissues such as blood and connections binding the various tissues together to form functional
organs.
4. Portray the viewpoint of a louse or flea, where a human body is a collection of ecosystems—
from a temperate forest of hair on the head, to tropical jungles in the armpits and pubic area,
to barren grasslands of a forearm. While we tend to think of the human body as uniform,
head lice and pubic lice are well aware of the climatic differences, both in temperature,
moisture, and amount of skin flakes.
5.
Most students recognize cartilage from the milky-colored end of the keel under chicken-breast meat. They may
notice that this cartilage has no blood vessels running through it. It is therefore inert compared to the bone
tissues that can be very bloody. Bone operations may even require blood transfusions.
6. Some students will not recognize that most of the red meat in the meat market is simply
muscle cells, fairly pure in lean meat but interlaced with fat tissue in marbled meat.
Generally, fat adds flavor and a major problem with fat-free foods is their decreased flavor.
What students see, and usually trim off as “fat,” is truly fat tissue.
7. The critical role of connective tissue is difficult to demonstrate without visiting the meat
processing plant or bringing in sections of articulated limbs.
8. Our knowledge of blood is intertwined with much history. U.S. troops in WWII were given
plasma instead of whole blood due to the uncertainty surrounding blood types, while the
Russians effectively transplanted blood from cadavers.
9. Major changes have occurred in our understanding of the nervous system in the last two
decades, due to new technology for exploring brain function. This includes the revelation that
neuroglial cells, not standards neurons, compose a major portion of brain tissue.
10. Describe the internal sensors that operate in maintaining blood pressure, blood volume, blood
chemistry, and other characteristics, pointing out that sensory information is not always from
the external environment and is not always consciously recognized. This has implications for
evolution, where we tend to think only of adaptations to the external environment;
adaptations to parasites and bacterial and viral invaders may be more important to humans
who have mastered their external environment with heaters, air conditioners and clothing.
Students may be aware of separate hot and cold nerves as children, due to cold water feeling
warm after coming in from playing in the snow.
11. Discuss the functions of the skin, especially its protective functions of maintaining water in
the body and preventing bacterial infections. You might discuss how burns over much of the
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body are so dangerous in eliminating these functions of the skin, and complete destruction of
all skin can be fatal. As with other conditions, we often do not appreciate the value of skin
until we see a patient that lacks it.
12. Humans—the “naked apes”—have come to utilize sweat glands more than most animals;
many students will understand the effectiveness of heat loss through evaporation if this is
related to the chilling that occurs after you get out of a shower or tub without toweling off;
for water to go from liquid droplets to evaporated gas vapor requires heat energy that is taken
from the surface of the body (the same evaporative cooling that occurs in refrigerator coils).
Most U.S. non-science students will not understand this latent heat of evaporation since most
lack a strong physics background.
Critical Thinking
Question 1. Why would more skin pigment be selected in tropical regions and less skin
pigment be preponderant in polar regions?
Answer: Tropical regions have more intense sunlight and a higher risk of cancer; melanin and other pigments help
shield delicate cells from this radiation. Skin cells, however, are also responsible for providing the precursor
molecule that is converted to vitamin D in the body by exposure to sunlight; the sunlight available in polar regions is
very slight, and the importance of pigments for UV protection is far less than the need for vitamin D.
Question 2.
How do the feedback mechanisms of labor and childbirth differ from those of maintaining a
constant body temperature?
Answer: Labor involves an ever-increasing need for uterine muscle contractions, etc., leading to childbirth and
involves positive feedback that continues this process. The normal human body temperature is like the house
thermostat and needs to be kept within a narrow range; therefore, movement toward hotter or colder triggers
negative feedback mechanisms to reverse the processes and return the body temperature to within the normal range.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
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CHAPTER
32
CIRCULATION AND CARDIOVASCULAR SYSTEMS
This chapter presents a study of circulation in various forms of life, including invertebrates, vertebrates,
and humans. A detailed discussion of the anatomy and physiology of the human circulatory system
precedes a description of various cardiovascular diseases. The formed elements (cells) of human blood
are described. A Health Focus box looks at “Prevention of Cardiovascular Disease.” A section on blood
types is included.
Chapter Outline
32.1 Transport in Invertebrates
1.
Sponges, cnidarians and flatworms are groups of organisms with a thin body wall that make a
circulatory system unnecessary.
2. Hydras have each cells exposed to water and can independently exchange gases and excrete wastes.
3. Planaria have a trilobed gastrovascular cavity and a small, flat body where nutrients diffuse from cell
to cell.
4. Pseudocoelomates, such as nematodes, use the coelomic fluid of the body cavity to transport fluids.
5. Echinoderms rely on movement of coelomic fluid as a circulatory system.
A. Invertebrates with a Circulatory System
1. In a circulatory system, a pumping heart moves one of two types of circulatory fluids.
a. Blood is a circulatory fluid and is always contained within blood vessels.
b. Hemolymph is a circulatory fluid which flows into the hemocoel of certain arthropods and
molluscs; it is a mixture of blood and interstitial fluid.
2. Certain arthropods and molluscs have an open circulatory system.
a. Hemolymph is pumped by the heart into the body cavity or saclike sinuses.
b. Hemolymph bathes the internal organs and then drains back to the heart.
c. In grasshoppers, a dorsal heart pumps hemolymph into an aorta, which empties into the hemocoel.
d. Hemolymph is colorless (it lacks hemoglobin or other respiratory pigments); a system of tracheae
provides oxygen.
3. Some invertebrates, including earthworms and cephalopods, have a closed circulatory system in
which blood never leaves the heart or vessels.
a. Valves prevent any backward flow of the blood as it moves through vessels.
b. Earthworms have five pairs of anterior lateral vessels that pump blood to every segment.
c. Blood moves in capillaries where an exchange with tissue fluid takes place before returning in
veins.
d. Earthworms have a red respiratory pigment hemoglobin dissolved in the blood, not inside blood
cells.
e. With no special cavity for gas exchange, the gas must diffuse across a moist body wall.
32.2 Transport in Vertebrates
1.
2.
3.
Vertebrates have a closed circulatory system called a cardiovascular system.
The muscular heart keeps blood circulating through the blood vessels.
a. The atria are the chambers of the heart that receive blood.
b. The ventricles pump blood into arteries.
There are three kinds of blood vessels: arteries carry the blood away from the heart, capillaries are
where the exchange with tissue fluid takes place, and veins return the blood to the heart.
a. Arteries
1)
have thick walls and are resilient.
2)
expand to accommodate sudden increase in blood volume that results after heart
contraction.
3)
divide into small arterioles.
b. Arteriole constriction and dilation is regulated by the nervous system to affect blood pressure.
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c.
Capillaries are microscopic blood vessels with a wall formed of one layer of simple squamous
cells.
1) Capillary beds are so prevalent that, in humans, all cells are within 60–80 µm of a capillary.
2) Only 5% of the capillaries are open at one time; after an animal has eaten, the capillary beds
of the digestive system open.
3) Capillaries are so narrow that red blood cells must pass through them in single file.
4) Gas, nutrient, and waste exchange occurs across the thin capillary walls.
d. The venules are vessels that take blood from capillaries and join to form a vein.
e. Veins transport blood toward the heart.
1) The walls of veins are much thinner than those of arteries; there is a lower blood pressure in
veins than in arteries.
2) One-way valves open in the direction of the heart, then close to prevent backflow.
A. Comparison of Circulatory Pathways
1. In vertebrates, there are three different types of circulatory pathways.
2. Fishes have a one-circuit (single-loop) circulatory pathway.
a. The heart has a single atrium and a single ventricle and pumps the blood under pressure to the
gills.
b. Blood is oxygenated in the gills.
c. After passing through gills, blood is returned to the dorsal aorta, which distributes the blood
throughout the body.
3. Other vertebrates have a two-circuit (double-loop) circulatory pathway to breathe air on land.
a. The systemic circuit transports the blood to tissues.
b. The pulmonary circuit pumps the blood to lungs.
4. In amphibians and most reptiles, the heart has two atria and a single ventricle.
5. In most reptiles, and in all birds and mammals, the heart is divided into left and right halves.
a. With two atria and two ventricles, the oxygenated blood is always separate from the deoxygenated
blood.
b. The right ventricle pumps blood to the lungs; the left ventricle pumps blood to the rest of the body.
c. This arrangement provides adequate blood pressure for both the pulmonary and the systemic
circulations.
32.3 Transport in Humans
A. The Human Heart
1. The pumping of the heart keeps the blood moving in arteries.
2. Skeletal muscle contraction is responsible for the blood movement in veins.
3. The heart is a cone-shaped, muscular organ about the size of a fist.
4. It is located between the lungs directly behind the sternum and is tilted so that the apex is oriented to
the left.
5. The myocardium is a major portion of the heart consisting mostly of cardiac muscle; its muscle fibers
are branched and tightly joined together.
6. The heart lies within the pericardium, a sac that secretes a lubricating fluid.
7. The endocardium lines the inner surface of the heart; it consists of connective tissue and endothelial
tissue.
8. An internal wall called the septum separates the heart into right and left halves.
9. The heart has two upper, thin-walled atria and two lower, thick-walled ventricles.
10. Heart valves direct the flow of blood and prevent any backward movement.
a. Valves are supported by strong fibrous tendons (chordae tendineae) which support the valves and
prevent them from inverting when the heart contracts.
b. Atrioventricular valves between the atria and ventricles prevent any back flow from the ventricle
to the atrium.
c. The right atrioventricular (tricuspid) valve on right side of the heart consists of three cusps or
flaps.
d. The left atrioventricular (bicuspid or mitral) valve on left side consists of two cusps or flaps.
e. Semilunar valves are located between the ventricles and their attached vessels.
1) The pulmonary semilunar valve lies between the right ventricle and the pulmonary trunk.
2) The aortic semilunar valve lies between the left ventricle and the aorta.
B. Path of Blood Through the Heart
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1.
The route of blood through the heart is as follows:
a. Oxygen-poor blood enters the right atrium from both the superior vena cava and the inferior vena
cava.
b. The right atrium sends blood through the right atrioventricular (tricuspid) valve to the right
ventricle.
c. The right ventricle sends blood through the pulmonary semilunar valve into the pulmonary trunk
and arteries to the lungs.
d. Oxygen-rich blood returns from the lungs through pulmonary veins and is delivered to the left
atrium.
e. The left atrium sends blood through the left atrioventricular (bicuspid or mitral) valve to the left
ventricle.
f. The left ventricle sends blood through the aortic semilunar valve into the aorta and on to the body
proper.
2. The heart is therefore a double pump serving the lungs and body circulations simultaneously; O 2-poor
blood and O2-rich blood never mix.
3. Since the left ventricle has the harder job of pumping blood throughout the body, its walls are thicker;
accordingly, blood pressure is greatest in the aorta.
4. Blood pressure decreases as the cross-sectional area of the arteries and arterioles increases.
C. The Heartbeat
1. The human heart contracts (beats) about 70 times a minute (2.5 billion times in a lifetime); each
heartbeat lasts about 0.85 seconds.
2. The heartbeat or cardiac cycle consists of phases.
3. The atria contract first while the ventricles relax (0.15 sec.), then the ventricles contract while atria
relax (0.30 sec.), and then all chambers rest (0.40 sec.).
4. Systole refers to the contraction of heart chambers and diastole refers to the relaxation of the heart
chambers.
5. The heart is in diastole about 50% of the time.
6. The short systole of the atria is needed only to send blood into the ventricles.
7. When the term “systole” is used alone, it refers to the left ventricle systole; the volume of blood that
the left ventricle pumps per minute into the systemic circuit is called the cardiac output.
8. When the heart beats, the familiar lub-dub sound is heard as the valves of the heart close.
a. Lub is caused by the vibrations of the heart when the atrioventricular valves close.
b. Dub is heard when the vibrations occur due to the closing of semilunar valves.
9. The pulse is a wave effect that passes down the walls of arterial blood vessels when the aorta expands
and then recoils following ventricular systole.
10. Since there is one arterial pulse per ventricular systole, the arterial pulse rate can be used to determine
the heart rate.
11. Rhythmic contraction of the heart is due to the cardiac conduction system.
a. The sinoatrial (SA) node is the “pacemaker” found in the upper dorsal wall of the right atrium; it
initiates the heartbeat by sending out an excitatory impulse every 0.85 seconds to cause the atria to
contract.
b. The atrioventricular (AV) node is found in the base of the right atrium very near the septum; when
stimulated by impulses from the SA node, it sends out impulses through the septum to cause the
ventricles to contract.
c. Although the beat of the heart is intrinsic, it is regulated by the nervous system which can increase
or decrease the heartbeat rate.
d. The SA node is called the cardiac pacemaker because it usually keeps the heartbeat regular.
e. The hormones epinephrine and norepinephrine also stimulate the heart.
12. An electrocardiogram (ECG) is a recording of the electrical changes that occur in the myocardium
during a cardiac cycle; it is used as a diagnostic tool to identify abnormal cardiac function.
13. Normal Cardiac Cycle
a. The P wave represents excitation and occurs just before atrial contraction.
b. The QRS complex signals that the ventricles are about to contract.
c. The electrical changes that occur as the ventricular muscle fibers recover produce the T wave.
14. Ventricular fibrillation is uncoordinated contraction of the ventricles; with the application of a strong
electric current, the SA node may reestablish a coordinated beat.
D. Vascular Pathways
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
1.
The human cardiovascular system has two major circular pathways.
The Pulmonary Circuit
a. The pulmonary circuit circulates blood to the lungs.
b. Oxygen-poor blood from the body collects in the right ventricle, which pumps it to the pulmonary
trunk.
c. The pulmonary trunk divides into right and left pulmonary arteries to carry blood to each lung.
d. In the lungs, carbon dioxide (CO2) is unloaded and O2 is picked up by blood.
e. Oxygen-rich blood from the lungs is returned through pulmonary veins to the left atrium.
2. The Systemic Circuit
a. The aorta and vena cavae are main pathways for blood in the systemic circuit.
b. Transport of oxygenated blood moves from the left ventricle through the aorta out to all tissues.
c. Deoxygenated blood returns from all tissues via the vena cavae.
d. In a systemic circuit, arteries contain bright red oxygen-rich blood; the veins contain dull red
oxygen-poor blood that appears blue when viewed through the skin.
3. The coronary arteries serve the heart muscle itself.
a. Coronary arteries originate from the base of the aorta just above the aortic semilunar valve.
b. Coronary arteries lie on the external surface of the heart; they branch into arterioles and
capillaries.
c. Capillary beds enter the venules that join to form the cardiac veins.
d. Coronary veins collect oxygen-poor blood from the capillaries and empty it into the right atrium.
4. The portal system is a pathway of blood flow that begins and ends in capillaries.
a. The hepatic portal vein transports blood from capillaries in the small intestinal villi to capillaries
in the liver.
b. The hepatic vein leaves the liver and enters the inferior vena cava.
c. In the liver, substances absorbed by the intestine are modified, toxins and bacteria are removed,
and the normal composition of blood is monitored.
5.
Tracing the Path of Blood
a.
Branches from the aorta go to the organs and major body regions.
b.
Generally, the artery and the vein that serve the same region are given the same name.
c. Once blood goes through the artery, it travels to the arterioles, then into branching capillaries
where gas exchange occurs, and then venules join to form the vein that enters a vena cava.
E. Blood Pressure
1. Systolic pressure results from blood being forced into the arteries during ventricular systole.
2. Diastolic pressure is the pressure in arteries during ventricular diastole.
3. Human blood pressure is measured as the force pushing against the wall of the brachial artery of the
upper arm.
a. Blood pressure is measured by a sphygmomanometer, which has a pressure cuff.
b. Clinical blood pressure measures pressures produced by contraction and relaxation of the left
ventricle.
c. Blood pressure is stated in millimeters of mercury (e.g., 120/80 mm Hg) for systolic/diastolic.
4. As blood flows from the aorta into arteries and arterioles, the blood pressure falls.
5. The difference in pressure between systolic and diastolic pressures gradually diminishes.
6. Capillaries have a slow, even blood flow due to the high total cross-sectional area.
7. Blood pressure in the veins is low and cannot move blood back to heart, especially from the limbs.
8. Venous return is dependent on these factors:
a. Skeletal muscle contraction on the walls of veins with valves, preventing backflow of blood, is
responsible for the flow of blood in veins.
b. Varicose veins are abnormal dilations that develop when the valves become weak and ineffective.
c. A repiratory pump helps blood flow from the higher pressure (when we exhale) to lower pressure
(when we inhale).
F. Cardiovascular Disease

Cardiovascular disease (CVD) is the leading cause of untimely death in Western
countries.

The risk of CVD can be reduced by following guidelines for a heart-healthy life-style.
1.
Hypertension
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a. An estimated 20% of Americans suffer from hypertension (high blood pressure).
b. Under the age of 45, a reading above 130/90 is hypertensive.
c. Beyond the age of 45, a reading above 140/95 is hypertensive.
d. The diastolic pressure is what is emphasized when medical treatment is considered.
2. Atherosclerosis
a. Hypertension is seen in individuals with atherosclerosis (formerly called arteriosclerosis).
b. Soft masses of fatty materials, mostly cholesterol, accumulate beneath the inner linings of arteries.
c. As this plaque accumulates, it protrudes into the vessel and interferes with blood flow.
d. Atherosclerosis develops in early adulthood but the symptoms may not appear until age 50 or
older.
e. Plaque can cause a blood clot to form on irregular arterial walls.
f. As long as a clot remains stationary, it is a thrombus.
g. If a clot dislodges, it is an embolus, a blood clot that moves in the blood.
h. In some families, atherosclerosis is inherited as familial hypercholesterolemia.
3. Stroke and Heart Attack
a. Stroke, heart attack, and aneurysm are associated with hypertension and atherosclerosis.
b. A stroke can result in paralysis or death; a small cranial arteriole bursts or is blocked by an
embolus.
1)
A stroke is also called a cardiovascular accident (CVA).
2) Whether paralysis or death occurs depends on the extent of the portion of the brain that lacks
O2.
3) Warning symptoms that foretell stroke include: numbness in hands or face, difficulty
speaking, blindness in one eye, etc.
c. A myocardial infarction (MI) is also called heart attack.
1) This occurs when a portion of heart muscle dies due to a lack of O2; this may be caused by a
thromboembolism blocking a coronary artery.
2) A partially blocked coronary artery causes angina pectoris causing pains or a flash of
burning.
3)
Nitroglycerin and related drugs dilate the blood vessels and relieve pain.
G. Prevention of Cardiovascular Disease (Health Focus box)
1.
The Don’ts
a.
Smoking
1) Smoking contritubes to hypertension.
2) When a person smokes, nicotine enters the bloodstream, causing arterioles to constrict
and blood pressure to rise.
3) Restricted bloodflow and cold hands are associated with smoking.
4) The heart then must pump harder to send the blood through the lungs at the time when
oxygen-carrying capacity of the blood is reduced.
b.
Drug Abuse
1)
Drugs, particularly stimulants, can lead to heart attacks and strokes.
2) Drinking two to four drinks a week for men and one to three drinks for women may
actually lower the risk of heart disease.
c.
Weight Gain
1) People who are more than 20% above their recommended weight have a higher risk of
hypertension.
2) In overweight people, more tissue needs servicing, and the heart sends the extra blood out
under greater pressure.
2.
The Do’s
a.
Healthy Diet
1)
Eating food with monounsaturdated and polyunsaturdated fats may lower LDL
cholesterol.
2)
A diet high in antioxidants may prevent cardiovascular disease.
b.
Cholesterol Profile
1) An LDL level above 160 mg/100 ml and an HDL level below 40 mg/100 ml are matters
of concern.
2)
An LDL level of below 100 mg/100 ml is recommended.
3)
Medications may be prescribed for individuals who do not meet these minimum
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guidelines.
c.
Exercise
1)
Exercise may reduce the risk of cardiovascular disease.
2)
Exercsie helps keep weight under control, may help minimize stress, and reduce
hypertension
32.4 Blood, a Transport Medium
The blood of mammals has two components: plasma and formed elements (cells and platelets).
1. Plasma contains water and many types of molecules, including nutrients, wastes, salts, and proteins.
2. Salts and proteins buffer the blood.
a. They effectively keep the blood pH near 7.4.
b. They maintain the blood osmotic pressure so water has a tendency to enter capillaries.
3. Some plasma proteins are involved in blood clotting.
4. Some plasma proteins assist in transporting large organic molecules in the blood.
a. Lipoproteins that transport cholesterol are globulins.
b. Albumin, a common plasma protein, transports bilirubin, a breakdown product of hemoglobin.
A. Formed Elements
1. Formed elements are of three types: red blood cells (RBCs), white blood cells (WBCs), and platelets.
2. Red Blood Cells
a. Red blood cells (erythrocytes) are small biconcave disks.
b. When mature, RBCs lack a nucleus and contain hemoglobin.
c. There are 6 million RBCs per mm3 of whole blood.
d. Each RBC contains about 250 million hemoglobin molecules.
1) Hemoglobin contains four globin protein chains, each with an iron-containing heme group.
2) The iron atom of a heme group loosely binds with an O2 molecule; thus, blood carries oxygen.
3) Anemia is either a lack of enough RBC or insufficient hemoglobin; an individual suffers from
a tired, run-down feeling.
e. RBCs are manufactured in the red bone marrow of the skull, ribs, vertebrae, and the ends of long
bones.
f. The growth factor erythropoietin is produced when an enzyme from the kidneys acts on a
precursor made by the liver and stimulates production of red blood cells; as a drug it helps people
with anemia.
g. Before being released from bone marrow, the RBCs lose their nucleus and synthesize hemoglobin.
h. Red blood cells have a life span of about 120 days; then they are destroyed chiefly in the liver and
spleen.
i. When the RBCs are destroyed, the hemoglobin is released; the iron is recovered and returned to
the bone marrow where it is reused.
j. The heme portions undergo chemical degradation and are excreted by the liver as bile pigments; it
colors the feces.
3. White Blood Cells
a. White blood cells (leukocytes) differ from RBCs in being larger and in having a nucleus.
b. WBCs lack hemoglobin and appear translucent without staining.
c. Granular leukocytes contain conspicuous granules in their cytoplasm and have a lobed nucleus.
1) Neutrophils have granules that stain slightly pink; they are amoeboid, spherical cells that
readily squeeze through capillary walls and phagocytize foreign material.
2) Eosinophils have granules that take up the red dye eosin.
3) Basophils have granules that take up a basic dye, staining them deep blue.
d. A newly discovered stem cell growth factor (SGF) increases the production of all WBCs, which
helps patients with low immunity.
e. Agranular leukocytes lack granules in their cytoplasm and have a circular or indented nucleus.
1) Monocytes are amoeboid and able to enter tissues where they transform into macrophages.
2) Macrophages release white blood cell growth factors that increase the number of leukocytes.
3) Pus is a thick, yellowish fluid that contains a large proportion of dead WBCs that have fought
infection.
4) Lymphocytes play a key role in fighting infection and include two types.
a) T cells are lymphocytes that directly attack virus-infected cells.
b) B cells can be stimulated to produce one type of antibody specific for one type of
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antigen.
5) An antigen is any substance stimulating production of antibodies; antigen is foreign to the
body.
6) Antibodies combine with antigens to promote their being phagocytized by a macrophage.
7) A person is actively immune when many B cells produce a specific antibody for an infection.
4. Platelets
a. Platelets (thrombocytes) result from fragmented giant cells (megakaryocytes) in the bone
marrow.
b. 200 billion platelets are produced a day; blood contains 150,000–300,000 platelets per mm3.
c. Platelets are involved in blood clotting, or coagulation.
5. Blood Clotting
a. When a blood vessel is damaged, platelets clump at the site of the puncture and partially seal the
leak.
b. The platelets and damaged tissue cells release a clotting factor called prothrombin activator.
c. With calcium ions, prothrombin activator catalyzes a reaction converting prothrombin to
thrombin.
d. Thrombin acts as an enzyme to sever two amino acid chains from each fibrinogen molecule.
e. These activated fragments join end-to-end forming long threads of fibrin.
f. Fibrin threads wind around the platelet plug and provide a framework for a clot.
g. RBCs are trapped within the fibrin threads, making the clot appear red.
h. When blood vessel repair is initiated, plasmin destroys the fibrin network and restores plasma
fluidity.
B. Capillary Exchange
1. Two forces control the movement of fluid through the capillary walls.
a. Osmotic pressure tends to cause water to move from tissue fluid to the blood.
b. Blood pressure tends to cause water to move from the blood to tissues.
c. At the arterial end of a capillary, blood pressure is higher than osmotic pressure: water exits and
moves into tissues.
d. Along the capillary, O2 and nutrients diffuse out into the tissue fluid, while CO 2 and other
metabolic wastes diffuse into the capillaries from the tissue fluid.
2. Midway along a capillary, there is no net movement of water.
3. The tissue fluid is intercellular fluid that surrounds the cells; the circulatory system exchanges
materials with this fluid.
4. The exchange between the blood and tissue fluid occurs by diffusion through the one-cell-thick
capillary walls.
a. At the venule end, osmotic pressure is higher than blood pressure and water moves back into the
blood.
b. Almost the same amount of fluid that left the capillary returns to it; there is always some excess
tissue fluid collected by the lymphatic capillaries.
5. The tissue fluid within lymphatic vessels is lymph.
6. Lymph returns to the systemic venous blood when lymphatic vessels enter the subclavian veins in the
shoulder.
7. Not all capillary beds are open at the same time; precapillary sphincters shunt blood along various
pathways.
8. Through capillary dilation and constriction, blood also distributes heat to body parts and conserves
heat when cold.
C. Blood Types
1. ABO System
a. The presence or absence of type A and B antigens on red blood cells determine a person’s blood
type.
b. If the person has type A blood, the A antigen is on the red blood cells; if the person is type B, the
B antigen is on the red blood cells.
c. In the ABO system, there are four blood types: A, B, AB, O.
1) Individuals have naturally-occurring antibodies to blood type antigens not present on their
blood cells; these are called anti-A and anti-B
2) RBCs with a particular antigen agglutinate when exposed to corresponding antibodies, e.g.,
type A RBCs will agglutinate in the presence of anti-A antibody (as would be found in the
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blood of a type B individual).
3) Agglutination is the clumping of red blood cells due to a reaction between antigens on the
red blood cells.
4) To receive blood, the recipient’s plasma must not have an antibody that causes donor cells to
agglutinate.
a) Recipients with type AB blood can receive any type blood; they are the universal
recipient.
b) Recipients with type O blood cannot receive A, B, or AB; but they are a universal donor.
c) Recipients with type A blood cannot receive B or AB.
d) Recipients with type B blood cannot receive A or AB.
D. Rh System
1. Rh factor is an important antigen in human blood types.
2. Rh positive (Rh+) has the Rh factor on red blood cells; Rh negative (Rh -) lacks the Rh antigen on
RBCs.
3. Rh-negative individuals do not have antibodies to Rh factor but make them if exposed to Rh + blood.
4. Rh factor is particularly important during pregnancy.
a.
Hemolytic disease of the newborn is possible if the mother is Rh negative and the father
is Rh positive.
b.
Rh positive is a genetically dominant trait; an Rh negative mother and an Rh positive
father pose a Rh conflict.
c.
The child’s Rh positive RBCs can leak across the placenta into the mother’s circulatory
system when the placenta breaks down.
d.
The presence of the “foreign” Rh positive antigens causes the mother to produce anti-Rh
antibodies.
e.
Anti-Rh antibodies pass across the placenta and destroy the RBCs of the Rh positive
child.
Lecture Enrichment Ideas
Experience Base: Because of the prevalence of heart disease, this topic usually generates
interest because most students have family members or relatives affected by it. In spite of high
school biology curricula, few college students seem to understand the mechanism by which
blood pressure is taken. Blood donor drives are commonly sponsored on campus and some of
blood biology can be related to the experiences students will have had if they donated blood.
1. Describe why most molluscs would have an open circulatory system but why cephalopods
(octopus, squid, and nautilus) would have a closed circulatory system. Consider the
circulatory needs of the predator as opposed to the sedentary filter feeder or the herbivore.
2. Draw the improvements in efficiency from the one-circuit heart of the fish to the partially
separated circulation in most reptiles, to the completely separated two-circuit heart of birds
and mammals. It is possible to allude to the evolutionary modifications that led to these
changes when drawing the diagrams and merely adding a blood vessel shunt or extending a
heart wall to make a three-chambered heart a four-chambered heart. Consider activity levels,
homeothermy, and similar functions that depend on oxygen and nutrient delivery.
3. Sometimes a human baby is born without complete closure of the hole between the ventricles
(foramen ovale); without surgery, this is fatal and illustrates that we cannot live with
incomplete separation of oxygenated and deoxygenated blood, as can a frog.
4.
Consider the size, shape, and structure of the human heart and why it has a thicker wall on the left side, which
delivers blood to the systemic circuit, as opposed to the right side, which sends blood only to the nearby
pulmonary circuit. Students may have been taught to pledge with their hand over their heart far to the left; it is
more central. As a general rule, it is as big as the person’s double-up fist.
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5. Discuss the actions of the valves and why valvular damage can cause heart murmurs,
varicose veins in the legs, and even death. Heart murmurs add sounds to the lub-dub
depending upon which valve is failing to close and allowing blood to backwash with a shhh
sound, as in “lub-dub-shhh.”
6. An actual demonstration of taking blood pressure will go far to clear the mystery of how the
blood pressure cuff works and what is actually heard during the reading of a nonelectronic
BP meter.
7.
Many disorders and diseases have symptoms that slowly get worse, but stroke and heart attack are usually
critical immediately. Ask students why this is.
8. Varicose veins are common enough that most students will recognize the term, as well as the
symptoms among older individuals. Discussion items can include: 1) what is the blue
spiderlike pattern seen near the surface with varicose veins; 2) why are people with sedentary
jobs (where they stand or sit often) more prone to varicose veins; 3) how can an operation for
stripping out varicose veins or replacing a section of vein with another segment be done if it
leaves an area without that venous blood vessel; 4) if there are fewer valves in veins on the
left than on the right, where are varicose veins more likely?
9. Red blood cells are produced by stem cells that defy the Hayflick limit on number of cell
divisions and continually produce RBCs. Current research is probing the possible ability of
some stem cells to “back up” and produce other tissues.
10. Lecture question: If people living at higher altitudes with “thinner air” and therefore less
oxygen adapt by increasing their red blood cells and/or hemoglobin, what are the
physiological effects (concerning sports, for instance) when they compete at sea level? What
are the effects when a sea level person travels to the Rocky Mountains, for instance? Tie this
in with the concept of “blood doping.”
11. Some students may have detected attached leeches after swimming and witnessed the strong
anticoagulant action in the saliva of the leech; others may have noticed mosquito bites bleed
more freely from a weak anticoagulant.
12. Because carbon monoxide is odorless and toxic, not only do some homeowners install CO
detectors, but forest firefighters have time limitations for staying on the front lines where it is
not possible to accurately detect the levels of carbon monoxide.
13. Some students believe that venous blood is blue. Oxygen-poor blood is a dark red and
becomes brighter red with oxygen. The blue appearance of blood in veins is due to the
tissues; students who have given blood by IV can see the dark red (not blue!) venous blood in
the collecting bag.
14. The shunting of blood flow to and away from various tissues is a particularly important
mechanism in diving marine mammals. This ability accounts for the whale and other diving
mammals being able to stay below water far longer than we can.
15. While arteries and veins carry blood one direction only (away from and toward the heart,
respectively), footage of capillary flow often shows blood reversing. Capillaries can be
compared to side streets in a town: while there may be one major “artery” into a town, there
are many sidestreets for traffic flow before it exits the other side of town. As tissue is
compressed or as arteriole sphincters restrict flow in one area, blood may reverse in
capillaries to move through a network of other “side streets.” The network is called an
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“anastomosis,” and regular routes to bypass capillary beds are arteriovenous shunts.
16. In smaller classrooms, both models and a real heart (sheep, beef, etc., from local meat
processing plants) can demonstrate the circulatory system during lecture; students who
actually get their hands on and inside the heart understand the flow pattern and the soft, wet
machinery, far better. If possible, take a field trip to a cadaver lab and allow students to
explore on the “real” human body.
17. Students familiar with streams and rivers will understand that velocity changes with stream
cross section; a river that is wide flows slowly but becomes rapid when forced through a
small channel—the cross section of all the capillaries is greater than the arteries and therefore
the flow is slower in capillaries.
18. Without any special equipment, a student who sinks down in the bathtub to where ears are
below water can soon hear their own heartbeat in clear detail.
Critical Thinking
Question 1.
The kidney receives about one-fifth of the blood flow; this translates into a red blood cell passing
through the kidney once for every four times it makes a circuit through the rest of the body. How does this compare
to blood flow through the lungs?
Answer: Because the heart is separated into four chambers with the right side shunting all blood to the pulmonary
circuit, a red blood cell will pass through the lungs every circuit or five times more often than it will pass through
the kidney.
Question 2.
The percentage of North American Indians with type B blood is only 1%, and virtually none has
type AB. Yet, anthropologists believe they originated from northern Asia, and the percentages of Americans with
types B and AB are 35% and 10%, respectively. What evolutionary mechanism could account for this?
Answer: Because of the high persistence of these blood types in human populations, there is unlikely to be a
significant advantage to having any particular blood type. However, if the migration of peoples from northern Asia
to North America across a land bridge gave rise to the North American Indian population, it is likely that the
founder effect led to a lack of alleles for type B blood. Small groups of immigrants mostly lacked this blood type.
Question 3.
The lung of a fetus is not yet functioning to absorb oxygen-rich blood from air; the placental
umbilical cord functions to deliver this through the abdomen to the fetus’s main blood vessels. Before birth, the fetal
heart therefore has an opening, called the foramen ovale, in the septum between the right and left ventricle. If this
does not close by birth, the infant may require heart surgery. (1) Why does this opening pose a problem for a
newborn only after birth, and what symptoms would you expect? (2) If this is such a potential danger, why hasn’t
evolution eliminated the opening completely?
Answer: (1) Oxygen-rich and oxygen-poor blood in the ventricles mix; this drastically decreases the effectiveness
of the heart chambers since some oxygen-rich blood is now sent to the lungs and some oxygen-poor blood is sent
back to the body, resulting in the appearance of a “blue baby” who cannot secure enough oxygen. While adequate
oxygen was provided by the mother, who is also “breathing for two” before birth, after birth all body systems are
starved for oxygen to carry out cell respiration. (2) The collapsed fetal lungs are merely developing tissue with no
more need for oxygen than other fetal tissues before birth; a fully divided four-chambered heart is not of any
advantage, and the foramen ovale is not a handicap, until the baby is breathing on its own.
Question 4.
The umbilical cord of the fetus is part of the fetal blood system and leads to the placental interface
where oxygen and carbon dioxide diffuse across, as well as food molecules and metabolic wastes. How are the
arteries and veins in the umbilical cord similar to the pulmonary circuit?
Answer: Arteries are defined as carrying blood away from the heart and veins return blood to the heart. Generally
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arteries carry oxygen-rich blood and veins carry oxygen-poor blood, but in the umbilical cord, this is reversed as it is
in the pulmonary circulation.
Question 5.
“Blood doping” occurs when an athlete or horse owner removes blood, stores it, and then returns it
after the body has had time to restore normal blood levels. Why is this sometimes done and how would this be
diagnosed?
Answer: This increases the amount of red blood cells and hemoglobin available to carry oxygen during the sport
event; a blood sample would show a higher-than-normal red blood cell count.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
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CHAPTER
33
LYMPH TRANSPORT AND IMMUNITY
The lymphatic system and immunity are detailed in this chapter. Topics include the anatomy of
the lymphatic system, nonspecific and specific defenses, active and passive immunity, and
autoimmune diseases. A Science Focus box examines “Antibody Diversity.” A Health Focus
box looks at “Opportunistic Infections and HIV.”
Chapter Outline
33.1 Lymphatic System
1.
2.
The mammalian lymphatic system consists of lymphatic vessels and lymphoid organs.
This system is closely associated with the cardiovascular system and has four main functions.
a. Lymphatic vessels absorb excess tissue fluid and return it to the bloodstream.
b. Lacteals receive lipoproteins at the intestinal villi and the lymphatic vessels transport these fats to
the bloodstream.
c. The lymphatic system is responsible for the production, maintenance, and distribution of
lymphocytes.
d. The lymphatic system helps defend the body against disease.
A. Lymphatic Vessels
1. Lymphatic vessels are extensive; most regions have lymphatic capillaries.
2. The structure of the larger lymphatic vessels resembles veins, including the presence of valves.
3. The movement of fluid is dependent upon skeletal muscle contraction; when the muscles contract,
fluid is squeezed past a valve that closes, preventing it from flowing backwards.
4. The lymphatic system is a one-way system that begins with lymphatic capillaries.
a. They take up fluid that has diffused out of the blood capillaries and has not been reabsorbed.
b. If excess tissue fluid is produced or not absorbed, it will accumulate and result in edema.
5. Once tissue fluid enters the lymphatic capillaries, it is called lymph.
6. Lymphatic capillaries join as lymphatic vessels that merge before entering one of two ducts.
a. The thoracic duct is larger than the right lymphatic duct.
1) It serves the lower extremities, abdomen, left arm, left side of the head and neck, and the left
thoracic region.
2) It then delivers lymph to the left subclavian vein of the cardiovascular system.
b. The right lymphatic duct is smaller.
1) It serves the right arm, the right side of the head and neck, and the right thoracic region.
2) It then delivers lymph to the right subclavian vein of the cardiovascular system.
B. Lymphatic Organs
1. The red bone marrow is the origin for all blood cells including all leukocytes that function in
immunity.
a. Some lymphocytes become mature B cells in the bone marrow.
b. Most bones of a child have red bone marrow but in adults, red bone marrow is
only in the skull, sternum, ribs, clavicle, pelvic bones, and vertebral column.
c. Red bone marrow consists of reticular fibers produced by reticular cells packed
around thin-walled sinuses.
d. Differentiated blood cells enter the bloodstream at these bone sinuses.
2.
The thymus gland is located along the trachea behind the sternum in the upper thoracic cavity.
a. T cells mature in the thymus. The thymus gland is larger in children than in adults
and may disappear completely in old age.
b.
The thymus produces thymic hormones, such as thymosin, that are thought to aid in T cell
maturation.
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3.
Lymph nodes are small (about 1–25 mm) ovoid structures located along lymphatic vessels.
a. A lymph node contains nodules, each packed with B lymphocytes and contain a
sinus.
b. Lymph nodes cluster in certain regions of the body and are named accordingly:
inunal nodes in the groin and axillary nodes in the armpits.
4.
5.
The spleen is located in the upper left abdominal cavity just below the diaphragm.
The spleen is similar to a lymph node but it is much larger, about the size of a fist.
Instead of cleansing the lymph, the spleen cleanses the blood.
A capsule divides the spleen into lobules which contain sinuses filled with blood.
Red pulp consists of blood vessels and sinuses where macrophages remove old and defective blood
cells; lymphocytes cleanse the blood of foreign particles.
White pulp consists of little lumps of lymphatic tissue.
If the spleen ruptures due to injury, it can be removed; its functions are assumed by other organs.
However, a person without a spleen is more susceptible to infections and may require antibiotic
therapy.
The tonsils are patches of lymphatic tissue located in a ring around the pharynx.
a. Peyer’s patches are located in the intestinal wall and the vermiform appendix is attached to the
cecum—both encounter pathogens that enter the body by way of the intestinal tract.
33.2 Nonspecific Defense Against Disease
Immunity is the ability to repel infectious agents, foreign cells, and cancer cells.
1. Immunity begins with nonspecific defenses.
2. The four nonspecific defenses include barrier to entry, protective proteins, natural killer cells, and
inflammatory response.
A. Barriers to Entry
Skin and the mucous membranes lining the respiratory, digestive, and urinary tracts are
non-chemical, mechanical barriers.
Ciliated cells lining the upper respiratory tract sweep mucous and particles up into the
throat to be swallowed.
Oil gland secretions inhibit the growth of bacteria on the skin.
The stomach has a low pH (1.2–3.0) that inhibits the growth of many bacteria.
B. Inflammatory Response
1. If tissue is damaged, a series of events known as the inflammatory response occurs.
2. The inflamed area has four symptoms: redness, pain, swelling, and heat.
3. Chemical signals, e.g., histamine, and mast cells, a type of white blood cell, cause
vasodilation and increased permeability of capillaries.
4. Enlarged capillaries produce redness and a local increase in temperature.
5. The swollen area stimulates free nerve endings, causing pain.
6. Neutrophils and monocytes migrate by amoeboid movement to the site of the injury;
they escape from the blood by squeezing through the capillary wall.
7. Dendritic cells and macrophages recognize the presence of pathogens and respond by
releasing cytokines.
8. The cytokines stimulate other immune cells.
9. Neutrophils, dendritic cells, and macrophages engulf pathogens.
10. As phagocytic cells die, they, along with dead bacteria, dead tissue cells, and living
white blood cells, form pus.
11. Dendritic cells and macrophages move to the lymph nodes and spleen, where they
activate B and T lymphocytes.
12. An inflammatory response may also involve the production of a fever, which serves
to inhibit the growth of microorganisms and stimulates immune cells.
C. Phagocytes and Natural Killer Cells
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1
Neutrophils are cells that are able to leave the bloodstream and phagocytize (engulf)
bacteria in connective tissue.
2. Eosinophils are phagocytic, but are better known for attacking animal parasites that
are too large to be phagocytized.
3. Macrophages and dendritic cells are the most powerful phagocytic white blood
cells that engulf pathogens.
4. Natural killer (NK) cells kill virus-infected cells and tumor cells by cell-to-cell
contact.
a. After stimulation by dendritic cells, they look for a self protein on the body’s
cells.
b. NK cells are not specific; they have no memory and their numbers do not increase
after stimulation.
D. Protective Proteins
1. Complement is composed of a number of plasma proteins designated by the letter C
and a subscript.
2. It “complements” certain immune responses, which accounts for its name.
3. It amplifies an inflammatory reaction by binding to mast cells, triggering histamine
release, and by attracting phagocytic cells to the site of infection.
4. Some complement proteins binds to antibodies already on the surface of pathogens,
thereby increasing the probability that pathogens will be phagocytized by a neutrophil
or macrophage.
5. Some complement proteins form a membrane attack complex that produces holes in
bacterial cell walls and plasma membranes; fluids and salts then enter to the point
where the cell bursts.
6. Interferons are cytokines, proteins produced by virus-infected animal cells.
33.3
Specific Defense Against Disease
1. If nonspecific defenses fail to prevent an infection, specific defenses activate against
a specific antigen.
2. We do not ordinarily become immune to our own cells; the immune system can tell
“self” from “nonself.”
3. The steps to accomplish a specific defense are:
a. The immune system is able to recognize particular molecules, called antigens.
1) An antigen is called a foreign antigen because the body does not produce
them; a self-antigen is an antigen that the body produces.
b. After recognizing antigens, the immune system can respond to them.
c. The immune system can remember antigens it has met before.
4. Specific immunity is primarily the result of the action of B lymphocytes (B cells) and
T lymphocytes (T cells).
a. B cells and T cells recognize antigens because they have antigen receptors—
plasma membrane proteins that allow them to combine with particular antigens.
b. B cells give rise to plasma cells that produce antibodies.
c. T cells differentiate into helper T cells, which regulate the immune response, or
cytotoxic T cells, which kill virus-infected and tumor cells.
5. B cells are responsible for antibody-mediated immunity.
a.
b.
Each type of B cell carries its specific receptor on its surface; this is called the B cell receptor
(BCR).
When a B cell in a lymph node of the spleen encounters an appropriate antigen, it is activated to
divide.
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c.
A.
The resulting cells are plasma cells, mature B cells that produce antibodies in the lymph nodes
and spleen; the antibodies are identical to the BCR of the B cell that produced them.
6. T cells are responsible for cell-mediated immunity.
a. Antigens must be presented to T cells by an antigen-presenting cell (APC).
b. T cells differentiate into either helper T cells, which release cytokines, or cytotoxic T cells, which
attack and kill virus-infected cells and cancer cells.
B Cells and Antibody-Mediated Immunity
1. The clonal selection theory states that the antigen selects the B cell to produce a clone of plasma cells.
a. Only one B cell has the BCRs that can combine with the specific antigen clone. The antigen
“selects” the B cell that will clone.
b. Defence by B cells is called antibody-mediated immunity because most members of the clone
become plasma cells that produce specific antibodies.
c. Some cloned B cells do not participate in antibody production but remain in the blood as memory
B cells.
2. Active Immunity occurs when an individual produces a supply of antibiotics.
a. However, active immunity is often induced when a person is well so that future infection is
prevented.
b. Immunization uses vaccines to provide the antigen to which the immune system responds.
c. To prepare vaccines, usually pathogens are treated so they are no longer virulent.
d. Genetically engineered bacteria can also produce antigen proteins from pathogens; the protein is
then used as a vaccine.
e. After a vaccine is given, the immune response is measured by the antibody level in serum—the
antibody titer.
1)
After the first exposure, a primary response occurs from no antibodies to a slow
rise in titer.
2) After a brief plateau, a gradual decline follows as antibodies bind to antigen or simply
break down.
3) After a second exposure, a secondary response occurs and the antibody titer rises rapidly
to a level much greater than before; this is a “booster.”
4) The high antibody titer is now expected to prevent any disease symptoms if the individual
is infected.
f.
Active immunity depends on memory B and memory T cells responding to lower doses
of antigen.
g.
Active immunity is usually long-lived although a booster may be required every so many
years.
3. Passive immunity occurs when an individual is given prepared antibodies to combat a disease.
a. It is short-lived because antibodies are not made by an individual’s own B cells.
b. Newborn infants are immune to some diseases because the mother’s antibodies have crossed the
placenta.
c. Breast-feeding also promotes passive immunity—the antibodies are in the mother’s milk.
d. Passive immunity is also needed when a patient is in immediate danger from an infectious disease
or toxin.
e. A person may be given a gamma globulin injection (serum that contains antibodies against the
agent) taken from an individual or animal who has recovered from it.
4. An antibody molecule is a Y-shaped protein molecule with two arms.
Each arm has a “heavy” and “light” polypeptide chain.
These chains have constant regions and variable regions.
The constant regions have amino acid sequences that do not change; the constant regions are not
identical among all antibodies.
d. The variable regions have portions of polypeptide chains whose amino acid sequence changes
providing antigen specificity; it forms the antigen binding sites of antibodies—their shape is
specific to antigen.
e. Another name for antibody is immunoglobulin (Ig).
f. IgG Antibodies
1) These are the major type in blood; less is in the lymph and tissue fluid.
2) IgG antibodies bind to pathogens and toxins.
a.
b.
c.
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g.
IgM Antibodies
1) These are pentamers; they contain five Y-shaped structures.
2) IgM appears in blood soon after an infection begins and disappears before it is over.
3) They are good activators of the complement system.
B. Monoclonal Antibodies
1. Every plasma cell derived from the same B cell secretes antibodies against the same antigen; these are
monoclonal antibodies.
2. Monoclonal antibodies can be produced in vitro.
a. B lymphocytes are removed from the body (usually mice are used) and exposed to a particular
antigen.
b. Activated B lymphocytes are fused with myeloma cells (malignant plasma cells that divide
indefinitely).
c. Fused cells are called hybridomas because they result from two different cells (hybrid) and one is
cancerous, therefore the suffix “-oma.”
3. Monoclonal antibodies are useful because of their extreme specificity for only a particular molecule.
4. They can be used to select out a specific molecule among many others.
5. Monoclonal antibodies are used for quick, reliable diagnosis of various conditions such as pregnancy.
6. They identify infections, sort out different T cells, and distinguish between normal and cancer cells.
7. They can distinguish cancerous from normal cells and can be used to carry isotopes or toxic drugs to
kill tumors.
C. Antibody Diversity (Science Focus box)
1. Immunologists and geneticists knew that each B cell makes an antibody especially equipped to
recognize the specific shape of a particular antigen.
2. However, what was not known was how the human genome contained enough genetic information to
permit the production of up to 2 million different antibody types needed to combat all of the pathogens
an individual is likely to encounter throughout their life.
3. Susumu Tonegawa examined the DNA sequences of lymphoblasts and compared them to mature B
cells.
4. He found that the DNA segments coding for the variable and constant regions of the antibodies were
present in each mature antibody-secreting B cells, where they randomly came together and coded for a
specific variable region.
5. Tonegawa became the first Japanese scientist to win the Nobel Prize in Physiology or Medicine.
D. T Cells and Cell-Mediated Immunity
1.
2.
3.
4.
5.
6.
7.
Like B cells, T cells have unique antigen receptors, called the T cell receptor, or TCR.
However, the receptors of cytotoxic and helper T cells cannot recognize antigen present in the tissues,
lymph, or blood.
Instead, antigen must be presented to them by an antigen-presenting cell (APC).
When an APC presents a viral or cancer cell antigen, the antigen is first linked to a major
histocompatibility complex (MHC) protein; together they are presented to a T cell.
a. The importance of the MHC was recognized when it was discovered it contributes to the difficulty
of transplanting tissues from one person to another.
b. When a donor and recipient are histocompatible, it is likely a transplant will be successful.
When a macrophage antigen is presented to a T cell, the T cell recognizes the antigen.
a. Once a helper T cell recognizes the antigen, it undergoes clonal expansion and produces cytokines
stimulating immune cells to remain active and perform their functions.
b. Once a cytotoxic T cell is activated, it undergoes clonal expansion and destroys any cell that
possesses antigen if the cell bears the correct HLA antigen presented earlier.
c. As the infection disappears, the immune reaction wanes and few cytokines are produced.
Apoptosis occurs in the thymus if the T cell bears a receptor to recognize a self antigen; if apoptosis
does not occur, T-cell cancers result (i.e., lymphomas and leukemias).
Types of T Cells
a. Cytotoxic T Cells
1) They destroy antigen-bearing cells (e.g., virus-infected or cancer cells).
2) They have storage vacuoles that contain perforin molecules.
3) Perforin molecules perforate a plasma membrane; water, salts, and enzymes (called
granzymes) then enter causing the cell to burst.
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b.
c.
4) This is called cell-mediated immunity.
Helper T Cells regulate immunity by enhancing the response of other immune cells.
1) When exposed to an antigen, they enlarge and secrete cytokines.
2) Cytokines stimulate the helper T cells to clone and other immune cells to perform their
functions.
HIV infections
1)
The primary host for an HIV is a helper T cell.
2)
The host helper T cell then produces viruses that go on to destroy more helper T
cells.
3) At first, the individual is able to stay ahead of the virus by producing enough helper T
cells to keep their number within the normal range.
4)
Gradually the T cell count drops to way below normal.
5)
The infected individual comes down with opportunistic infections.
6)
Now the individual has AIDS.
7) An HIV infection is presently a treatable disease, but the regimen is difficult to maintain,
and viral resistance to the medications is becoming apparent.
E. Opportunistic Infections and HIV (Health Focus box)
1. AIDS (acquired immunodeficiency syndrome) is caused by the destruction of the immune system,
followed by an HIV (human immunodeficiency virus) infection.
2. HIV eventually destroys the immune system cells and the person succumbs to infections that would
not cause disease in a person with a healthy immune system.
3. HIV kills helper T cells by directly infecting them, and also causes uninfected T cells to die by
mechanisms, including apoptosis.
4. It may take up to ten years for an individual helper T cell to become depleted so the immune system
can no longer organize a specific response to opportunistic infections (Ols).
5. Some of the illnesses HIV patients may be susceptible to are: Shingles, Candidiasis, Pneumoctosis
pneumonia, Kaposi’s sarcoma, toxoplasmic encephalitis, Mycobacterium avium complex (MAC), and
cytomegalovirus.
6. However, due to the development of powerful drug therapies, people infected with HIV in the U.S. are
suffering lower incidence of Ols than in the 1980s and 1990s.
F. Cytokines and Cancer Therapy
1. Cytokines are signaling molecules produced by either lymphocytes, monocytes, or other cells.
2. Cytokines stimulate white blood cell formation; they may work as adjunct therapy for cancer and
AIDS.
3. Interferon and interleukins are used to improve the ability of an individual’s T cells to fight cancer.
4. Cancer cells with altered proteins on their cell surface should be attacked by cytotoxic T cells.
5. Cytokines may awaken the immune system and lead to the destruction of cancer.
a. Researchers withdraw T cells from a patient and culture them in the presence of interleukin.
b. The T cells are re-injected into the patient; doses of interleukin then maintain the killer activity of
the T cells.
6. Interleukin antagonists may help prevent skin or organ rejection, autoimmune diseases, and allergies
when used as adjuncts for vaccines.
33.4 Immunity Side Effects
A. Tissue Rejection
1. Tissue rejection occurs because cytotoxic T cells cause disintegration of foreign tissue; this is a correct
distinguishing between self and nonself.
2. Selection of compatible organs and administration of immunosuppressive drugs prevent tissue
rejection.
3. Transplanted organs should have the same type antigens as in the recipient.
4. Cyclosporine and tacrolimus both act by inhibiting the response of T cells to cytokines.
5. Xenotransplantation, the transplantation of animal tissues and organs into human beings, is another
way to solve the problem of organ rejection.
B. Disorders of the Immune System
1. Autoimmune diseases result when cytotoxic T cells or antibodies mistakenly attack the body’s own
cells as if they bear foreign antigens.
2. The cause is not known but autoimmune diseases sometimes appear following recovery from an
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infection.
In myasthenia gravis, the neuromuscular junctions do not work properly and muscular weakness
results.
4. In multiple sclerosis (MS), the myelin sheath of nerve fibers is attacked.
5. Persons with systemic lupus erythematosus present many symptoms before dying from kidney
damage.
6. Heart damage following rheumatic fever and type I diabetes are also autoimmune diseases.
7. There are no cures for autoimmune diseases, but they are controlled by drugs.
C. Allergies
1. Allergies are hypersensitivities to substances such as pollen and other everyday substances.
2. A response to these antigens, called allergens, usually involves tissue damage.
3. Immediate and delayed allergic responses are two of four possible responses.
a. Immediate allergic responses occur within seconds of contact with an allergen.
b. Coldlike symptoms are common.
c. IgE antibodies are attached to the plasma membrane of mast cells in tissues and basophils in
blood.
d. When an allergen attaches to IgE antibodies on these mast cells, they release large amounts of
histamine and other substances, which cause the cold symptoms or even anaphylactic shock.
e. Anaphylactic shock is an immediate allergic response that occurs because the allergen has
entered the blood stream; it is characterized by a sudden, life-threatening drop in blood pressure.
4. Allergy shots sometimes prevent the onset of allergic symptoms.
a. Injections of the allergen cause the body to build up high quantities of IgG antibodies.
b. These combine with allergens received from the environment before they have a chance to reach
IgE antibodies located on the plasma membrane of mast cells and basophils.
5. Delayed Allergic Response
a. Delayed allergic responses are initiated by sensitized memory T cells at the site of allergen in the
body.
b. The allergic response is regulated by the cytokines secreted by both T cells and macrophages.
c. The tuberculin skin test is an example: positive test shows prior exposure to TB bacilli but
requires some time to develop reddening of tissue.
3.
Lecture Enrichment Ideas
Experience Base: Due to the success of modern medicine, American students have far less
memorable exposure to both serious infectious disease and vaccines (often administered in
infancy and early childhood). College-age students no longer have smallpox vaccination scars
nor memories of polio epidemics. The disorders that remain are often complex (lupus, allergic
responses, etc.) and not as simple to explicate as earlier infectious diseases. However, the recent
national concerns with biological warfare agents and bioterrorism will provide a new motivation
to understand these concepts since students may have to make their own decisions on smallpox
vaccinations, etc.
1. Inflammation is often overlooked as an important process; emphasize its role as one of the
nonspecific reactions that helps protect the body from invasion. Many people view
inflammation as bad; and we do purchase many over-the-counter drugs to reduce the
symptoms. However, ask students to consider if they really would want to eliminate the
inflammatory response.
2. Describe the specificity of the immune response, with special emphasis on the lock-and-key
mechanism of antigen-antibody interaction.
3. In the tropics, small worms (microfilaria) are injected by mosquitoes and they eventually
clog the lymphatic system, causing symptoms many students will recognize seeing in films
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or videos as “elephantiasis.” Students should now be able to explain why this enlargement
occurs, why it is not immediately fatal, and why it is not curable by surgery.
4. Edema is a fairly common condition; students can understand how a physician or nurse
diagnoses it: press against the skin until a pale fingerprint is left where the surface capillaries
have been pressed free of blood; before the pink color returns, feel if the fingerprint
depression remains; if so, the tissues are retaining water (edema). Such elaborations help
students understand both that the body does contain much body fluid outside the
bloodstream, and that medical professionals must learn an array of diagnostic techniques, in
this case by feel or palpation.
5. Another name for the lymph gland under the armpits or in the groin area was “bubo”; hence
the term bubonic plague due to the enlargement of these glands during infection. Doctors,
unaware of the function of these organs, nevertheless lanced them and drained the large
amount of pus. Students should now be able to describe, in modern terms, why this swelling
occurred.
6.
The critical role of natural bacteria on the skin was demonstrated when an antibacterial hexachloraphene soap
was marketed over three decades ago; it was very effective in reducing harmless bacteria that caused body odor,
but the absence of these natural bacteria allowed harmful bacteria to colonize the skin and caused serious skin
disorders—the soap was soon restricted from everyday use. Milder antibacterial soaps using milder agents have
since been marketed although researchers are still worried about the development of resistant bacterial strains.
7. Distinguish “complement” (meaning “added to for completion”) from compliment (meaning
“to flatter”), since few of today’s students will recognize the first meaning unless
experienced in geometry. The concept of “complement” is also difficult to understand
without use of visuals. Lecture question: Why isn’t “complement” considered an active agent
on its own?
8. Continue exploring in more detail how the immune process becomes more specific as it goes
on, so that later antibodies bind more tightly to the antigen than earlier ones. This is a part of
the clonal selection that occurs, and the memory cells are able to react more selectively
against the antigen if it should appear again. Discuss why you don’t get measles or chicken
pox more than once and how influenza viruses must continually change their surface proteins
to remain a problem for each of us.
9.
Contrast smallpox, which was eradicated, with influenza and malaria, which have not been. The limited
variation, restricted human host, economy of vaccine, etc., that made smallpox vulnerable is not the case for
most agents.
10. Ask why it takes so long (sometimes years) to produce allergy relief through allergy shots
and whether the shots must be continued when relief has occurred.
11. Emphasize the distinction between T and B cells. Discuss the maturation process of T cells in
the thymus and the presence of a separate organ (the bursa of Fabricius) in birds that has a
similar function in maturation of the B cells there. Athymic (nude) mice lack a thymus (and
hair) and can be grafted with practically any tissue (including chicken skin—they grow
feathers!) without rejection because the pre-T cells have no organ in which to mature.
12. Pasteur was working with chicken cholera, passing it from sick to healthy chickens, when he suspended
the research and went on vacation. When he returned, the culture he gave to healthy chickens failed to
kill them. He assumed his culture was bad and injected these same chickens with the fresh disease agent;
they remained healthy, much to his puzzlement. He eventually concluded that his first disease culture had
weakened (attenuated) over time, and could be used to provide immunity through vaccination. This soon
proved to be the case with his rabies tissues, which he used prematurely but with great luck to save a
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boy’s life. Pasteur worked out the concept of “attenuating bacteria” but did not know what the body was
actually doing to form immunity. How would your students explain to Pasteur what the chickens’ bodies
were doing to form immunity, in today’s language?
Critical Thinking
Question 1.
The procedure where a medical doctor uses his or her hands to feel tissues is called “palpation.”
Why would palpation of the lymph nodes under the arm or ear be a common diagnostic practice?
Answer: Infectious agents are often swept along the lymph ducts and end up collected at the lymph nodes when the
swelling indicates that the lymphocytes are fighting the infection.
Question 2. In elephantiasis, small filarial worms enter by mosquito bites and eventually clog
the lymph system, causing great distention of the legs, arms, scrotum, etc. It is untreatable. What
causes these symptoms and why is it virtually untreatable?
Answer: There is a gradual flow of body fluids, from serum seeping across the capillary wall through the extensive
lymph ducts and eventually dumping back into the bloodstream at the subclavian vein. The filarial worms clog the
lymphatic ducts and lymph backs up, greatly distending these tissues that, like veins, move lymph by valves and
muscular contractions from the lower extremities. It is not possible to remove the worms since the surgical intrusion
would be far more damaging, and the worms apparently present a cuticle surface that can avoid the immune
response.
Question 3.
If a drug such as Cyclosporine can prevent tissue rejection, why can’t it be used to cure the
autoimmune diseases?
Answer: Cyclosporine acts by inhibiting the production of interleukin-2. Interleukins improve
the ability of an individual’s own T cells to fight cancer and other foreign proteins. Autoimmune
diseases appear after infections and may be due to the T cells learning to attack the body’s own
tissues. If suppression of the T cells improved a patient with an autoimmune disease, the patient
would also be at higher risk for other infections.
Question 4.
Although we presumed we had eradicated smallpox as a disease, recent evidence indicates that the
virus had been cultured and weaponized, and that there is a risk that it could be released as a bioterrorism agent. The
duration of a smallpox vaccination is unknown, but protection is not ensured past ten years. The U.S. discontinued
smallpox vaccination of citizens about 1980. Smallpox is unique in that a person exposed to the virus can still be
protected if they receive the vaccine within the next 3–4 days. (1) Why would a bioterrorism attack in the U.S. be
met with an uneven occurrence of the disease, compared with the earlier natural outbreaks in third world countries?
(2) What is the effectiveness of this biological warfare agent as time goes by? (3) If the current U.S. stockpile of a
few million doses was diluted to allow more to be vaccinated, what would be the result?
Answers: (1) Previously vaccinated older persons would have varying titers of antibody depending on their personal
levels of immunity, although most would not be protected. (2) With more passage of time, the increase in
unvaccinated younger population and loss of protection in aging older people makes smallpox a more devastating
biological weapon. (3) In case of an attack, the administration of a diluted vaccine could reduce the severity of the
disease.
Question 5.
If you are bitten by an eastern diamondback rattlesnake, you could be given antivenin made from
the plasma of horses given injections of venom from this exact species, or a general rattlesnake antivenin made by
injecting a mixture of many different rattlesnake venoms. Which would you prefer for fast action and why is it faster
acting?
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Answer: Since this provides passive immunity, or borrows the antibodies already built up by the horse, this does not
involve the buildup of your memory cells but merely uses the antibodies immediately present in the horse serum.
The antivenin made from the exact same species would have the highest dose of these antibodies and would be more
effective; the mixture would contain a much lower dose since antibodies made to other proteins would not be
effective. The general antivenin is made for use when the identity of the rattlesnake is uncertain.
Question 6.
Most people in the United States trace ancestry to Europe where the population represents the
resistant survivors of many centuries of evolution with tuberculosis. Therefore, in the U.S., tuberculosis is not
prevalent, is usually a chronic disease, and we monitor it by using an allergy test to detect individuals who have
been exposed. However, in Asia, tuberculosis is much “newer” and the population has not had as many centuries to
select for resistant survivors. The tuberculosis cases are more acute and easily transmitted; therefore the health
officials vaccinate the public with a related BCG bacterium that provides some immune response. (1) Can Asia
monitor for tuberculosis using the patch test? (2) Which continent is using the correct strategy? (3) If you traveled
briefly to Asia, should you get the BCG vaccine?
Answers: (1) Once you receive the BCG vaccine, you will test positive for tuberculosis; thus the allergy test is not
useful. (2) The two strategies are appropriate for each respective region. (3) If you take the BCG vaccine, you would
always test positive after return to the U.S. and would have to have a chest x-ray to check for tuberculosis; the BCG
might be justified if you traveled to a tuberculosis ward or other “hotspot” but would otherwise be problematic.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
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CHAPTER
34
DIGESTIVE SYSTEMS AND NUTRITION
This chapter presents a study of the digestive systems of animals, with emphasis on the human
system. The activity of each part of the digestive system is described in detail. Various digestive
enzymes and their mode of action are described. The chapter concludes with a study of nutrition:
the various nutrients necessary for the human body and the functions in which they participate. A
Health Focus box describes the “Wall of the Digestive Tract.”
Chapter Outline
34.1 Digestive Tracts
1. Most animals need to digest food into small molecules that can cross plasma
membranes.
2.
Digestion provides the energy needed to carry out routine metabolic activities and maintain
homeostasis.
3. The digestive tract ingests food, breaks down food into small molecules that can cross plasma
membranes, absorbs these nutrient molecules, and eliminates nondigestible remains.
A. Incomplete Versus Complete Tracts
1. Planarians are organisms with an incomplete digestive tract, a single opening (usually called a
mouth).
a. Planaria are carnivorous and feed largely on smaller aquatic animals.
b. The digestive system contains only a mouth, a pharynx, and an intestine.
c. To feed, its pharynx extends far beyond the mouth to suck up minute quantities at one time.
d. Digestive enzymes in the tract allow some extracellular digestion.
e. Digestion is finished intracellularly by cells that line the digestive cavity; food then diffuses to
nearby cells.
f. The digestive system lacks regions of specialized function.
g. The tapeworm, relatives of planaria, lack a digestive system altogether; they absorb food through a
body wall with modified microscopic projections that absorb nutrients from the host.
2. In contrast, the earthworm has a complete digestive tract.
a. The digestive system is composed of a tube with a mouth and an anus.
b. Earthworms feed on decayed organic matter in the soil.
c. Different regions of the gut have specialized functions (e.g., ingestion, mechanical digestion, etc.).
d. A muscular pharynx draws in food with sucking action.
e. The crop is storage area with expansive walls.
f. The gizzard has thick muscular walls to grind food.
g. Digestion occurs in the intestine, outside of cells or “extracellular.”
h. The surface area for absorption is increased by an intestinal fold called the typhlosole.
i. The undigested remains exit the body at the anus.
B. Continuous Versus Discontinuous Feeders
1. Clams are continuous feeders, or filter feeders.
a. Water moves into a mantle cavity through an incurrent siphon and deposits particles on gills.
b. Ciliary action moves particles to the labial palps which direct them into the mouth and into the
stomach.
c. Digestive enzymes from a digestive gland help amoeboid cells in the tract complete digestion.
2. Marine fanworms are sessile filter feeders; only small particles are consumed while large particles are
rejected.
3. Baleen whales are active filter feeders; baleen (fringe) filters small krill from water.
4. Squids are an example of discontinuous feeders.
a. The head of a squid has ten arms; two arms seize the prey and bring it to the squid’s mouth.
b. Beaklike jaws and a radula (toothy tongue) reduce the food to pieces.
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c. The esophagus leads to a stomach that holds food until digestion is complete.
d. Discontinuous feeders require a storage region in the gut.
C. Adaptation to Diet
1. Animals are herbivores (eat plants only) or carnivores (eat only other animals) or omnivores (eat both).
2. Invertebrates demonstrate a wide variety of diets.
3. Mammal dentition differs according to their mode of nutrition.
a. Omnivores, including humans, have dentition that accommodates both a vegetable and meat diet.
b. Omnivore teeth include incisors (cutting and shearing), canines (tearing), premolars (crushing),
and molars (grinding).
c. Herbivores have large, flat premolars and molars for grinding plant matter.
d. Grazers (e.g., horses) have sharp incisors for clipping off grass and leaves.
e. Hard-to-digest plant material requires extensive grinding to disrupt the plant cell walls.
f. Animals that feed on plants may have long and complex digestive tracts and bacteria in their
digestive tracts that can digest cellulose, producing nutrients (glucose) that the animal can use.
g. Some grazers have a rumen to digest chewed grasses; partially digested cud is then re-chewed.
h. Carnivores’ pointed incisors and canines tear off pieces small enough to swallow.
i. Meat is rich in protein and fatty acids and is easier to digest than plant material.
j. Carnivores have fewer molars for grinding and a shorter digestive tract with less specialization.
34.2 Human Digestive Tract
1. The human digestive tract is a complete tube-within-a-tube system.
2. Each part of the digestive system has a specific function.
3. Food is never found within the accessory glands, only within the tract itself.
4. The digestion of food in humans is an extracellular process.
5. Enzymes are secreted into the digestive tract by nearby glands which never contain food themselves.
6. Digestion requires a cooperative effort by the production of hormones and the actions of the nervous
system.
A. Mouth
1. Food is chewed in the mouth (oral cavity) and mixed with saliva; the mouth is the beginning of the
digestive tract.
a. Three pairs of salivary glands secrete saliva by way of ducts into the mouth.
b. Salivary amylase is the enzyme that begins starch digestion; maltose is the common end product.
c. Food is manipulated by a muscular tongue containing both touch and pressure receptors.
d. Taste buds are located primarily on the tongue but also on the surface of the mouth; these
chemical receptors are stimulated by the chemical composition of food.
e. Food is chewed and mixed with saliva to form a bolus in preparation for swallowing.
B. The Pharynx and the Esophagus
1. The digestive and respiratory passages come together in the pharynx, and then separate.
a. During swallowing, the pathway of air to the lungs could be blocked if food entered the trachea.
b. The epiglottis covers the opening into the trachea as muscles move a bolus of food through the
pharynx into the esophagus.
2. The esophagus is a muscular tube that moves swallowed food to the stomach by peristalsis, a
rhythmical contraction that moves the contents along in tubular organs.
C. Stomach
1. The stomach stores liters of partially digested food, freeing humans from continual eating.
2. Dr. William Beaumont revealed much of the stomach’s functions in the mid-1800s.
a. Alexis St. Martin had an opening (fistula) into the stomach, received from a gunshot, through
which Dr. Beaumont could observe stomach activity.
b. Beaumont collected the gastric juice produced by cells of gastric glands.
c. Walls of the stomach contract vigorously and mix food with juices secreted when the food enters.
d. Beaumont found that gastric juice contains hydrochloric acid and another digestive substance,
pepsin.
e. He discovered gastric juices are produced independently of the protective mucous secretions.
f. His careful work pioneered the study of the physiology of digestion.
3. Hydrochloric acid (HCl) lowers pH of the gastric contents to about 2.
a. The epithelial lining of the stomach has millions of gastric pits leading to gastric glands.
b. This acid kills most bacteria and other microorganisms.
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c.
The low pH also stops the activity of salivary amylase and promotes the activity of pepsin, an
enzyme that digests large proteins to smaller peptides.
4. A thick layer of mucus protects the wall of the stomach from the HCl and pepsin.
5. Ulcers develop when the lining is exposed to digestive action; recent research indicates this is usually
due to infection by Helicobacter pylori bacteria.
6. Stomach contents, a thick, soupy mixture, is called chyme.
7. At the base of the stomach is a narrow opening controlled by a sphincter (a circular muscle valve).
a. When the sphincter relaxes, chyme enters the first part of the small intestine, called the duodenum;
a neural reflex causes the sphincter to contract, closing off the opening.
c. The sphincter relaxes and allows more chyme to enter the duodenum.
d. The slow, rhythmic pace with which chyme exits the stomach allows for thorough digestion.
D. The Small Intestine
1. The human small intestine is a coiled muscular tube about three meters long.
2. As chyme enters the duodenum, proteins and carbohydrates are partly digested but no fat digestion
occurs.
3. Additional digestion is aided by secretions from the liver and the pancreas.
a. Bile is a secretion of the liver temporarily stored in the gallbladder before being sent to
duodenum; bile emulsifies fat (allows fat droplets to disperse in water).
4. The lining of the small intestine has ridges and furrows; these surfaces are covered by villi (sing.
villus); the small intestine is specialized for absorption by the huge number of villi that line the
intestinal wall.
a.
Villi are fingerlike projections; their surface cells are covered by microvilli.
b.
Microvilli are minute projections, called the “brush border,” on the surface of the cells of
the intestinal villi.
c.
Ridges, furrows, villi, and microvilli greatly increase the effective surface area of the
small intestine.
5. Each villus contains blood vessels and a lymphatic capillary, called a lacteal.
a. A lacteal aids in the absorption of fats.
6. Sugars and amino acids enter villi cells and are absorbed into bloodstream.
7. Glycerol and fatty acids enter villi cells; reassembled into fat molecules, they move into lacteals.
8. After nutrients are absorbed, they are eventually carried throughout the body by the bloodstream.
E. Large Intestine
1. The large intestine has four parts: the cecum, colon, rectum, and anal canal.
2. It is larger in diameter but shorter in length than the small intestine and is the region following the
small intestine.
3. Appendix
a. The appendix is a fingerlike projection extending from the cecum, a blind sac at the junction of
the small and large intestine.
b. It may play a role in fighting infections.
c. If an infected appendix bursts, it results in general abdominal infection , called peritonitis.
4. The colon is subdivided into the ascending, transverse, descending, and sigmoidal colon.
5. About 1.5 liters of water enter the digestive tract daily from ingestion and another 8.5 liters enter from
various secretions.
a. About 95% of this total liquid is reabsorbed by the small intestine; most of the remainder is
absorbed by cells of the colon.
b. If the water is not reabsorbed, it causes diarrhea which can cause a serious dehydration and ion
loss.
6. In addition to water, the large intestine absorbs salts and some vitamins, including the vitamin K
produced by intestinal bacteria.
7. The large intestine terminates at the anus, the opening of the anal canal.
8. Feces
a. Feces consists of about 75% water and 25% solid matter.
b. One-third of the solid matter is intestinal bacteria.
c. The remainder is undigested wastes, fats, organic material, mucus, and dead cells from the
intestinal lining.
9. Intestinal polyps are small growths arising from the epithelial lining.
a. Whether they are benign or cancerous, polyps can be removed surgically.
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b.
A low-fat, high-fiber diet promotes regularity and may provide protection against mutagenic
agents.
F. Three Accessory Organs
1. The Pancreas
a. The pancreas lies deep within the abdominal cavity, just below the stomach, and rests on the
posterior abdominal wall.
b. It is an elongated and somewhat flattened organ.
c. As an endocrine gland, it secretes glucagon and insulin hormone into the bloodstream.
d. As an exocrine gland, it secretes pancreatic juice.
1) Pancreatic juice contains sodium bicarbonate that neutralizes acidic chyme.
2) Pancreatic enzymes digest carbohydrates, fats, and proteins.
2. The Liver
a. The liver, the largest gland in the body, fills the top of the abdominal cavity, just under the
diaphragm.
b. The liver has numerous functions:
1) It detoxifies blood by removing and metabolizing poisonous substances.
2) It makes plasma proteins including albumin and fibrinogen.
3) It destroys old red blood cells and converts hemoglobin to bilirubin and biliverdin in bile.
4) It produces bile stored in the gallbladder before it enters the duodenum to emulsify fats.
5) It stores glucose as glycogen and breaks down glycogen to maintain a constant blood glucose
concentration.
6) The liver produces urea from amino groups and ammonia.
c. Blood vessels from both the large and small intestines lead to the liver as the hepatic portal vein.
d. The liver maintains the blood glucose level at 0.1% by removing glucose from the hepatic portal
vein to store as glycogen; when needed, glycogen is broken down and glucose re-enters the
hepatic vein.
e. Amino acids can be converted to glucose but deamination (removal of amino groups) must occur.
f. By a complex metabolic pathway, the liver converts amino groups to urea, the nitrogenous
breakdown product of amino acids.
g. Urea is the most common human nitrogenous waste; it is transported by the blood to the kidneys.
3. Liver Disorders
a. Jaundice is a symptom involving a yellowish skin due to a large amount of bilirubin in blood.
1) Jaundice can also result from hepatitis, inflammation of the liver.
b. Viral hepatitis is a viral liver infection.
1) Hepatitis A results from eating contaminated food.
2) Hepatitis B and C are spread by blood transfusions, kidney dialysis, and unsterile needle use.
3) All three can be caused from sexual contact.
c. Cirrhosis is a chronic disease where the liver tissue is replaced by fatty tissue and then scar tissue;
alcoholics provide too much alcohol for the liver to break down.
4. The Gallbladder
a. The gallbladder is a pear-shaped, muscular sac attached to the surface of the liver.
b. Bile, produced in the liver, is stored in the gallbladder.
c. When needed, bile leaves the gallbladder and goes into the duodenum via the common bile duct.
G. Wall of the Digestive Tract (Health Focus box)
 The wall of the digestive tract has four layers.
1.
Mucosa (mucous membrane)
a.
The mucosa is the first layer of the wall next to the lumen.
b. The mucosa produces mucus, which protects the wall from the digestive enzymes inside the
lumen.
c. Diverticulosis is the condition in which portions of the mucosa have pushed through the other
layers and formed pouches where food can collect.
d. When the pouches become inflamed or infected, the condition is called diverticulitis.
2.
Submucosa
a.
The submucosa is the second layer in the digestive wall.
b.
This layer is a broad band of loose connective tissue that contains blood vessels,
lymphatic vessels, and nerves.
c. Because the submucosa contains blood vessels, it can be the site of an inflammatory response that
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leads to inflammatory bowel disease (IBD), which is categorized by chronic diarrhea, abdominal
pain, fever, and weight loss.
3.
Muscularis
a.
The muscularis is the third layer of the digestive tract wall.
b.
This layer contains two layers of smooth muscle.
c.
The contraction of these muscles accounts for movement of digested food from the
esophagus to the anus.
d. The muscularis can be associated with Irritable Bowel Syndrome (IBS), in which contractions of
the wall cause abdominal pain, constipation, and/or diarrhea.
4.
Serosa
a.
The serosa is the fourth layer of the digestive tract wall.
b.
This layer secretes a serous fluid and is part of the internal lining of the abdominal cavity.
c.
An inflamed appendix (appendicitis) has to be removed because, should the appendix
burst, the result can be peritonitis, a life-threatening infection of the peritoneum.
34.3 Digestive Enzymes
1. Salivary amylase is the enzyme that begins starch digestion; maltose is the common end product.
a. Maltose cannot be absorbed in the small intestine; additional digestive action breaks the maltose
into glucose, which can be absorbed.
2. Protein digestion begins in the stomach.
a. Pepsinogen is converted to pepsin when exposed to the HCl in the stomach; pepsin breaks
proteins into smaller peptides.
3. Starch, proteins, nucleic acids, and fats are enzymatically broken down in the small intestine.
a. Pancreatic amylase digests starch to maltose.
b. Trypsin, also a pancreatic enzyme, digests protein to peptides.
c. Peptidases and maltase, produced by the small intestine, complete the digestion of protein to
amino acids and starch to glucose, respectively.
d. Lipase, another pancreatic enzyme, digests fat droplets to glycerol and fatty acids.
34.4 Nutrition
A. Carbohydrates
1. Carbohydrates are present in food in the form of sugars, starch, and fiber.
2. After being absorbed from the digestive tract into bloodstream, all sugars are
converted to glucose for transport in the blood and use by cells.
3. Good sources of starch are: beans, peas, cereal, grains, and potatoes.
4. Fiber includes various undigestible carbohydrates derived from plants.
a. Food sources rich in fiber are: beans, peas, nuts, fruits, vegetables, and wholegrain products.
b. Fiber is not a nutrient because it cannot be digested to small molecules that enter
the bloodstream.
c. Insoluble fiber (e.g., as found in wheat bran) has a laxative effect and may guard
against colon cancer.
d. Soluble fiber (e.g., as found in oat bran) combines with bile acids and cholesterol
in the intestine and prevents their absorption.
e. The recommended daily intake of fiber is 25 g for women and 38 g for men. The
average American is consuming 15 g of fiber per day.
5. Complex carbohydrates (e.g., whole grain cereals, breads, etc.) are recommended
because they are digested to sugars and contain fiber.
B. Lipids
1. Triglycerides (fats and oils) supply energy for cells, but fat is stored for the long
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term in the body.
2. Nutritionists recommend that people include unsaturated (i.e., canola and soybean
oil), rather than saturated (i.e., butter, meat, cheese), fats in their diets.
3. Cholesterol is a lipid and can be synthesized by the body.
a. Cholesterol is not found in plant foods; only animal foods (chesse, egg yolk, liver,
shrimp and lobster) are rich in cholesterol.
4. Trans fatty acids are more likely to cause CV disease than saturated fats—any
packaged goods that contain partially hydrogenated vegetable oils (“shortening”) will
likely contain trans fat.
C. Proteins
1. Dietary proteins are digested to amino acids, which cells use to synthesize hundred
of cellular proteins.
2. Of the 20 different amino acids required for protein synthesis, 8 (9 in children) cannot
be synthesized by the body and are thus termed essential amino acids.
3. Some foods do not provide all the essential amino acids—vegetarians should combine
two or more plant products to acquire all the essential amino acids.
4. A high-protein diet can harm the body.
a. Dehydration can occur.
b. Calcium loss in the urine can occur, leading to kidney stones.
D. Diet and Obesity
1. Type 2 Diabetes
a. Diabetes mellitus is indicated by the presence of glucose in the urine.
b. Glocuse has spilled over into the urine because there is too high a level of glucose in the blood.
c. In type 2 diabetes, the person is usually obese and displays imparied insulin production and insulin
resistance.
d. The body’s cells can’t take up glucose, even when insulin is present; the blood level exceeds the
norrnal level and glucose appears in the urine.
e. Type 2 diabetes is increasing rapidly in most industrial countries of the world.
f. A healthy diet, increased physical activity, and weight loss have been seen to improve the ability
of insulin to function properly in type 2 diabetes.
2. Cardiovascular Disease
a. Cardiovascula disease (including hypertension, heart attack, and stroke) is among the leading
causes of death in the United States.
b. Cardiovascular is often due to arteries blocked by plaque.
c. Consuming saturated fats tend to raise LDL cholesterol levels, while eating unsaturated fats
lowers LDL cholesterol levels.
d. The American Heart Association recommends limiting total cholesterol intake to 300 mg per day.
E. Vitamins and Minerals
1. Vitamins are essential organic compounds the body cannot make but still requires for metabolic
activities.
a. Many vitamins are portions of coenzymes: e.g., niacin is part of NAD+ and riboflavin is a part of
FAD.
b. Coenzymes are needed in small amounts because they are used repeatedly.
c. Vitamin A is not a coenzyme but a precursor for the visual pigment that prevents night blindness.
d. Lack of vitamins results in vitamin deficiencies.
e. The 13 vitamins are divided into those that are fat soluble and those that are water soluble.
2. Humans require 20 minerals (e.g., calcium, phosphorus).
a. They are constituents of cells and body fluids and structural components of tissues.
b. Calcium is needed to build bones and teeth and for nerve conduction and muscle contraction.
c. Many people consume too much sodium, which can cause water retention and contribute to
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hypertension.
Lecture Enrichment Ideas
Experience Base: A science instructor may want to browse through a nutrition store to gain an
idea of the extent of local use of supplements and the alluded-to claims of dietary supplements
that do not have full research support. This is one area where students do not have substantially
less experience than earlier generations. Much of the content of this chapter will be meaningful
to most students.
1.
It is difficult to discuss the digestive systems of organisms meaningfully without an awareness of the lifestyle,
habitat, etc., of those organisms. Describing the complete and incomplete guts, and continuous and
discontinuous feeders is much easier with visual slides or videotapes showing the organisms in action.
2. Describe the differences in teeth and digestive tracts among mammalian omnivores,
herbivores, and carnivores. For fossil animals, sometimes the only evidence paleontologists
have to deduce the life history of organisms is their dentition.
3. In comparing cnidaria and planaria to earthworms and insects, describe the increased need
for separation and sophistication of the digestive, respiratory, and circulatory systems. Note
that parasitic forms may often simplify from their complex ancestors.
4.
Compare the earthworm’s stomach, which is divided into crop and gizzard, to both the cow’s stomach, with its
four separate areas to hold and digest food, and the simpler stomach of a human or a cat. Ask students how
these specializations are related to diet.
5. The effects of malnutrition were and are a window into the function of both the human body
and the role of various nutrients. Examples can be drawn from cases of starvation in the
world, and from history as in the case of the prison camp at Andersonville; reading factual
symptomology can make vivid the effects of lack of proper nutrition.
6. Most students will be familiar with vitamins and other supplements. This is an opportunity to
extend discussion into the science necessary to establish minimum daily requirements,
genuine bioactivity, and solid guidelines to avoid deficiency or overdose.
7. Most people consider diarrhea a nuisance, but what would be the biological consequences of
not having diarrhea when an amoeba or other parasite that produces toxins was residing in
the intestine?
8. Models of the digestive system provide superior hands-on understanding to students.
9.
The kidney (not yet studied in this textbook sequence) was one of the first organs to be successfully
transplanted since it has a single generalized function, one major artery and vein, and is compartmentalized in
connective tissue. Ask students to suggest why the liver is one of the last organs to be successfully
transplanted—a question that will require recognition of its many complex metabolic functions and intricate
blood vessel supply. Ask why a person who drinks alcohol excessively throughout early life might die from cat
scratches or other minor infections from trivial bacteria that would not cause more than a minor fever in most
people.
10. Pancreatic cancer is generally fatal. Ask students to describe the functions that would be
required of a hypothetical artificial pancreas machine.
11. When a person who has an inflamed gallbladder consumes food high in fats, they often begin
to have pains from the active gallbladder. Lecture question: How does the gallbladder
“know” that fats are “on their way?”
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12. The use of actual experimental results is the best opportunity to also help students think scientifically. In using
this “cookbook” experiment with four tubes with HCl, enzyme, both and neither, is the control really necessary
and why? If digestion occurs with an enzyme and no HCl, does that prove that HCl was inert; what additional
experiment would be necessary to confirm this?
13. The attitudes of many citizens who are not obese contribute to the “ideal body image” in
advertisements, beauty pageants, etc. Sensitization of more students to the adverse
consequences can contribute to the eventual solution.
Critical Thinking
Question 1. Sometimes a section of intestine must be removed when it is found to be
cancerous. Which would most adversely affect the functioning of the total digestive system: the
loss of one foot of duodenum, one foot of the lower small intestine, or one foot of the large
intestine? Why?
Answer: There would be substantial portions of small and large intestine left over to perform the functions of
nutrient and water absorption, respectively. However, one foot of duodenum would constitute most of this section.
In addition, it is the region that must accommodate the influx of stomach contents, bile salts, and pancreatic juices.
Question 2.
Although we have developed artificial kidneys and hearts, there really is no machine that can
function as an artificial liver. Why is it so difficult to replicate all of the functions of the liver?
Answer: Unlike the heart that has the single responsibility to pump blood, or the kidney that mainly filters blood,
the liver has six major and distinct functions: detoxifying blood of metabolic poisons, making blood proteins,
destroying old red blood cells and converting hemoglobin to bile products, producing bile to emulsify fats, storing
and releasing glucose/glycogen, and producing urea from amino groups. An artificial liver would have to consist of
six complex machines.
Question 3.
There are a few populations of people on the globe who have a long tradition of many more
individuals living to great old age (over 100). When emigrants from these regions live in modern settings with ample
diets, their survivorship is lower and near average levels. The one common factor appears to be a much lower caloric
intake, perhaps 70% of the average daily allowance considered “normal.” Discuss this in the light of “antioxidants.”
Answer: A person who lives on less food would produce fewer free radicals and suffer less
cellular damage. However, they would risk malnutrition by living nearer the minimal threshold
for vital nutrients and vitamins.
Question 4.
If the food product is appetizing to a person, why is it unwise to focus on a diet of just this one
food item, even if it is high in protein like soybeans?
Answer: A food can be high in protein but have a limited number of the essential acids that cannot be formed by
body metabolism and must be in the diet. The amino acids provided by soy products would not include all of these
amino acids and another source would be necessary to prevent deficiency diseases.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
262
CHAPTER
35
RESPIRATORY SYSTEMS
This chapter studies the respiratory systems of animals, beginning with those that inhabit aquatic
environments and progressing to the human respiratory system, described in detail. The
biochemistry of gas transport and gas exchange is described. A Science Focus box answers,
“Questions About Tobacco, Smoking, and Health.” A number of human diseases of the
respiratory system are discussed.
Chapter Outline
35.1 Gas Exchange Surfaces
Respiration is the sequence of events that results in gas exchange between the environment and the body’s cells.
1. Ventilation (breathing) includes inspiration (bringing air in) and expiration (moving
air out).
2.
3.
4.
5.
6.
7.
External respiration involves gas exchange with the external environment in the lungs.
Internal respiration involves gas exchange between the blood and tissue fluid.
An effective gas exchange region must be moist, thin, and large in relation to the size of the body.
Some animals are small and shaped to allow their surface to be an adequate gas-exchange surface.
Larger animals are complex and have a specialized gas-exchange surface.
Diffusion improves with vascularization; gas delivery to cells is promoted if the blood contains
hemoglobin.
A. External Gas-Exchange Surfaces
1. It is more difficult for animals to obtain O2 from water than from air.
a. Water fully saturated with air contains only a fraction of the O2 as the same volume of air.
b. Water is more dense than air; therefore aquatic animals must use more energy to respire.
c. Fishes use up to 25% of their energy to respire, whereas land mammals use only 1–2% of their
energy output.
2. Hydras and planaria have a large surface area in comparison to their size.
a. Gas exchange occurs directly across their body surface.
b. The hydra’s outer cell layer contacts the environment; an inner layer exchanges gases with the
water in the gastrovascular cavity.
c. The flat body of planaria permits cells to exchange gases with the external environment.
3. A tubular shape and vascularized parapodia extensions in polychaete worms provide surface areas for
diffusion.
4. The earthworm is an invertebrate that uses its body surface for respiration.
a. An earthworm expends energy to secrete mucus and release fluids from excretory pores.
b. The earthworm is also behaviorally adapted to stay in the moist soil during the day when air is
driest.
5. Aquatic animals often pass water over gills.
a. Gills are finely divided and vascularized outgrowths of either an outer or inner body surface.
b. Among clams, water is drawn into the mantle cavity and flows over gills.
6. Insects have a system of air tubes called tracheae through which oxygen is delivered directly to the
cells without entering the blood.
7. Terrestrial vertebrates usually have lungs, which are vascularized outgrowths fron the lower
pharyngeal region.
a. The lungs of birds and mammals are subdivided into smaller passageways and spaces.
B. The Gills of a Fish
1. Decapod gills are located in brachial chambers under the exoskeleton; water is circulated by special
mouthparts.
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2.
3.
4.
Fish gills are outward extensions of the pharynx organized into arches.
Ventilation is the result of the combined action of the mouth and gill covers.
When the mouth is open, the opercula are closed and water is drawn in; the mouth then closes and the
opercula open, drawing water from the pharynx through gill slits located between the gill arches.
5. To the outside of the gill arches are gill filaments folded into platelike lamellae, each of which contains
capillaries; the result is a tremendous surface area for gas exchange.
a.
Blood in capillaries of gill lamellae flows in a direction opposite to that of water.
b.
This countercurrent flow of water and blood increases the amount of O2 and
CO2 exchanged.
c.
Such a countercurrent mechanism extracts about 80–90% of the initial dissolved
O2 in the water.
C.
The Tracheal System of Insects
1. Insects and certain terrestrial arthropods utilize air tubes called tracheae.
a. Oxygen enters a tracheal system at spiracles, which are valvelike openings at each side of the
body.
b. The tracheae branch and rebranch to end in tiny tracheoles that are in direct contact with body
cells.
c. Larger insects have air sacs located near major muscles to keep air moving in and out of the
trachea.
d. The tracheae effectively deliver adequate oxygen to the cells of insects; the circulatory system has
no role in gas transport.
2.
Larger insects have air sacs located near major muscles.
a.
Contraction and relaxation of these muscles draw in and empty air.
3. For insects that spend their larval or adult stages in water, the tracheae do not receive air by way of
spiracles.
a. Diffusion of oxygen across the body wall supplies the tracheae with oxygen.
D. The Lungs of Humans
1. The human respiratory system includes everything that conducts air to and from the lungs; the lungs lie
deep within the thoracic cavity for protection against drying.
2. Air moves into the nose, crosses the pharynx, flows through the glottis (an opening into the larynx or
voice box) to the trachea, bronchi, bronchioles, and finally the alveoli, where gas exchange occurs.
a. This process filters debris, warms the air, and adds moisture.
b. When the air reaches the lungs, it is at body temperature and is saturated with water.
c. The trachea and bronchi are lined with cilia that beat upward carrying mucus, dust, and any food
particles that went the wrong route.
d. The hard and soft palates separate the nasal cavities from the mouth.
e. When food is being swallowed, the glottis is closed by the epiglottis, and the soft palate covers the
entrance of the nasal passages into the pharynx.
f. At the edges of the glottis are vocal cords; as air passes across them, these tissues vibrate creating
sounds.
g. From the larynx, air flows down the trachea to the bronchi.
1) The larynx is held open by cartilage that forms the Adam’s apple.
2) The trachea walls are reinforced with C-shaped rings of cartilage.
h. The trachea divides into two bronchi; C-shaped rings of cartilage diminish as bronchi branch.
i. Within the lungs, each bronchus branches into numerous bronchioles that conduct air to alveoli.
j. Alveoli are microscopic air sacs.
E. Questions About Tobacco, Smoking, and Health (Science Focus box)
1. There is no safe way to smoke. All cigarettes can damage the human body.
2. Cigarette smoking contains nicotine, which is addictive.
3. Smoking does cause cancer: Tobacco use accounts for about one-third of all cancer deaths in the
United States.
4. Cigarette smoke affects the lungs by causing chronic bronchitis, emphysema, and chronic obstructive
pulmonary disease (as well as cancer).
5. Smokers have a “smoker’s cough” because cigarette smoke contains chemicals that irritate the air
passages and lungs.
6. If you smoke, but don’t inhale, there is still danger. Nonsmokers are breathing the second hand smoke
from smokers and are still at risk for lung cancer.
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7.
8.
Cigarette smoking affects the heart by increasing the risk of heart disease.
Smoking while pregnant is linked to a greater chance of miscarriage, premature delivery, stillbirth,
infant death, low birth weight, and sudden infant death syndrome (SIDS).
9. Some of the short-term effects of smoking cigarettes include shortness of breath, a nagging cough,
diminished ability to smell and taste, premature aging of skin, and increased risk of sexual impotence
in men. Long-term effects include cancer, heart disease, aneurysms, bronchitis, emphysema, and
stroke.
10. Environmental tobacco smoke (ETS) causes about 3,000 lung cancer deaths and about 35,000–40,000
deaths from heart disease each year (in nonsmokers).
11. Chewing tobacco and snuff are not safe alternatives to cigarette smoking. The risk of other cancers is
still increased.
35.2 Breathing and Transport of Gases
A. Breathing
1. Inspiration (or inhalation) is the act of moving air into the lungs.
2. Expiration (or exhalation) is the act of moving air out of the lungs.
3. Mammals have both a rib cage and a diaphragm.
a. The diaphragm is a horizontal muscle that divides the thoracic cavity from the abdominal cavity.
b.
As the thoracic cavity expands, the lung volume increases; air flows in due to
the difference in air pressure.
c. By lowering the ribs, pressure is exerted on the lungs, which forces air out.
d. All terrestrial vertebrates, except birds, use a tidal ventilation mechanism; air moves in and out by
the same route.
1) The lungs of reptiles, amphibians, and mammals are not completely emptied during each
breathing cycle.
2) With incomplete ventilation, entering air mixes with used air in the lungs.
3) This conserves moisture but decreases gas-exchange efficiency.
e. The high oxygen requirements of flying birds require a one-way ventilation mechanism.
1) Incoming air is carried past the lungs by a trachea that takes it to a set of posterior air sacs.
2) Air then passes forward through the lungs into a set of anterior air sacs and is finally expelled.
3) The one-way flow means that oxygen-rich air does not mix with used air; this maximizes gas
exchange.
4. Adults typically have a breathing rate of 12 to 20 ventilations per minute.
5. The rhythm of ventilation is controlled by a respiratory center in the medulla oblongata of the brain.
6. Even though the respiratory center automatically controls the rate and depth of breathing, the activity
can be influenced by nervous and chemical inputs.
a. The chemoreceptors in the carotid bodies, located in the carotid arteries, and in the aortic bodies,
located in the aorta, will stimulate the respiratory center during intense exercise.
B. Gas Exchange and Transport
1. Respiration includes external and internal respiration.
2. Gas exchange between the air in the alveoli and the blood in the pulmonary capillaries is primarily by
diffusion.
3. The amount of pressure each gas exerts is called the partial pressure (PO2 and PCO2).
a. CO2 diffuses from higher concentration in the blood across the walls of alveolar capillaries to
lower concentration in the air in the alveoli.
b. Oxygen diffuses from higher concentration in alveoli across the walls of the alveolar capillaries to
the lower concentration in the blood.
C. Transport of Oxygen and Carbon Dioxide
1. Most O2 entering the pulmonary capillaries combines with hemoglobin (Hb) to form oxyhemoglobin
(HbO2).
Hb
+
O2
→
HbO2
deoxyhemoglobin
2.
oxygen
oxyhemoglobin
Each hemoglobin molecule has four polypeptide chains; each chain folds over an iron-containing
heme.
a. Each RBC has 250 million hemoglobin molecules.
b. Each RBC can carry a billion molecules of O2 oxyhemoglobin.
c. The iron atom of a heme group loosely binds with an O 2 molecule.
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3.
4.
5.
The oxygen-binding ability of hemoglobin can be graphed.
a. The percentage of oxygen-binding sites of hemoglobin carrying O2 varies with partial pressure
of O2 in the immediate environment.
b. The partial pressure is the amount of pressure exerted by a particular gas among all of the gases
present.
c. At a normal partial pressure of O2 in lungs, hemoglobin becomes practically saturated with O2.
d. At the O2 partial pressures in the tissues, oxyhemoglobin quickly unloads much of its O 2.
HbO2 → Hb + O2
e. The acid pH and warmer temperature of the tissues also promote this dissociation.
In tissues, some hemoglobin combines with CO2 to form carbaminohemoglobin.
However, most CO2 is transported in the form of bicarbonate ion (HCO3 -).
a. First, CO2 combines with water, forming carbonic acid (H2CO3).
b. This then dissociates to a H+ and a HCO3CO2 + H2O → H2CO3 → H+ + HCO3carbonic acid bicarbonate ion
c. Carbonic anhydrase, an enzyme in red blood cells, speeds this reaction.
d. Release of H+ ions could drastically lower blood pH; however, the hydrogen ions are absorbed by
the globin portions of hemoglobin and the HCO3- diffuses out of the RBCs and into the plasma.
e. Hemoglobin combines with H+ ions as reduced hemoglobin (HHb); HHb plays a vital role in
maintaining normal blood pH.
f. As blood enters the pulmonary capillaries, most of the CO2 is in plasma as HCO3-.
g. The little free CO2 remaining diffuses out of the blood across the walls of the pulmonary
capillaries and into alveoli.
h. Any decrease in plasma CO2 concentration causes the following reaction also catalyzed by
carbonic anhydrase:
H+ + HCO3- → H2CO3 → CO2 + H2O
i. At the same time, hemoglobin unloads H + and HHb becomes Hb.
35.3 Respiration and Health
A. Lower Respiratory Tract Disorders
1. Acute bronchitis is an infection of the primary and secondary bronchi and is usually preceded by a
viral upper respiratory infection.
2. Pneumonia
a. Pneumonia is usually caused by a bacterial or viral lung infection.
b. The bronchi and alveoli fill with fluid.
c. Pneumonia can be localized in specific lobules.
d. AIDS patients are subject to a rare form of pneumonia caused by the protozoan Pneumocystis
carinii.
3. Pulmonary Tuberculosis
a. Pulmonary tuberculosis is caused by the tubercle bacillus, a type of bacterium.
b. A TB skin test is a highly diluted extract of the bacilli injected into the patient’s skin; if a person
has been exposed, the immune response will cause an area of inflammation.
c. Bacilli that invade lung tissue are isolated by the lung tissue in tiny capsules called tubercles.
d. If the person is highly resistant, the imprisoned bacteria die.
e. If resistance is low, the bacteria can eventually be liberated.
f. A chest X-ray detects active tubercles.
g. Appropriate drug therapy can ensure localization and the eventual destruction of live bacteria.
h. Resurgence has accompanied increases in AIDS, homeless, and poor.
i. The new strains are resistant to standard antibiotics.
C. Disorders
1. Pulmonary Fibrosis
a. Inhaling particles of silica, coal dust, fiberglass and asbestos can lead to pulmonary fibrosis.
b. These agents result in a build up of fibrous connective tissue—the lungs then cannot inflate
properly.
c. Asbestos was used widely for fireproofing and widespread exposure occurred; it is estimated that a
possible 2 million deaths could be caused by asbestos between 1990 and 2020.
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2.
3.
4.
5.
Chronic Bronchitis
a. In chronic bronchitis, airways are inflamed and filled with mucus; often a cough brings mucus up.
b. The bronchi degenerate, losing cilia and normal cleansing action and making an infection likely.
c. Smoking cigarettes and cigars is the most common cause, but other pollutants are also involved.
Emphysema
a. Emphysema is a chronic and incurable disorder; it involves distended and damaged alveoli.
b. The lungs often balloon due to trapped air and ineffective alveoli.
c. Emphysema is often preceded by chronic bronchitis.
d. The elastic recoil of the lungs is reduced and the airways are narrowed, making expiration
difficult.
e. Since the surface area for gas exchange is reduced, insufficient O2 reaches the heart and the brain.
f. This triggers the heart to work furiously to force more blood through the lungs; this can then lead
to a heart condition.
g. Lack of oxygen to the brain makes the patient feel depressed, sluggish, and irritable.
h. Exercise, drug therapy, and supplemental oxygen may relieve the symptoms and slow the
progress.
Asthma
a. Asthma is a disease of the bronchi and bronchioles; it causes wheezing, breathlessness, and a
cough.
b. The airways are sensitive to specific allergens (e.g., pollen, dust, cold air, etc.).
c. Exposure to the irritant causes the smooth muscle in bronchi to spasm; chemical mediators given
off by the immune cells in the bronchioles result in the spasms.
d. Bronchial inflammation reduces the diameter of the airways.
e. Special inhalers can control the inflammation and sometimes prevent an attack; other inhalers can
stop muscle spasms.
Lung Cancer
a. Formerly more common in men, lung cancer now surpasses breast cancer as the cause of death in
women due to smoking.
b. Lung cancer develops in the lung tissue in steps.
1) First, a thickening and callusing of the cells lining the bronchi appears.
2) Cilia are lost so it becomes impossible to prevent dust and dirt from settling in the lungs.
3) Next, cells with atypical nuclei appear in the callused lining.
4) A tumor consisting of disordered cells with atypical nuclei develops as cancer in situ (cancer
at one location).
5) When some tumor cells break free and penetrate other tissue (metastasis), the cancer spreads.
6) A tumor may grow until the bronchus is blocked, cutting off the air supply to the lungs.
7) The entire lung then collapses; the trapped secretions become infected causing pneumonia or
lung abscess.
c. The only treatment is surgery (pneumonectomy) where a lobe or whole lung is removed before
metastasis occurs.
Lecture Enrichment Ideas
Experience Base: Ambiguous use of the term “respiration” in previous biology classes and in
common usage will need to be countered by careful and correct classroom usage, discerning
ventilation from aerobic respiration/mitochondrial activity. Students will have some experiences
with asthma, bronchitis, and laryngitis but visuals will be needed for tuberculosis, iron lungs, and
other respiratory health concerns more common at an earlier time in human history.
1. Compare the amount of oxygen needed by an animal with little movement versus an animal
that moves about a lot, and discuss how the respiratory system is adapted for those needs. In
particular, comparisons should be made between invertebrates (such as a hydra or an
earthworm) versus flying insects, and between ectothermic vertebrates (such as a frog) and
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an endothermic bird that must metabolize up to ten times more food.
2. Diagram why a countercurrent exchange mechanism in a fish’s gills allows maximal
efficiency in absorbing oxygen from the water. This can be related to the same concept seen
in the next chapter in the function of the kidney. Countercurrent exchange also functions in
the evening out of the temperature extremes at the tips of limbs of arctic foxes with a hot core
body temperature and cold foot pads, and is an important biological concept cutting across
several other systems.
3. Consider why a bird has a complete ventilation method but a mammal does not; consider
what different metabolic requirements might have led birds to develop their more efficient
mechanism over time. There will be an assumption that humans are always the most
efficient, and that evolution always moves organisms toward the most efficient systems
possible. Slow metabolism organisms, however, do not need this additional efficiency in
respiration.
4. Most students are familiar with the flight attendant’s required narration on safety equipment
at the beginning of a high-altitude flight. Lecture question: Provide a correct scientific
description of the physiological events that would occur to a passenger in the case of cabin
decompression.
5. Lecture question: Consider that a scientist discovers a hemoglobin substitute that is far more
efficient in attracting oxygen and holds it with greater affinity; why is this not better than
hemoglobin?
6. Explain how a doctor can listen for dead spots that indicate lobes of lungs have filled with
fluid (pneumonia); most students have been patients and will remember “Breathe deeply . . .
again.” The dead spots would most likely be in the bottom lobes where fluid would move by
gravity.
7. Query students on the consequence of our “wearing” our lung on the outside as one large flat
gill surface exposed to the air (a surface the size of several classroom walls). Problems would
include physical trauma, massive dehydration, infection, etc.
8. Before our understanding of the various forms of lung diseases and disorders, the term
“consumption” was used for certain lung ailments. This is an excellent opportunity to show
how our progress in medical knowledge both produces a preciseness in our scientific
language, and this preciseness in turn helps us generate the next level of research.
9. Models of the respiratory system can be brought into the classroom to illustrate the
respiratory parts. A bell jar with a rubber sheet at the bottom and, at the other end, a cork
with a glass tube connected to a balloon can be created very inexpensively to demonstrate
positive and negative pressure effects on the mechanism of breathing. Simulation of how the
vocal cords work can be demonstrated by bringing to class a guitar and plucking at the
different size wires and tension.
10. A hospital tour can illustrate the equipment used in assisting patients in respiratory distress.
11. In contrast to the many illustrations in biology that show many body tubes open and pipelike,
all tubes in the body are collapsed flat unless there is something in them. The exception to
this is the trachea that, like a vacuum cleaner hose, has rings to keep it from collapsing.
12. A “sucking chest wound” is a terribly excruciating puncture of the thoracic cavity that allows
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air to flow into the pleural region. Ask students why this prevents normal breathing and what
is the only solution to correct “pneumothorax.” When, after exhalation or after inhalation,
should the airtight bandage be placed to seal the chest wound?
13. Lecture question: How do we know CO2 is monitored instead of oxygen? If we constantly
breathe into and out of the same bag, the CO2 level would rise and the O2 level would fall.
But if we breathe out air that was then bubbled through a solution that extracted the CO2, and
then breathe the resultant air again, only the O2 level would fall and, like most mammals, we
do not increase our breathing rate and would faint from lack of oxygen to the brain.
14. In CPR, a person assisting a victim breathes into the victim only a partial breath of “already
used” air from their own lungs. Lecture question: Why is this, nonetheless, effective?
15. Diving sea mammals face an unusual situation in regulating their breathing compared to
terrestrial animals. While land mammals always see CO2 levels increase concurrent with O2
depletion, diving mammals can shut off blood flow to large portions of tissues and these
tissues no longer contribute CO2 to the blood. Therefore, what system for regulating
breathing rate should a diving mammal evolve?
Critical Thinking
Question 1.
Most people are familiar with hospital “oxygen tents” used to increase the percentage of oxygen
inhaled in patients who have respiratory or circulatory problems. Why would breathing such a rich oxygen
atmosphere not be recommended for a healthy person, or even for such patients over a prolonged time? Consider all
gases involved in the respiratory process.
Answer: The amount of ventilation also determines the amount of carbon dioxide that is removed. If a person can
breathe slower because the air is enriched in oxygen, the amount of carbon dioxide would theoretically be higher.
However, the breathing rate is regulated by chemical sensors that mainly detect the levels of carbon dioxide.
Therefore, the breathing rate will be kept higher than oxygen demand would require and the blood would be
oxygenated at higher levels than normal, with the symptoms of “hyperventilation.”
Question 2.
In Hong Kong, China, the population has not been exposed to tuberculosis, and most Chinese
residents are not resistant to the bacteria, in comparison to Westerners whose European ancestors have been the
survivors of tuberculosis for millennia. Therefore, health officials have chosen to give Hong Kong children BCG, a
vaccine of related harmless bacteria that bestows some immunity to tuberculosis. Therefore, our Western “TB test”
does not work with Hong Kong residents and screening for TB has to be done with more expensive X-rays.
Meanwhile, in the U.S. we use the “TB test” and do not use BCG. Why don’t they adopt our system; or why don’t
we adopt theirs?
Answer: Having evolved a resistant population by coexisting with and surviving tuberculosis, the mainly Western
population in the U.S. is more resistant, has a lower incidence of tuberculosis, and TB is transmitted only with
difficulty. Therefore, it is not necessary to immunize everyone with BCG to keep the numbers low, and infected
individuals can be discovered with an inexpensive test. In Hong Kong, failure to use BCG in such a crowded city of
people who have no evolutionary resistance to TB would assure the disease was rapidly spread. Use of BCG makes
immune individuals who thereafter test positive for tuberculosis and the TB test is therefore useless. If the U.S.
moved to BCG, the cost of the vaccine would greatly exceed the benefit of conferring immunity on a few
individuals in a more resistant population, and the loss of the inexpensive TB test would require widespread use of
more expensive X-rays. Both populations are using the medical protocol appropriate for their evolutionary and
immunological situation.
Question 3.
Seals and whales have sensors that detect the oxygen level of the blood in addition to the carbon
dioxide levels as seen in humans. Why?
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Answer: As diving animals, they shut off blood flow to major portions of their bodies in order to use the circulating
oxygen in the critical tissues. Since there is not as much CO2 coming from the tissue metabolism, it is necessary to
monitor the oxygen levels directly in order to detect when the diving animal must surface as oxygen is running out.
Question 4.
How is breathing affected in a passenger riding in a plane that is not pressurized?
Answer: The plane is limited in how high it can fly and keep the pilot and passengers alive. The diffusion of oxygen
from the atmosphere into the body is affected by the air pressure: the greater the pressure gradient, the faster the rate
of diffusion. At high altitudes in an airplane, there is less air pressure, and therefore, less diffusion of oxygen into
the blood would take place.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
270
CHAPTER
36
BODY FLUID REGULATION AND EXCRETORY
SYSTEMS
This chapter presents a study of the mechanisms by which animals regulate body fluid
concentrations. Included are sections of aquatic animals and terrestrial animals, and the forms of
nitrogenous waste produced by prototype animals. The various organs of excretion in animals are
characterized. The human urinary system is described in detail.
Chapter Outline
36.1 Excretion and the Environment
A. Nitrogenous Waste Products
1.
2.
3.
4.
5.
6.
7.
8.
9.
The breakdown of nucleic acids and amino acids results in nitrogenous wastes.
Amino acids derived from protein synthesize body proteins or nitrogen-containing molecules.
Unused amino acids are oxidized to generate energy or are stored as fats or carbohydrates.
In both cases, amino groups (—NH2) must be removed.
Nitrogenous wastes are excreted as ammonia, urea, or uric acid, depending on the species.
The removal of amino groups requires a fairly constant amount of energy that differs for each
conversion.
Amino groups removed from amino acids form ammonia (NH3) by adding a third hydrogen ion (H+).
a. This requires little or no energy.
b. Ammonia is quite toxic but it is water soluble; it requires the most water to wash it away from the
body.
c. Bony fishes, aquatic invertebrates, and amphibians excrete this ammonia through gills and skin
surfaces.
Terrestrial amphibians and mammals usually excrete urea as their main nitrogenous waste.
a. Urea is much less toxic than ammonia; excreted in a moderately concentrated solution, it also
conserves body water.
b. Production of urea requires energy; it is produced in the liver as a product of the energy-requiring
urea cycle.
c. In the urea cycle, carrier molecules take up carbon dioxide and two molecules of ammonia, finally
releasing urea.
Insects, reptiles, and birds excrete uric acid as their main nitrogenous waste.
a. Uric acid is not very toxic and is poorly soluble in water; uric acid is readily concentrated for
water conservation.
b. In reptiles and birds, a dilute solution of uric acid passes from the kidneys to the cloaca, a
common reservoir for products of the digestive, urinary, and reproductive systems.
c. After any water is absorbed by the cloaca, the uric acid passes out with the feces.
d. Reptile and bird embryos are enclosed in eggshells; the uric acid that builds up is nontoxic in
storage.
e. Uric acid is synthesized by enzymatic reactions using even more ATP than urea synthesis.
f. Therefore, there is a trade-off between water conservation and energy expenditure.
B. Excretory Organs Among Invertebrates

Most animals have tubular organs to regulate salt-water balance and excrete metabolic wastes.
1. Planaria have two strands of branching excretory tubules that open to the outside through excretory
pores.
a. Located along the tubules are flame cells (photonephridia) containing tufts of cilia that appear to
flicker.
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b.
The cilia beat back and forth, propelling a hypotonic fluid through canals emptying at the body
surface.
c. This system functions in the excretion of excess water and wastes.
2. An earthworm’s body is divided into segments and nearly every segment has a pair of nephridia.
a. A nephridium is a tubule with a ciliated opening, the nephridiostome, and an excretory
nephridiopore.
b. Fluid from the body cavity is propelled through this tubule by cilia.
c. Nutrients are reabsorbed and carried away by the network of capillaries surrounding the tubules.
d. The nephridia form urine that contains only metabolic wastes, salts, and water.
e. Each day, an earthworm produces urine equal to 60% of its body weight, enough water that it can
safely excrete ammonia.
3. Insects have a unique excretory system consisting of long, thin Malpighian tubules attached to the
gut.
a. The Malpighian tubules take up metabolic wastes and water from the hemolymph—these follow
the salt gradient established by active transport of K+ ions.
b. At the rectum, water and other useful substances are reabsorbed.
c. Uric acid remains and eventually passes out the anus.
d. Insects that live in the water or that eat large quantities of moist food reabsorb very little water.
e. Insects in dry climates reabsorb most of the water and excrete a dry, semisolid mass of uric acid.
f. Crayfish (an arthropod) have antennal glands (green glands) in the head which perform selective
secretion of certain salts.
g. In shrimp and pillbugs, the excretory organs, called maxillary glands, are located in the maxillary
segments.
h. Arachnids (spiders, scorpions, etc.) have coxal glands used in excretion near the joint of one or
more appendages.
C. Osmoregulation Among Aquatic Vertebrates
1. Most vertebrates osmoregulate—maintain particular ion concentrations in their blood.
2. The excretory system regulates body fluid concentrations by regulating the water and ions in body
fluids.
3. This regulation depends on the concentration of mineral ions (i.e., Na +, CI-, K+, and HCO3- ).
4. Only cartilaginous fishes (sharks, rays) and marine invertebrates (molluscs, arthropods) have body
fluids that are isotonic to seawater, yet those fluids do not contain the same amount of salt as seawater
a. Their blood has high concentrations of urea to match the tonicity of seawater; for some unknown
reason, this is not toxic to the fishes.
5. Marine bony fishes have a moderate salt level compared to seawater; their common ancestor probably
evolved in fresh water.
a. They are prone to water loss and could become dehydrated, therefore they constantly drink
seawater.
b. They swallow water equal to 1% of their weight every hour to counteract dehydration.
c. To remove excess salt, they actively transport Na+ and CI- ions (salt) at the gills.
6.
The body fluids
of freshwater bony fish are hypertonic to freshwater; they passively gain water.
a. Freshwater fishes never drink water.
b. They take in salts at the gills and pass large quantities of dilute, hypotonic urine.
c. They discharge a quantity of urine equal to one-third their body weight each day.
d. Because this causes them to lose salts, they actively import Na + and CI- ions into the blood at the
gills.
D. Osmoregulation Among Terrestrial Vertebrates
1. The kangaroo rat lives in the desert and is threatened by dehydration.
a. Their fur prevents loss of water, and during the day, it remains in a cool burrow.
b. The kangaroo rat has convoluted nasal passages that absorb moisture from exhaled air.
c. Most terrestrial animals must drink fresh water often; however, the kangaroo rat completely avoids
drinking water.
1) It forms a very concentrated urine.
2) It defecates fecal matter that is almost completely dry.
3) It meets its water requirements with the metabolic water derived from aerobic respiration.
2. Some terrestrial animals that live near oceans are able to drink seawater despite its high osmolarity.
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a.
Such birds and reptiles have a salt gland that excretes concentrated salt solution.
36.2 Urinary System in Humans

A.
B.
C.
D.
E.
The human urinary system is an organ system with four parts.
1. The human kidneys are two bean-shaped, reddish brown organs, each about the size of a fist.
a. They are located on either side of the vertebral column, below the diaphragm, and partly protected
by the lower rib cage.
b. The kidneys are the sites of urine formation.
2. Each kidney is connected to a ureter; each conducts urine from a kidney to the urinary bladder.
3. The urinary bladder stores urine until it is voided from the body through the urethra.
4. A single urethra conducts urine from the urinary bladder to the exterior of the body.
5. The male urethra runs through the penis and also carries semen; in females, it opens the ventral to the
vaginal opening.
Kidneys
1. If a kidney is sectioned longitudinally, three major regions can be distinguished.
a. The renal cortex is the thin, outer region and it appears granular.
b. The renal medulla consists of the striped, pyramid-shaped regions that lie on the inner side of the
cortex.
c. The renal pelvis is the innermost hollow chamber.
2. Each human kidney is composed of about one million tiny tubules called nephrons.
3. Some nephrons are located primarily in the cortex but others dip down into the medulla.
Nephrons
1. Each nephron is comprised of several parts.
2. The end of a nephron pushes in to form a cuplike structure called the glomerular capsule.
a. The outer layer is composed of simple squamous epithelium.
b. The inner layer is made of specialized cells that allow easy passage of molecules.
3. Nearest the glomerular capsule is the proximal convoluted tubule, lined by cells with many
mitochondria and tightly packed microvilli.
4. Simple squamous epithelium forms the loop of the nephron, the middle portion of the nephron tubule
with a descending and ascending limb.
5. The distal convoluted tubule is the distal portion of the nephron tubule; several join to deliver the
urine into collecting ducts.
6. The loop of nephron and the collecting duct give pyramids of the medulla their striped appearance.
7. Each nephron has its own blood supply.
a. The renal artery branches into smaller arteries, which then branch into afferent arterioles, one for
each nephron.
b. Each afferent arteriole divides to form a capillary bed or glomerulus.
c. The glomerular capillaries drain into an efferent arteriole which branches into a peritubular
network.
d. The peritubular capillaries drain into a venule; the venules from many nephrons drain into a small
vein; small veins join to form the renal vein, a vessel that enters the inferior vena cava.
Urine Formation
1. Urine production requires three distinct processes.
2. Glomerular filtration occurs at the glomerular capsule.
3. Tubular reabsorption occurs at the proximal convoluted tubule.
4. Tubular secretion occurs at the distal convoluted tubule.
Glomerular Filtration
1. When blood enters the glomerulus, blood pressure moves small molecules from the glomerulus across
the inner membrane of the glomerular capsule into the lumen of the glomerular capsule; this is
glomerular filtration.
2. The glomerular walls are 100 times more permeable than the walls of most capillaries.
3. Molecules that leave the blood and enter the glomerular capsules are the glomerular filtrate.
4. Plasma proteins and blood cells are too large to be part of the glomerular filtrate.
5. Failure to restore fluids would soon cause death from loss of water, nutrients, and low blood pressure.
Tubular Reabsorption
1. Tubular reabsorption of fluids from the nephron back to the blood occurs through the walls of the
proximal convoluted tubule.
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2.
Reabsorption recovers much of the glomerular filtrate.
a. The osmolarity of the blood equals the filtrate so osmosis of water does not occur.
b. Sodium ions are actively reabsorbed; chlorine ions follow passively.
c. This changes the osmolarity of the blood so that water moves passively from the tubule back to the
blood.
d. About 60–70% of salt and water are reabsorbed at the proximal convoluted tubule.
3. Cells of the proximal convoluted tubule have numerous microvilli, increasing the surface area for
absorption, and numerous mitochondria, which supply the energy needed for active transport.
4. Only molecules with carrier molecules for them are reabsorbed.
5. If there is more glucose, for example, than carriers, excess glucose will appear in the urine.
6. In diabetes mellitus, there is too much glucose because the liver fails to store glucose as glycogen.
F. Tubular Secretion
1. Tubular secretion moves substances from the blood to the tubular lumen by other than glomerular
filtration.
2. Secretion back into the filtrate is primarily associated with the distal convoluted tubule.
3. This helps rid the body of potentially harmful compounds that were not filtered into the glomerular
capsule.
4. Uric acid, hydrogen ions, ammonia, and penicillin are eliminated this way.
G. The Kidneys and Homeostasis
1. The kidneys regulate the water balance of the blood, thereby maintaining the blood volume and blood
pressure.
H. Maintaining the Salt-Water Balance
1. The long loop of nephron is comprised of a descending limb and an ascending limb.
2. Salt (NaCl) passively diffuses out of the lower portion of the ascending limb, but the upper, thick
portion of the limb actively transports salt out into the tissue of the outer renal medulla.
3. Less salt is available for transport from the tubule as fluid moves up the thick portion of the ascending
limb; the ascending limb is impermeable to water.
4. Urea leaks from the lower portion of the collecting ducts, causing the concentration in the lower
medulla to be highest.
5. Because of the solute concentration gradient within the renal medulla, water leaves the descending
limb of the loop of nephron along its length.
6. The decreasing water concentration in the descending limb encounters an increasing solute
concentration; this is a countercurrent mechanism.
7. Fluid received by a collecting duct from the distal convoluted tubule is isotonic to the cells of the
cortex; as this fluid passes through the renal medulla, water diffuses out of the collecting duct into the
renal medulla.
8. The urine finally delivered to the renal pelvis is usually hypertonic to the blood plasma.
9. Antidiuretic hormone (ADH) is released from the posterior lobe of the pituitary.
a.
ADH acts on the collecting ducts by increasing its permeability to H 2O, thereby
increasing H2O retention.
b.
When ADH is released, more water is reabsorbed and there is less urine.
c.
When ADH is not released, more water is excreted and more urine forms.
d.
If an individual does not drink much water, the pituitary releases ADH; if hydrated, ADH
is not released.
e.
Diuresis means increased urine; antidiuresis means a decreased amount of urine.
10. Reabsorption of Salt
a. Usually more than 99% of the sodium filtered out at the glomerulus is returned to the blood.
b. Most is reabsorbed at the proximal tubule, 25% is extruded by the ascending limb of the loop of
nephron, and the rest is from the distal convoluted tubule and collecting duct.
c. Aldosterone is a hormone secreted by the adrenal cortex.
1) It acts on the distal convoluted tubules to increase the reabsorption of Na + and the excretion of
K+.
2) Increased Na+ in the blood causes water to be reabsorbed, increasing blood volume and
pressure.
3) If the blood pressure is insufficient to promote glomerular filtration, the afferent arteriole cells
secrete renin.
4) Renin catalyzes the conversion of angiotensinogen (a protein produced by the liver) into
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I.
angiotensin I.
5) Later, angiotensin I is converted to angiotensin II by angiotensin-converting enzyme.
6) Angiotensin II stimulates cells in the adrenal cortex to produce aldosterone.
7) Angiotensin II increases the blood pressure as a vasoconstrictor.
11. The Atrial natriuretic hormone (ANH) is produced by the atria of the heart when cardiac cells
stretch.
a.
When blood pressure rises, the heart produces ANH to inhibit the secretion of renin and
the release of ADH.
b.
Therefore, this hormone decreases blood volume and pressure.
Maintaining the Acid-Base Balance
1. The bicarbonate buffer system and breathing work together to maintain blood pH.
2. The excretion of H+ ions and NH3, and reabsorption of bicarbonate ions (HCO3 - ) is adjusted.
a. If the blood is basic, fewer hydrogen ions are excreted and fewer sodium and bicarbonate ions are
reabsorbed.
H+ + HCO3-  H2CO3  H2O + CO2
b. If the blood is acidic, H+ ions are excreted with ammonia, while Na+ and HCO3- ions are
reabsorbed; Na+ ions promote formation of hydroxide ions and bicarbonate takes up H+ ions when
carbonic acid is formed.
NH3 + H+ → NH4+
c. Ammonia is produced in the tubule cells by deamination of amino acids.
3. Reabsorption or excretion of ions by the kidneys is a homeostatic function that maintains the pH of the
blood and osmolarity.
Lecture Enrichment Ideas
Experience Base: Students may know the location of kidneys from the boxing rules about
kidney punches. Some international students, especially from British Commonwealth countries
may be familiar with kidney structure from its more common use in food. Visuals are critical in
explaining the nephron system. Using models or media to demonstrate the gross anatomy of the
kidney, bladder, etc., is also important.
1. Describe the role of the excretory system in excretion and why it is so important that
nitrogenous wastes be removed from the body; only the American bear can recycle amino
acid metabolic wastes to usable proteins during hibernation; all other organisms must rid
themselves of these toxins. Bears in hibernation still undergo metabolism although at a
slower rate. Students may be able to postulate the problems that must be overcome.
2. A freshwater fish placed in the ocean, or a marine fish placed in fresh water are under
physiological stress. A contrast of these two systems helps emphasize the modifications
found in organisms.
3. Compare the countercurrent mechanism in fish gill oxygenation with the same concept found
in the human kidney.
4.
There is an overall principle in excretory systems of pushing out most fluids and then reabsorbing the useful
nutrients and water. Use this as a “running theme” as flame cells, Malpighian tubules, and nephrons are
described.
5. Students may recoil at old concepts of using warm urine to melt earwax or trapped miners
surviving on urine. This is a place to emphasize that, in contrast to the digestive system, the
urinary system is a filtered and sterile system unless there is a urinary tract infection. The
first human trials on the antibiotic penicillin, produced in small quantities, involved
extracting the penicillin in the patient’s urine and recycling it. “Smelly-urine” restrooms are
275
the result of bacterial action on urine waste molecules after the urine has exited the body.
6. Birds must remain lightweight and cannot store large quantities of heavy water. Lecture
question: What is the likely system in birds? Note the variation in toxicity and solubility of
ammonia, urea, and uric acid pastelike wastes.
7. The variations in length of the urethra are related to the ease of entry of bacterial infections;
students should be able to speculate on whether males or females are more prone to
bladder/kidney infections.
8. Students may confuse “ureter” and “urethra” due to their similarity; a phonics distinction is
that “ureters” sounds correct in the plural but “urethras” does not.
9. Textbook and overhead outlines of sagittal sections of the pelvic organs usually illustrate the
substantially smaller space allotted for the female urinary bladder, thus accounting for a very
real difference in mileage between bathroom stops during long-distance trips. Lecture
question: What is likely to happen in “bladder space” after a hysterectomy?
10. On occasion, a person develops four kidneys in the same space as two; however, this means
there are twice as many renal arteries and veins. Ask what the consequence would be in
processing time for a soft drink, for instance. This is a common enough anomaly that a large
lecture class is very likely to have one person with four kidneys, although they may not know
it. Why is this so common, from an evolutionary perspective? But, if it is in fact
advantageous, why do we not all have four kidneys?
11. Lecture question: Why was the kidney one of the first organs to be successfully transplanted?
(Answer: It is encapsulated and has a limited number of connections [artery, vein, ureter]).
12. Have students keep an approximated record of their fluid intake and urinary output for a 24hour period and explain any discrepancies between the two values.
13. Albumin is a large protein molecule in blood plasma; students will relate to it and its
“thickness” as a molecule in simple raw egg white.
14. Lecture question: Why must a person continue to take tablets or capsules for a medication to
remain in the blood at levels to be effective? The rate at which a medication is removed is
one of the critical items in drug research, and is the basis for the prescription line: twice-aday, once-a-day, etc. The faster the kidney removes the drug from the blood, the more we
have to re-dose.
15. The concept of “countercurrent” is not readily apparent because there are no general visible
experiences that operate as countercurrents in daily life. Therefore, it is necessary to walk
through the two reversed gradients and show the effect with and without the countercurrent
process.
16. Many blood pressure medicines act through changing the blood volume and amount of urine
produced. At the end of this section, students should be able to deduce why. A diagram
showing the kidneys tapping into a major portion of the systemic circulation may also help—
the kidneys intercept approximately 20% of blood flow.
17. Hemodialysis involving artificial mechanisms is not effective in removing all metabolic
waste molecules. Lecture question: Why do patients on dialysis machines have to adhere to
limited diets?
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18. Chapter concepts here should now make evident what a physician is looking for with urine
tests for sugar, protein, etc.
Critical Thinking
Question 1.
Why do birds produce a white paste in bird feces, and why is it so damaging to car paint?
Answer: Birds have a common opening for digestive, excretory and reproductive systems. The white pastelike
material is uric acid, an excretory product that is combined with feces before it is eliminated. The uric acid allows
nontoxic storage and conserves water for the bird that cannot store large reserves of heavy water and fly. Uric acid is
acidic, and therefore caustic to paint.
Question 2.
urine volume?
If a diuretic such as tea decreased reabsorption of water from 99% to 98%, what would happen to
Answer: Urine volume would double. Previous urine represented 1% after 99% is reabsorbed; a drop to 98% leaves
2% or double the product.
Question 3.
How are kidneys related to blood pressure?
Answer: Nephrons regulate the water-salt balance of the blood. The more salts in the blood, the greater the diffusion
of water into the blood; the greater blood volume requires the heart to work harder and causes greater blood
pressure.
Question 4.
Tea is an excellent diuretic; if you drink one cup of tea, you will probably urinate several times
that in water. Why does a small change in the percentage of water reabsorbed produce a large change in the volume
of urine produced?
Answer: If 99% of the water is being reabsorbed, the urine constitutes the 1% not reabsorbed. If reabsorption drops
1% to 98% the urine will constitute 2%, or double the urine production.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
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CHAPTER
37
NEURONS AND NERVOUS SYSTEMS
This chapter presents a detailed study of the nervous system. After a description of the evolution
of the nervous system, the chapter focuses exclusively on the human system. The structure and
function of neurons are described in detail. The various areas of the brain and their functions, and
nervous system physiology are described. The biochemistry of impulse transmission is
discussed. A Science Focus box discusses “Five Drugs of Abuse.”
Chapter Outline
37.1 Evolution of the Nervous System
A. Invertebrate Nervous Organization
1.
Comparative study shows the evolutionary steps leading to the centralized nervous system of
vertebrates.
2. Even primitive sponges, with only a cellular level of organization, respond by closing the osculum.
3. Hydra (cnidarians) possesses two cell layers separated by mesoglea.
a. The hydra can contract, extend, and move tentacles to capture prey and even turn somersaults.
b. A simple nerve net extends throughout the hydra body within the mesoglea.
c. The hydra nerve net is composed of neurons in contact with one another and with contractile
epitheliomuscular cells.
d. The more complex cnidaria (sea anemones and jellyfish) may have two nerve nets.
1) A fast-acting nerve net enables major responses, particularly in times of danger.
2) Another nerve net coordinates slower and more delicate movements.
4. The planarian nervous system is bilaterally symmetrical.
a. It has two lateral nerve cords that allow rapid transfer of information from anterior to posterior.
b. The nervous system of planaria is called a ladderlike nervous system.
c. The nervous system of planaria exhibits cephalization; at their anterior end, planaria have a
simple brain composed of a cluster of neurons or ganglia.
d. Cerebral ganglia receive input from photoreceptors in eyespots and sensory cells in auricles.
e. The transverse nerve fibers between the sides of the ladderlike nerve cords keep the movement on
both sides of a planarian body coordinated.
f. Bilateral symmetry plus cephalization are important trends in nervous system development.
g. The organization of the planarian nervous system foreshadows both the central and peripheral
system of vertebrates.
5. The annelids, arthropods, and mollusks are complex animals with true nervous systems.
a. The nerve cord has a ganglion in each segment of the body that controls muscles of that segment.
b. The brain still receives sensory information and controls the activity of the ganglia so the entire
animal is coordinated.
c. The presence of a brain and other ganglia indicate an increased number of neurons among
invertebrates.
B. Vertebrate Nervous Organization
1. Vertebrate nervous systems exhibit cephalization and bilateral symmetry.
a. The vertebrate nervous system is composed of both central and peripheral nervous systems.
1) The central nervous system (CNS) develops a brain and spinal cord from the embryonic
dorsal nerve cord.
2) The peripheral nervous system consists of paired cranial and spinal nerves.
b. Paired eyes, ears, and olfactory structures gather information from the environment.
c. A vast increase in number of neurons accompanied evolution of the vertebrate nervous system; an
insect may have one million neurons while vertebrates may contain a thousand to a billion times
more.
2. The Vertebrate Brain
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a.
b.
C.
The vertebrate brain is at the anterior end of the dorsal tubular nerve cord.
The vertebrate brain is customarily divided into the hindbrain, midbrain, and forebrain.
1) A well-developed hindbrain regulates organs below a level of consciousness; in humans it
regulates lung and heart function even when sleeping; also, it coordinates motor activity.
2) The optic lobes are part of a midbrain which was originally a center for coordinating reflex
responses to visual input.
3) The forebrain receives sensory input from the other two sections and regulates their output.
4) The cerebrum is highly developed in mammals and is associated with conscious control; the
outer layer, called the cerebral cortex, is large and complex.
The Human Nervous System
1. Three specific functions of the nervous system are to:
a. receive sensory input;
b. perform integration; and
c. generate motor output to muscles and glands.
2. The central nervous system (CNS) consists of the brain (in the skull) and the spinal cord (in the
vertebral column).
3. The peripheral nervous system (PNS) lies outside the CNS and contains the cranial and spinal
nerves.
4. The PNS is divided into the somatic and autonomic systems.
a. The somatic system controls the skeletal muscles.
b. The autonomic system controls the smooth muscles, cardiac muscles, and glands.
5. The CNS and PNS of the human nervous system are connected and work together to perform the
functions of a nervous system.
37.2 Nervous Tissue
 Nervous tissue is made up of neurons (nerve cells) and neuroglia (which support and
nourish the neurons).
A. Neurons and Neuroglia
1. Neurons vary in appearance, depending on their function and location, but they all
have three parts.
a.
b.
c.
2.
The cell body contains the nucleus and other organelles.
Dendrites receive information and conduct impulses toward the cell body.
A single axon conducts impulses away from the cell body to stimulate or inhibit a neuron, muscle,
or gland.
1) A long axon is called a nerve fiber.
2) The long axons are covered by a white myelin sheath.
Types of Neurons
a. Motor (efferent) neurons have many dendrites and a single axon; they conduct impulses from the
CNS to muscles or glands.
b. Sensory (afferent) neurons are unipolar; they conduct impulses from the periphery toward the
CNS.
1) The process that extends from the cell body divides into two processes, one going to the CNS
and one to periphery.
c. Interneurons (association neurons) are multipolar.
1) They have highly-branched dendrites within the CNS.
2) Interneurons convey messages between the various parts of the CNS.
3) They form complex brain pathways accounting for thinking, memory, language, etc.
B. Transmission of the Nerve Impulses
1.
2.
Julius Bernstein (early 1900s) proposed that the nerve impulse is the movement of unequally
distributed ions on either side of an axonal membrane, the plasma membrane of an axon.
A. L. Hodgkin and A. F. Huxley later confirmed this theory.
a. They and other researchers inserted a tiny electrode into the giant axon of a squid.
b. The electrode was attached to a voltmeter and an oscilloscope to trace a change in voltage over
time.
c. The voltage measured the difference in the electrical potential between the inside and outside of
the membrane.
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d. An oscilloscope indicated any changes in polarity.
Resting Potential
a. When an axon is not conducting an impulse, an oscilloscope records a membrane potential equal
to negative 70 mV, indicating that the inside of the neuron is more negative than the outside.
b. This is the resting potential because the axon is not conducting an impulse.
c. This polarity is due to the difference in electrical charge on either side of the axomembrane.
1) The inside of the plasma membrane is more negatively charged than the outside.
2) Although there is a higher concentration of K + ions inside the axon, there is a much higher
concentration of Na+ ions outside the axon.
3) The plasma membrane is more permeable to K+ ions, so this gradient is less and the K + ion
potential is less.
4) The sodium-potassium pump maintains this unequal distribution of Na+ and K+ ions.
d. The sodium-potassium (Na+-K+) pump is an active transport system that moves Na+ ions out and
K+ ions into the axon.
e. The pump is always working because the membrane is permeable to these ions and they tend to
diffuse toward the lesser concentration.
f.
Since the plasma membrane is more permeable to potassium ions than to sodium ions, there are
always more positive ions outside; this accounts for some polarity.
g. The large negatively charged proteins in the cytoplasm of the axon also contribute to the resting
potential of – 70 mV.
4. Action Potential
a. When an axon conducts a nerve impulse, the rapid change in the membrane potential is the action
potential.
b. Protein-lined channels in the axomembrane open to allow either sodium or potassium ions to pass;
these are sodium and potassium gated ion channels.
c. The oscilloscope goes from –70 mV to +40 mV in a depolarization phase, indicating the
cytoplasm is now more positive than the tissue fluid.
d. The trace returns to –70 mV again in the repolarization phase, indicating the inside of the axon is
negative again.
5. Propagation of Action Potentials
a. If an axon is unmyelinated, an action potential stimulates an adjacent axomembrane to produce an
action potential.
b. In myelinated fibers, the action potential at one neurofibril node causes action potential at the next
node.
1) The myelinated sheath has neurofibril nodes, gaps where one neurolemmocyte ends and the
next begins.
2) The action potential “leaps” from one neurofibril node to another—this is called saltatory
conduction.
3) Saltatory conduction may reach rates of over 200 meters/second, compared to 1 meter/second
without it.
c. As each impulse passes, the membrane undergoes a short refractory period before it can open the
sodium gates again; this ensures a one-way direction to the impulse.
C. Transmission Across a Synapse
1. The minute space between the axon bulb and the cell body of the next neuron is the synapse.
2. A synapse consists of a presynaptic membrane, a synaptic cleft, and the postsynaptic membrane.
a. Synaptic vesicles store neurotransmitters that diffuse across the synapse.
b. When the action potential arrives at the presynaptic axon bulb, synaptic vesicles merge with the
presynaptic membrane.
c. When vesicles merge with the membrane, neurotransmitters are discharged into the synaptic
cleft.
d. The neurotransmitter molecules diffuse across the synaptic cleft to the postsynaptic membrane
where they bind with specific receptors.
e. The type of neurotransmitter and/or receptor determines if the response is excitation or inhibition.
f. Excitatory neurotransmitters use gated ion channels and are fast acting.
g. Other neurotransmitters affect the metabolism of the postsynaptic cells and are slower.
3. Neurotransmitters and Neuromodulators
a. At least 100 different neurotransmitters have been identified.
3.
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b.
Acetylcholine (ACh) and norepinephrine (NE), dopamine, and serotonin are present in both
the CNS and the PNS.
1) ACh can have either an excitatory or an inhibitory effect, depending on the tissue.
2) NE is important to dreaming, waking, and mood.
3) Dopamine is involved in emotions, learning, and attention.
4) Serotonin is involved in thermoregulation, emotions, and perception.
c. Once a neurotransmitter is released into a synaptic cleft, it initiates a response and is then removed
from the cleft.
d. In some synapses, the postsynaptic membrane contains enzymes that rapidly inactivate the
neurotransmitter.
e. Acetylcholinesterase (AChe) breaks down acetylcholine.
f. In other synapses, the presynaptic membrane reabsorbs the neurotransmitter for repackaging in
synaptic vesicles or for molecular breakdown.
g. The short existence of neurotransmitters in a synapse prevents continuous stimulation (or
inhibition) of postsynaptic membranes.
h. Many drugs that affect the nervous system act by interfering with or potentiating the action of
neurotransmitters.
i. Neuromodulators are molecules that block the release of a neurotransmitter or modify a neuron’s
response to one.
1) Substance P is released by sensory neurons when pain is present; endorphins block the release
of substance P and therefore act as natural painkillers.
D. Synaptic Integration
1. A neuron has many dendrites and may have one to ten thousand synapses with other neurons.
2. A neuron receives many excitatory and inhibitory signals.
3. Excitatory neurotransmitters produce a potential change (signal) that drives the neuron closer to an
action potential; inhibitory signals produce a signal that drives the neuron further from an action
potential.
4. Thus, excitatory signals have a depolarizing effect and inhibitory signals have a hyperpolarizing effect.
5. Integration is the summing up of excitatory and inhibitory signals.
a. If a neuron receives many excitatory signals, or at a rapid rate from one synapse, the axon will
probably transmit a nerve impulse.
b. If both positive and inhibitory signals are received, the summing may prohibit the axon from
firing.
37.3 Central Nervous System: Brain and Spinal Cord
1.
The central nervous system (CNS) is where sensory impulses are received and motor control is
initiated.
2. Both the brain and the spinal cord are protected by bone.
3. Both are wrapped in three connective tissue coverings called meninges; meningitis is a disease caused
by many different bacteria or viruses that invade the meninges.
4. The spaces between the meninges are filled with cerebrospinal fluid to cushion and protect the CNS.
5. The cerebrospinal fluid is contained in the central canal of the spinal cord and within the ventricles of
the brain.
6. The ventricles are interconnecting spaces that produce and serve as reservoirs for the cerebrospinal
fluid.
A. The Spinal Cord
1. The spinal cord has two main functions.
a. It is the center for many reflex actions (automatic responses to external stimuli).
b. It provides the means of communication between the brain and the spinal nerves.
2. The spinal cord is composed of white and gray matter.
a. Gray Matter
1) The unmyelinated cell bodies and short fibers give gray matter its color.
2) In a cross section, the gray area looks like a butterfly or the letter H.
3) It contains portions of sensory neurons and motor neurons; short interneurons connect them.
b. White Matter
1) Myelinated long fibers of interneurons run together in tracts and give the white matter its
color.
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c.
2) Tracts conduct impulses between the brain and the spinal nerves; ascending tracts are dorsal
and descending tracts from the brain are ventral.
3) Tracts cross over near the brain; therefore, the left side of the brain controls the right side of
the body.
If a spinal cord injury occurs in the cervical region, the condition of quadriplegia (paralysis of all
four limbs) results.
If the injury is in the thoracic region, the lower limbs may be paralyzed (paraplegia).
d.
B. The Brain
1. The brain has four ventricles: two lateral ventricles and a third and fourth ventricle.
2. The cerebrum is associated with the two lateral ventricles, the diencephalon with the third, and the
brain stem and cerebellum with the fourth.
3. The Cerebrum
a. The cerebrum, also called the telencephalon, is the largest part of the brain in humans.
b. It is the last center receiving sensory input and carrying out integration to command motor
responses.
c. The right and left cerebral hemispheres (the two halves of the cerebrum) are connected by a
bridge of nerve fibers, the corpus callosum; different functions are associated with the two
hemispheres.
d. The outer portion is a highly convoluted cerebral cortex consisting of gray matter containing cell
bodies and short unmyelinated fibers.
e. The cerebral hemisphere is divided into four surface lobes:
1) Frontal lobe—located towards the front of the hemispheres and is associated with motor
control, memory, reasoning, and judgment.
a.
The left frontal lobe has Broca’s area for our ability to speak.
2) Parietal lobe—located behind the frontal lob and involved with sensory reception and
integration, and taste.
3) Temporal lobe—located laterally and receives information from our ears.
4) Occipital lobe—located in the back side of the brain, it receives information from our eyes.
f.
The cerebral cortex contains motor, sensory, and association areas.
1) The human hand takes up a large proportion of the primary motor area.
2) The ventral to the primary motor area is a premotor area that organizes motor functions
before the primary area sends signals to the cerebellum.
3) Sensory information from the skin and skeletal muscles arrives at a primary somatosensory
area.
4) A general interpretation area receives information from all of the sensory association areas
and allows us to quickly integrate signals and send them to the prefrontal area for immediate
response.
g. Basal Nuclei
1) Aside from the tracts, there are masses of gray matter located deep within the white matter.
2) These basal nuclei integrate motor commands; malfunctions cause Huntington and
Parkinson disease.
4. The Diencephalon
a. The hypothalamus and thalamus are in a portion of the brain known as the diencephalon, where
the third ventricle is located.
b. The hypothalamus forms the floor of the third ventricle.
c. The hypothalamus maintains homeostasis.
1) It is an integrating center that regulates hunger, sleep, thirst, body temperature, water balance,
and blood pressure.
2) It controls the pituitary gland and thereby serves as a link between the nervous and endocrine
systems.
d. The thalamus consists of two masses of gray matter in the sides and roof of the third ventricle.
1) It is the last portion of the brain for sensory input before the cerebrum.
2) It is a central relay station for sensory impulses traveling up from the body or from the brain
to the cerebrum.
3) Except for smell, it channels sensory impulses to specific regions of the cerebrum for
interpretation.
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e. The pineal gland, which secretes the melatonin hormone, is in the diencephalon.
The Cerebellum
a. The cerebellum is separated from the brain stem by the fourth ventricle.
b. The cerebellum is in two portions joined by a narrow median portion.
c. It receives information from the eyes, inner ear, muscles, etc., indicating body position, integrates
the information, and sends impulses to muscles maintaining balance.
d. The cerebellum assists in the learning of new motor skills, as in sports or playing the piano; it may
be important in judging the passage of time.
6. The Brain Stem
a. The brain stem contains the medulla oblongata, pons, and midbrain.
b. Besides acting as a relay station for tracts passing between the cerebrum and spinal cord or
cerebellum, the midbrain has reflex centers for visual, auditory, and tactile responses.
c. The pons (“bridge”) contains bundles of axons traveling between the cerebellum and rest of the
CNS.
1) The pons functions with the medulla to regulate the breathing rate.
2) It has reflex centers concerned with head movements in response to visual or auditory stimuli.
d. The medulla oblongata lies between the spinal cord and the pons, anterior to the cerebellum.
1) It contains reflex centers for regulating heartbeat, breathing, and vasoconstriction.
2) It contains reflex centers for vomiting, coughing, sneezing, hiccupping, and swallowing.
3) It contains nerve tracts that ascend or descend between the spinal cord and the brain’s higher
centers.
7. The Reticular Activating System (RAS)
a. The reticular formation consists of gray matter and nerve fibers that extend the length of the brain
stem and is a major component of the RAS.
b. The RAS receives sensory signals that it sends up to higher centers, and motor signals that it sends
to the spinal cord.
c. The RAS sources the cerebrum and causes a person to be alert.
d. The RAS can filter out unnecessary sensory stimuli, which explains why you can study with the
TV on.
e. General anesthetics function by suppressing the RAS.
f. A severe injury to the RAS can cause a person to be comatose, from which recovery may be
impossible.
8. The Limbic System
a. The limbic system is a complex network of tracts and nuclei that incorporate medial portions of
cerebral lobes, subcortical nuclei, and diencephalon.
b. It blends higher mental functions and primitive emotions.
c. Its two major structures are the hippocampus and amygdala, essential for learning and memory.
1) The hippocampus makes prefrontal area aware of past experiences stored in association areas.
2) The amygdala causes experiences to have emotional overtones.
3) Inclusion of the frontal lobe in the limbic system allows reasoning to keep us from acting out
strong feelings.
d. Learning and Memory
1) Memory is the ability to hold thoughts in the mind and to recall past events.
2) Learning takes place when we retain and utilize past memories.
3) The prefrontal area in the frontal lobe is active in short-term memory (e.g., telephone
numbers).
4) Long-term memory is a mix of semantic memory (numbers, words) and episodic memory
(persons, events).
5) Skill memory is the ability to perform motor activities.
6) The hippocampus serves as a go-between to bring memories to mind.
7) The amygdala is responsible for fear conditioning and associates danger with sensory stimuli.
8) Long-term potentiation (LTP) is an enhanced response at synapses within the hippocampus.
9) LTP is essential to memory storage; excited postsynaptic cells may die due to a glutamate
neurotransmitter.
10) Extinction of too many cells in the hippocampus is the underlying cause of Alzheimer disease.
37.4 Peripheral Nervous System
5.
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1.
The peripheral nervous system lies outside the CNS.
a. Cranial nerves connect to the brain.
b. Spinal nerves lie on either side of the spinal cord.
2. Axons in nerves are called nerve fibers.
3. The cell bodies of neurons are found in the CNS or in ganglia.
4. Ganglia are collections of cell bodies in the PNS.
5. Humans have 12 pairs of cranial nerves attached to the brain.
a. Sensory nerves only contain sensory nerve fibers.
b. Motor nerves only contain motor nerve fibers.
c. Mixed nerves contain both sensory and motor nerve fibers.
d. Cranial nerves mostly connect to the head, neck, and facial regions.
e. The vagus nerve also branches to the pharynx, larynx, and some internal organs.
6. Humans have 31 pairs of spinal nerves emerging from the spinal cord.
a. The paired spinal nerves leave the spinal cord by two short branches, or roots.
b. The dorsal root contains fibers of sensory neurons conducting nerve impulses to the spinal cord;
the cell body of a sensory neuron is in the dorsal root ganglion.
c. The ventral root contains the axons of motor neurons that conduct nerve impulses away from the
spinal cord.
d. All spinal nerves are mixed nerves that conduct impulses to and from the spinal cord.
e. Spinal nerves are mixed nerves with sensory and motor fibers; each serves its own region.
A. Somatic System
1. The peripheral nervous system has two divisions: somatic and autonomic.
2. The somatic system includes the nerves that carry sensory information to the CNS and motor
commands away from the CNS to skeletal muscles.
3. Any voluntary control of muscles involves the brain; reflexes, involuntary responses to stimuli,
involve the brain or just the spinal cord.
4. Outside stimuli can initiate reflex actions, some of which involve the brain.
B. The Reflex Arc
1. A reflex arc involves the following pathway:
a. Sensory receptors generate an impulse in a sensory neuron that moves along sensory axons toward
the spinal cord.
b. Sensory neurons enter the cord dorsally and pass signals to interneurons.
c. Impulses travel along motor axons to an effector, which brings about a response to the stimulus.
d. The immediate response is that muscles contract to withdraw from source of pain.
2. Reflex response occurs because the sensory neuron stimulates several interneurons.
3. Some impulses extend to the cerebrum, which makes a person conscious of the stimulus and the
reaction.
C. Autonomic System
1. The autonomic system is a part of the PNS and regulates cardiac and smooth muscle and glands.
2. There are two divisions: the sympathetic and parasympathetic systems.
a. Both function automatically and usually in an involuntary manner.
b. Both innervate all internal organs.
c. Both utilize two neurons and one ganglion for each impulse.
1) The first neuron has a cell body within the CNS and a preganglionic fiber.
2) The second neuron has a cell body within the ganglion and a postganglionic fiber.
d. Breathing rate and blood pressure are regulated by reflex actions to maintain homeostasis.
3. Sympathetic Division
a. Most preganglionic fibers of the sympathetic system arise from the middle (thoracic-lumbar)
portion of the spinal cord and almost immediately terminate in ganglia that lie near the cord
(thoracic-lumbar portion).
b. Therefore the preganglionic fiber is short, but the postganglionic fiber that contacts an organ is
long.
c. The sympathetic system is especially important during emergency situations (the “fight or flight”
response).
d. To defend or flee, muscles need a supply of glucose and oxygen; the sympathetic system
accelerates heartbeat, and dilates bronchi.
e. To divert energy from less necessary digestive functions, the sympathetic system inhibits
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digestion.
The neurotransmitter released by the postganglionic axon is mainly norepinephrine, similar to
epinephrine (adrenaline) used as a heart stimulant.
4. Parasympathetic Division
a. The parasympathetic system consists of a few cranial nerves, including the vagus nerve, and
fibers that arise from the bottom craniosacral portion of the spinal cord.
b. In this case, the preganglionic fibers are long and the postganglionic fibers are short.
c. This system is a “housekeeper system”; it promotes internal responses resulting in a relaxed state.
d. The parasympathetic system causes the eye pupil to constrict, promotes digestion, and retards
heartbeat.
e. The neurotransmitter released is acetylcholine.
D. Drugs of Abuse (Science Focus box)
1. Alcohol
a. Alcohol consumption is the most socially accepted form of drug use worldwide.
b. Alcohol (ethanol) acts as a depressant on many parts of the brain where it affects neurotransmitter
release or uptake.
c. Depending on the amount consumed, the effects of alcohol on the brain can lead to a feeling of
relaxation, lower inhibitions, impaired concentration and coordination, slurred speech, and
vomiting.
d. Coma or death can occur if blood level of alcohol becomes too high.
e. Chronic consumption can damage the frontal lobes, decrease overall brain size, and increase
ventricle size.
f. Prolonged alcohol use can permanently damage the liver, even to a point where a liver transplant
may be necessary.
2. Nicotine
a. When tobacco is smoked, nicotine is delivered to the CNS where it binds to the neurons, causing
the release of dopamine.
b. In the PNS, nicotine acts as a stimulant by mimicking acetylcholine and increasing heart rate,
blood pressure, and muscle activity.
c. Nicotine withdrawal symptoms include irritability, headache, insomnia, poor cognitive
performance, the urge to smoke, and weight gain.
3. Club and Date Rape Drugs
a. The chemical structure of methamphetamine (also called meth or speed) is similar to that of
dopamine, and its stimulatory effects mimic that of cocaine.
b. The initial rush of methamphetamine is typically followed by a state of high agitation that may
lead to violent behavior.
c. Chronic use is characterized by paranoia, auditory and visual hallucinations, self-absorption,
irritability, and aggressive, erratic behavior.
d. Excessive intake can lead to hyperthermia, convulsions, and death.
e. Ecstasy is a drug with similar effects to those of methamphetamine.
f. Ecstasy has an overstimulatory effect on neurons that produce serotonin which elevates our
moods.
g. Date rape drugs have sedative effects such as relaxation, amnesia, and disorientation.
h. They are also popular at clubs because they enhance the effect of heroin and Ecstasy.
4. Cocaine
a. Cocaine is a stimulant in the CNS that interferes with the up-take of dopamine at synapses,
resulting in a rush of well-being that lasts 5–30 minutes.
b. Result of cocaine use is sleeplessness, lack of appetite, increased sex drive, tremors, and “cocaine
psychosis.”
c. Cocaine related deaths are usually due to cardiac and/or respiratory arrests.
5. Heroin
a. Heroin acts as a depressant in the nervous system.
b. Opiates (of which heroin belongs) depress breathing, block pain pathways, cloud mental function,
and sometimes cause nausea and vomiting.
c. Long-term effects are addiction, hepatitis, HIV/AIDS, and bacterial infections due to using shared
needles.
6. Marijuana
f.
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a.
b.
7.
The active ingredient in marijuana is the resin that is rich in THC (tetrahydrocannabinol).
When THC reaches the CNS, the person experiences mild euphoria, along with alterations in
vision and judgment.
c. In heavy users, hallucinations, anxiety, depression, rapid flow of ideas, body image distortions,
paranoia, and psychotic symptoms can result.
d. Regular usage of marijuana can cause cravings that make it difficult to stop.
Treatment for Addictive Drugs
a. Treatment for addiction to drugs consists mainly of behavior modifications.
b. New treatment techniques involve administering antibodies to block the effects of cocaine and
methamphetamine.
c. A vaccine for cocaine that would stimulate antibody production is being tested.
Lecture Enrichment Ideas
Experience Base: Our understanding of the functioning of the brain is advancing rapidly since
the 1990s; the first “decade of the brain,” as PET scans and other new techniques allow us to
achieve much understanding of brain function without surgical intervention. It is likely that some
of the concepts explained here will be modified and refined during the next two years. However,
this research fosters news stories which can be used for class discussion.
1. Describe why the development of a more complex nervous system would have been
advantageous to animals early in their evolution.
2. Not all neurons have the classic spiderlike appearance; illustrations of rods in the retina and
other receptor neurons can provide a perspective of the variations on the general theme.
3. Dominoes can be lined up and “rippled” to illustrate that an impulse can deliver a message
while no one domino actually has to transverse the whole length, just as no calcium ion has
to move the length of an axon; the setting up of the dominoes is representative of the sodium
pump moving the sodium ions outside the membrane, and the falling down is their flow
inside the membrane—the “ripple” is the impulse. The number of times you can send a
domino “impulse” depends on how fast you can reset the dominoes, just as the number of
nerve impulses per second depends on how fast the axon membrane can pump ions back
outside.
4. Query students as to why lateralization (different functions in the two cerebral hemispheres)
would evolve.
5. Detail the advantages of segmental ganglia in annelids and arthropods. When a fly is raised
off of its feet, its wings begin beating. When it is lowered and its feet touch the floor, its
wings stop beating. This effect still occurs when the fly’s head and small “brain” has been
removed!
6. The myelin sheath is usually viewed as mere insulation and it is necessary to describe how
myelination makes the electrical transmission of nervous impulses more rapid.
7. Clarify how the somatic system has motor functions that deal with skeletal muscles, while
the autonomic system’s motor neurons cause the contraction of smooth or cardiac muscle and
effect the release of glandular secretions.
8. Because of its development in layers based on function, researchers sometimes refer to the
various layers of the brain as the “reptilian brain,” etc. A sequence of brain models across
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fish, amphibians, reptiles, and mammals will show progressive development with the
complex cerebrum being the distinguishing human development. For instance, there is a
“reptilian brain” in us that represents the basic ability to maintain body functions and this
portion is essentially the same as in reptiles.
9. Ask students why a linkage of emotions to memory would be an evolutionary advantage, as
perhaps in the case of remembering how one successfully evaded a predator. The effect of
emotions upon retaining images in long-term memory can be illustrated by asking students
where they were on September 11, 2001; middle-aged students, where they were when they
heard the space shuttle had exploded; and for even older nontraditional students, where they
were when they heard Kennedy had been shot.
10. Impulses along an axon are analogous to current along a copper wire. Yet a radio is not a
mess of wire and a human behavior is not just one continuous nerve impulse. Wires lead to
and from the antenna and speaker magnets, etc., and neurons connect sensory cells with other
neurons and with muscles and glands. Therefore, what happens at the synapse is critical to
understanding the effects. Media and/or models can demonstrate how the nerve impulse and
synapse generate behavior.
11. Some students are familiar with the effects of common aerosol insecticides on mosquitoes,
flies, beetles, etc.: the insect dies in spasms of uncontrolled twitching. Coverage of the action
of neurotransmitters at a synapse gives students the background to now understand what is
happening: a common family of insecticides destroys acetylcholinesterase and the insect is
unable to disable its natural neurotransmitter; continued impulses override any ability to
control normal body functions.
12. Most of our understanding of neuron physiology was researched on the very large neurons of
the squid and snail. Note how today, animal research remains critical in this discipline as
well as throughout biomedical research.
13. This may be the first time that students have encountered specific terms for various types of
memories. Distinguishing differences and being more exact with terminology is part of
science progress. Terms such as “think” or “memory” are less useful as we discern them to
consist of many distinct processes.
14. Students who are bilingual from birth will “store both languages” in Broca’s area, but if they
learn the second language after ages 3–5, the second will be “stored” in Wernicke’s region.
Accidents that damage Wernicke’s region eliminate second language competency in the
second case only. Lecture question: What does this biological fact suggest about children
growing up in multilingual Europe versus monolingual American society?
15. Use two variable points, such as spreading apart the tips of two keys on your key ring, to detect the density of
sensory nerves on the human body; a student can feel two close points on a fingertip but cannot distinguish
points much further apart on the back of the arm, etc. High densities exist in the mouth and lips, fingers, and
genital regions.
16. Studying organs individually misses the fact that they must act in concert. Lecture question:
What might happen if organs were not controlled by a coordinated sympathetic and
parasympathetic system? Such a “physiologically confused” organism would have little
chance of survival. Two families of nervous system drugs affect one or the other of these
systems.
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Critical Thinking
Question 1.
synapse?
What keeps a nerve impulse from flowing “backward” in a neuron and across a
Answer: Generally, sensory neurons are structured to receive stimuli from the ends of dendrites. As a nerve impulse
moves along, pores open in a cascade during depolarization but take a few milliseconds to repolarize; this keeps the
sodium gates from backward propagating. At the synapse, the axon end secretes the neurotransmitter that is received
by the dendrite end of the next neuron; this chemical transmitter-receiver system prevents backward propagation
across the synapse.
Question 2.
Why do most people remember where they were when they heard the space shuttle Challenger
had exploded or (for older students) when President Kennedy was shot?
Answer: The limbic system is involved in stimulating memory during emotionally-charged events.
Question 3.
How can a person be declared “brain dead” when he/she is still breathing?
Answer: This definition refers to cerebral brain activity; the vital centers for heartbeat, breathing, and some other
functions are located in the brainstem.
Question 4.
Why is it difficult to diagnose the cause of an inability to speak?
Answer: Speech requires not only the functioning of Broca's or Wernicke's area, but also accurate hearing, memory
for word associations, and various motor areas to generate speech, all be integrated correctly.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
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CHAPTER
38
SENSE ORGANS
This chapter describes the structures involved with the senses of taste, smell, vision, and hearing.
The biochemistry involved with the sensory perceptions is described. A Health Focus box looks
at “Protecting Vision and Hearing.”
Chapter Outline
38.1 Chemical Senses
A. Chemoreceptors are responsible for taste and smell by being sensitive to chemicals in food,
liquids, and air.
1. Chemoreception is found universally in animals; it is thought to be the most primitive sense.
2. Chemoreceptors are present all over a planarian but concentrated in the auricles at the side of the head.
3. Insects, such as houseflies, taste with their feet.
4. Crustacea have chemoreceptors on their antennae and appendages.
5. In amphibians, chemoreceptors are located in the nose, mouth, and all over the skin.
6. In mammals, receptors for taste are in the mouth, and receptors for smell are in the nose.
B. Sense of Taste
1. Human taste buds are located primarily on the tongue.
2. Many lie along the walls of papillae, the small elevations on the surface of the tongue.
3. Isolated ones are present on the surfaces of the hard palate, pharynx, and epiglottis.
4. Taste buds are embedded in tongue epithelium and open at a taste pore.
5. Taste buds have supporting cells and elongated taste cells that end in microvilli.
6. Microvilli bear receptor proteins for certain chemicals.
a. Molecules bind to receptor proteins and impulses are generated in associated sensory nerves.
b. Nerve impulses go to the brain cortical areas which interpret them as tastes.
7. Humans have four primary types of taste.
a. Taste buds for each are concentrated in particular regions.
1) Sweet receptors are most plentiful near the tip of the tongue.
2) Sour receptors occur primarily along the margins of the tongue.
3) Salty receptors are most common on the tip and upper front portion.
4) Bitter receptors are located near the back of the tongue.
5) A fifth type, called umami, may exist for certain flavors (MSG, cheese, beef broth, seafood).
b. The brain appears to take an overall “weighted average” of taste messages as the perceived taste.
C. Sense of Smell
1. The sense of smell depends on olfactory cells located in the olfactory epithelium high in the roof of
the nasal cavity.
2. Olfactory cells are modified neurons.
3. Each cell has a tuft of five olfactory cilia that bear receptor proteins for an odor molecule.
a. There are around 1,000 different types of odor receptors; many olfactory cells carry the same type.
b. Nerve fibers from like olfactory cells lead to the same neuron in the olfactory bulb.
c. An odor activates a characteristic combination of cells; this information is pooled in the olfactory
bulb.
d. Interneurons communicate this information via the olfactory tract to areas of the cerebral cortex.
4. Olfactory bulbs are directly connected with the limbic system; smells are associated with emotions and
memory.
5. Taste and smell supplement each other.
a. “Smelling” food also involves the taste receptors.
b. Losing taste when you have a cold is usually due to a loss of smell.
38.2 Sense of Vision
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A. Animals lacking photoreceptors, sensory receptors sensitive to light, depend on their senses of hearing and
smell rather than sight.
B. Photoreceptors vary in complexity.
1. In its simplest form, a photoreceptor indicates only the presence of light and its intensity.
2. “Eyespots” of planaria allow flatworms to determine direction of light.
3. Image-forming eyes occur in four invertebrate groups: cnidaria, annelids, molluscs, and arthropods.
4. Arthropods have compound eyes composed of many independent visual units (ommatidia), each
possessing all of the elements needed for light reception.
a. The cornea and crystalline cone of each visual unit focus rays toward the photoreceptors.
b. Photoreceptors generate nerve impulses, which pass to the brain by way of optic nerve fibers.
c. The image resulting from all stimulated visual units is crude; the small size of compound eyes
limits the number of visual units.
d. Insects have color vision but utilize a narrower range of the electromagnetic spectrum and can see
some ultraviolet.
5. Some fishes, reptiles, and most birds are believed to have color vision, but among mammals, only
humans and other primates have color vision; this is adaptive for day activity.
6. Vertebrates and certain molluscs (e.g., the squid and the octopus) have a camera-type eye.
a. Molluscs and vertebrates are not closely related; therefore this is convergent evolution.
b. A single lens focuses an image of the visual field on closely packed photoreceptors.
c. In vertebrates the lens changes shape to aid in focusing; in molluscs the lens move back and forth.
d. The human eye is considerably more complex than a camera.
7. Animals with two eyes facing forward have three-dimensional, or stereoscopic vision.
8. Animals with eyes facing sideways (e.g., rabbits) have panoramic vision—the visual field is wide.
C. The Human Eye
1. The human eye is an elongated sphere 2.5 cm in diameter with three layers.
2. The sclera is the outer, white fibrous layer that covers most of the eye; it protects and supports the
eyeball.
3. The cornea is a transparent part of the sclera at the front of the eye; it is the window of the eye.
4. The conjunctiva is a thin layer of epithelial cells that covers the sclera and keeps the eyes moist.
5. The middle, thin, dark-brown layer is the choroid containing many blood vessels and pigments
absorbing stray light rays.
6. To the front of the eye, the choroid thickens and forms a ring-shaped ciliary body and finally becomes
the iris that regulates the size of an opening called the pupil.
7. The lens divides the cavity into two portions: aqueous humor fills the anterior cavity and vitreous
humor fills the posterior.
8. Retina
a. The inner layer is the retina that contains photoreceptors called rod cells and cone cells.
b. The fovea centralis is a small area of retina that contains only cones; this area produces acute
color vision in daylight.
c. Cone cells are not very sensitive in low intensity light; at night, the rods are still active.
9. Focusing of the Eye
a. Light rays enter the eye through the pupil and are focused on the retina.
b. Focusing involves light passing through the cornea, the lens, and the humors.
c. Because of refraction, the image on the retina is inverted 180º from actual but is perceived righted
in the brain.
d. The shape of the lens is controlled by the ciliary muscle.
e. The lens is flatter when the ciliary muscle is relaxed when we are viewing distant objects.
f.
The lens is naturally elastic and becomes rounder for viewing near objects where light rays must
bend to a greater degree.
g. This change is called visual accommodation.
h. An aging lens loses its ability to accommodate for near objects and we may need reading glasses
by middle age.
i.
The lens is also subject to cataracts, or becoming opaque; surgery, using a cryoprobe, is the only
current treatment.
j.
Persons who can see well close up but not far away are nearsighted (myopia).
1) They often have an elongated eyeball that focuses a distant image in front of the retina.
2) They can wear corrective concave lenses to refocus the image on the retina.
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3) Radial keratotomy is a new treatment that surgically cuts and flattens the cornea.
Persons who can see far away but not up close are farsighted (hyperopia).
1) They often have a shortened eyeball that focuses near images behind retina.
2) They can wear corrective convex lenses to refocus the image on retina.
l.
When the cornea or lens is uneven, the image is fuzzy; this is astigmatism corrected by an
unevenly ground lens to compensate for unevenness.
D. Photoreceptors of the Eye
1. Vision begins when light has been focused on photoreceptors in the retina.
2. Rod cells and cone cells have an outer segment joined to an inner segment by a stalk.
3. The outer segment contains stacks of membranous disks (lamellae) with many molecules of
rhodopsin.
4. The rhodopsin molecules contain a protein opsin and the pigment molecule retinal derived from
Vitamin A.
5. When a rod absorbs light, rhodopsin splits into opsin and retinal, leading to a cascade of reactions and
the closure of ion channels in the rod cell plasma membrane.
6. This stops the release of inhibitory molecules from the rod’s synaptic vesicles and starts signals that
result in impulses to brain.
7. The rods are stimulated by low light and provide night vision.
8. Because rods are distributed throughout the retina, rods detect our peripheral vision and motion but not
color or detail.
9. Cones located primarily in the fovea centralis are activated by bright light and detect detail and color.
10. The three kinds of cones contain blue, green, or red pigment.
11. Each pigment is composed of retinal and opsin, but the structure of opsin varies among the three.
12. Combinations of cones are stimulated by intermediate colors; the combined nerve impulses are
interpreted in the brain.
E. Integration of Visual Signals in the Retina
1. The retina has three layers of neurons.
a. The rods and cones are nearest the choroid.
b. Bipolar cells form the middle layer.
c. Ganglion cells, whose fibers become the optic nerve, form the innermost layer.
2. Since only rod and cone cells are sensitive to light, light must penetrate through the ganglion cells.
3. Rods and cones synapse with bipolar cells which pass the impulse to ganglion cells.
4. There are more rods and cones than nerve fibers leaving ganglionic cells.
5. Up to 150 rods may synapse with a ganglion cell; this results in indistinct vision.
6. Each cone synapses with one ganglionic cell; this accounts for the detailed images of cones, mostly
found in the fovea.
7. Integration occurs as signals pass from the bipolar to the ganglion cells.
a. If all rod cells in a receptive field are stimulated, the ganglion cell is weakly stimulated or neutral.
b. If only the center is lit, it is stimulated; if only the edge is lit, it is inhibited.
c. Therefore considerable processing occurs in the retina before an impulse is sent to the brain.
8. The blind spot is an area where the optic nerve passes through the retina; it lacks rods and cones.
F. Protecting Vision and Hearing (Health Focus box)
1. Preventing a Loss of Vision
a. The most frequent loss blindness is caused from reinal disorders, glaucoma, and cataracts.
b. Diabetic retinopathy results in the bursting of capillaries to the retina.
1)
Careful blood glucose level regulation may protect them from diabetic retinopathy.
c. Macular degeneration results in thickened choroids vessels not functioning and destroying the
cones.
d. Glaucoma results when the drainage system of the eyes fail and fluid builds up and destroys nerve
fibers that are responsible for peripheral vision.
e. Cataracts are cloudy spots on the lens of the eye and eventually pervade the whole lens.
1) In addition, men smoking 20 cigarettes or more a day, and women who smoke more than 35
cigarettes a day, double their risk of cataracts.
f. Additional preventative measures include wearing large lens, glass sunglasses to absorb ultraviolet
light.
2. Preventing a Loss of Hearing
a. In the ear, the mobility of ossicles decreases with age.
k.
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b.
c.
d.
e.
f.
g.
h.
i.
38.3
In a condition called otosclerosis, new filamentous bone grows over the stirrup, impeding its
movement.
1)
The only remedy for this type of conduction deafness is surgical treatment.
Age-associated nerve deafness due to stereocilia damage from exposure to loud noises is
preventable.
Noise above a level of 80 decibels could result in damage to the hair cells of the organ of Corti.
The sterocilia and then the hair cells disappear completely.
Noise that can result in hearing loss includes: loud music, noisy indoor or outdoor equipment,
motorcycles and recreational vehicles, burst of gunfire.
Additional causes of hearing loss can result from taking medicines such as some anticancer drugs
and certain antibiotics.
Symptoms include temporary hearing loss, a “full” feeling in the ears, muffled hearing, or ringing
in the ears.
Preventative measures include reducing exposure to the noise, wearing noise-reduction earmuffs,
wearing compressible earplugs.
Senses of Hearing and Balance
1. The ear has two sensory functions: hearing and balance (equilibrium).
2. The sensory receptors for both are in the inner ear, and each consists of hair cells
with stereocilia that are sensitive to mechanical stimulation; they are
mechanoreceptors.
A. Anatomy of the Ear
1. The human ear has three divisions: an outer, middle, and inner ear.
2. The outer ear consists of the pinna (external flap) and the auditory canal.
a. The auditory canal opening is lined by fine hairs that filter air.
b. Modified sweat glands in the auditory canal secrete earwax to guard against foreign matter.
3. The middle ear begins at the tympanic membrane and ends at a bony wall with membrane-covered
openings (the oval window and the round window).
a. It contains small bones called ossicles: malleus (hammer), incus (anvil), and stapes (stirrup).
b. The malleus adheres to the tympanum; the stapes touches the oval window.
c. The auditory (eustachian) tube extends from the middle ear to the pharynx to equalize the inside
and outside air.
4. The inner ear has three regions: the semicircular canals, vestibule, and cochlea.
5. The cochlea resembles a snail shell because it spirals.
B. Process of Hearing
1. The process of hearing begins when sound waves enter the auditory canal, causing the ossicles to
vibrate.
2. Sound is amplified about 20 times by the size difference between the tympanic membrane and the oval
window.
3. The stapes strikes the membrane of the oval window, passing pressure waves to the fluid in the
cochlea.
4. Three canals are located within the cochlea: vestibular canal, cochlear canal, and tympanic canal.
5. The vestibular canal connects with the tympanic canal, which leads to the oval window membrane.
6. Along the basilar membrane are hair cells whose stereocilia are embedded in a tectorial membrane.
7. The hair cells of the spiral organ (organ of Corti) synapse with nerve fibers of the cochlear (auditory)
nerve.
8. When the stapes strikes the membrane of the oval window, pressure waves move from the vestibular
canal to the tympanic canal and across the basilar membrane, and the round window bulges.
9. The basilar membrane vibrates up and down bending the stereocilia of hair cells embedded in the
tectorial membrane.
10. This generates nerve impulses in the cochlear nerve that travel to the brain stem.
11. When they reach the auditory areas of the cerebral cortex, this is interpreted as sound.
12. The spiral organ is narrow at its base and widens at its tip; each part is sensitive to different pitches.
13. Nerve fibers from each region (high pitch base or low pitch tip) lead to slightly different regions of the
brain, producing the sensation of pitch.
14. Sound volume is caused by more vibration; the increased stimulation is interpreted as louder sound
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intensity.
15. Tone is an interpretation by the brain based on the distribution of the hair cells stimulated.
C. Sense of Balance
1. The sense of balance is divided into:
a. rotational equilibrium (angular or rotational movement of the head), and
b. gravitational equilibrium (vertical or horizontal movement).
2. Rotational equilibrium utilizes the semicircular canals.
a. The semicircular canals are oriented at right angles to one another in three different planes.
b. The enlarged base of each semicircular canal is called an ampulla.
c. Fluid flowing over and displacing a cupula causes the stereocilia of the hair cells to bend; the
pattern of impulses carried by the vestibular nerve to the brain changes.
d. Continuous movement of the fluid in the semicircular canals causes vertigo motion sickness.
e. By spinning and stopping, we see a room still spin; this indicates that vision is also involved in
balance.
3. Gravitational equilibrium utilizes the utricle and saccule.
a. A vestibule or space between the semicircular canals and the cochlea contains the utricle and the
saccule.
b. The utricle and saccule are small membranous sacs, each of which contains hair cells.
c. Hair cell stereocilia are embedded within a gelatinous material called the otolithic membrane.
d. Calcium carbonate granules (otoliths) rest on this membrane.
e. The utricle is sensitive to horizontal movements; the saccule responds best to up-down
movements.
f. When the head is still, otoliths in the utricle and saccule rest on the otolithic membrane above the
hair cells.
g. As the head bends or the body moves, otoliths are displaced and the otolithic membrane sags,
bending the larger stereocilia (kinocillium) of hair cells beneath; this tells the brain the direction of
movement.
D. Sensory Receptors in Other Animals
1. The lateral line system of fish and amphibians detects water currents and pressure waves.
2. Primitive fishes have the system on the surface; advanced fishes enclose it in a canal on the side.
3. The lateral line receptor is a collection of hair cells with cilia embedded in a mass of gelatinous
material (cupula).
4. Static equilibrium organs called statocysts are found in cnidaria, mollusks, and crustacea.
5. A small particle called a statolith stimulates cilia that generate impulses, indicating the position of the
head.
Lecture Enrichment Ideas
Experience Base: Many of the auditory phenomena described here are relevant to most students
via their enjoyment of music, speech classes, etc. However, the anatomy will require some visual
media. Many will have music playing experience and be familiar with pitch, etc., but most will
still appreciate audio tracks of various frequencies from below to above the average hearing
range. This range narrows as you age and the instructor may not have the range of most students.
Most students will have had experience with vision and hearing tests
1.
Compare the kinds of receptors involved in perceiving sensation with the kinds of energy for each sensation.
Sound, smell and taste require a molecular medium; sight does not. The speed of light is fastest, sound is
relatively slower, and smell and taste require diffusion and contact with actual molecules.
2. Examine the concept that different receptors can be stimulated to send a message to the
wrong area of the brain, which is the final interpreter of the stimulus. Pain is a particularly
good topic for discussion, since too much sound, light, pressure, heat, cold, etc., can send the
message that there is pain. You might also discuss the fact that a blow to the head may make
you “see stars” or a light, even though the visual receptors are not stimulated.
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3. Discuss color blindness as a genetic defect in the operation of the cones, and ask students to
try to determine how it might arise. Mention the relatedness of the photosynthetic pigment,
bacteriorhodopsin, and ask students to determine why a similar molecule would be involved
in our visual apparatus and in the chemical that absorbs the energy of light in bacteria.
Projection slides can be used in class for students to test their color vision and the blind spot
in class.
4. It is useful to point out that some other animals possess sensory organs and an ability to
perceive stimuli different from or beyond those we can perceive, including sound frequencies
beyond our range, light waves beyond our range, and probably magnetic fields. Not all
people have the same sensitivity to light, pain threshold, etc., and most certainly we do not
perceive the world in a manner identical to other species because we do not have the same
sensory organs or ranges as other animals.
5. The example of “seeing stars” after a blow to the head is one of many examples that could be
given demonstrating that sensory perception depends upon where the nerve impulse
originates and where it arrives in the brain, not on any distinct or uniquely different nerve
impulse.
6. Pain is a seriously underrated sense, although most students would initially state they would
prefer to live without pain. Relating the life of individuals who lack the ability to feel pain
may provide some value of this sense. The sense of pain is critical to not turning a jar lid too
hard and shredding hand tissues, etc.
7. The vision disk, a type of protractor held to the forehead and available as a human anatomy
project from Hubbard, allows a teacher to demonstrate the fovea and retina of a student’s
eye: sliding a card from 90 degrees to center, a student can only read the letters on it, or see
color, as it approaches the center of view.
8. Lecture question: What does a street scene look like at night? Except for some color around a
street lamp or store window or sign, it is black-and-white. Why? This is one good example of
how the different number of neurons has a visual effect we can “see.”
9. Lecture question: How far can you determine stereoscopic depth? Most of us fail to perceive
depth beyond 20 feet as the eyes are directed nearly parallel. This is also found in focusing a
camera.
10. The integration of vision example can be carried further than the text description: the
integration allows us to focus on edges. When rod cells are stimulated, they also inhibit those
rod cells next to them. Therefore, if the whole field is stimulated, the general integrated
signal is subdued. However, a rod cell at an edge is not inhibited by the rods next to it that
are in the dark; thus its signal is less inhibited. Such processing serves to pre-process the
input and help us respond to edges or changes “where the action is.”
11. Slides are available that show color dot patterns; students with color blindness may be unable
to read various numbers based on contrasting color dots. A “blind spot” exercise can be
photocopied and handed out; students can adjust the sheet so that staring at one dot, a second
dot falls in the blind spot and it disappears as the mind fills in with the white paper
background—again a demonstration that we perceive with our mind based on past learned
experiences.
12. Pit vipers have an extended and detailed vision into the infrared light spectrum where we
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only generally feel heat. This allows them to visualize bird and mammal prey by body heat at
night in total darkness. There is an assumption that humans are superior in all body system,
including senses. But this is just one of many cases where other animals can sense stimuli we
cannot detect.
13. We also have stereo hearing and can detect direction of sound from variable loudness if both
ears are healthy.
14. Most students will know that a dog can hear higher frequencies than humans can hear. In
addition, the human ability to hear the highest and lowest notes gradually decreases with age.
Therefore elementary school children can hear lower and higher notes than college students,
who in turn can hear a wider range than an old biology professor, unless they have damaged
their hair cells with very loud music.
15. Smells arouse memories because they are directly connected to the limbic system. Unlike
most neurons, these neurons do continually grow and regenerate, although slowly. Thus our
sense of smell (and taste) may change over our lifetime and children who hated the taste of
peas when young grow into adults that like the taste.
16. The statolith system of crayfish uses a particle from the environment, perhaps a sand grain,
inside a round chamber lined with sensory hairs to determine which direction is down. This
chamber lining is shed each time the crayfish molts and a new particle must be utilized.
Researchers can raise crayfish in clean aquariums where the only particles are iron filings.
After one molt, the statolith particle is now an iron filing and when researchers turn on an
overhead electromagnet, the crayfish immediately invert and swim upside down! Such an
example reinforces this concept while demonstrating that researchers also have fun.
Critical Thinking
Question 1.
There are about 1,000 different odor receptor proteins and only three types of cones for color.
Why, then, do we not just see three different colors and smell just a thousand odors?
Answer: An odor is a complex of molecules that activates a combination of cells; this information is pooled in the
olfactory bulb. The brain then determines the pattern of the types of receptors activated. For colors, various
combinations of cones are stimulated by intermediate shades of color; the combined nerve impulses are interpreted
in the brain as a specific color. Both systems are somewhat like the limited number of piano keys that can
nevertheless play a much wider range of chords in combinations.
Question 2.
When the eardrum or tympanic membrane is impacted by an external sound wave, it moves less
than a fraction of the diameter of the smallest atom. How can such a small pressure wave be detected by our
neurons?
Answer: The sequence of ossicles act as levels to magnify the vibrations about 20 times by the time they move from
the tympanic membrane to the stapes.
Question 3.
A student places one hand in hot water and the other hand in cold water. After adjustment, the
student places both hands in a third bowl of warm water and notes the hand that was in cold water now feels very
warm, and the hand that was in warm water now feels a little cold although both hands are now in the same
temperature of water. What is happening?
Answer: There are two different sets of free nerve endings responding. Because there are more cold nerve endings,
there are more of them that are now feeling warmer. If both hot and cold feelings were generated by a single type of
sensory nerve that reported a calibration, the sensations would be the same when both hands returned to the same
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temperature of water.
Question 4.
If the nerve impulse coming from the pain receptor in the skin of the finger is the same as the
individual nerve impulse coming from the organ of Corti for hearing, how do you account for the different
sensations?
Answer: Even though a nerve impulse has the same electrochemical charge coming from a pain receptor or from the
organ of Corti, the brain is responsible for the type of sensation felt and for the localization of the sensation. If the
impulse goes to the temporal lobe of the cerebral cortex, we interpret that sensation as sound.
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296
CHAPTER
39
LOCOMOTION AND SUPPORT SYSTEMS
A comparison of the diversity of animal skeletal systems precedes an examination of the human
musculoskeletal system. The structure and function of whole muscles and muscle fibers are also
detailed, as is the biochemistry of muscle contraction. A Heath Focus box is included on “You
Can Avoid Osteoperosis.”
Chapter Outline
39.1 Diversity of Skeletons
1. Three types of skeletons occur in the animal kingdom.
2. A hydrostatic skeleton occurs in cnidarians, flatworms, roundworms, and annelids.
3. An exoskeleton is found in molluscs and arthropods, respectively.
4. An endoskeleton is found in sponges, echinoderms, and vertebrates.
A. Hydrostatic Skeleton
1. A fluid-filled gastrovascular cavity or coelom can act as a hydrostatic skeleton.
2. It offers support and resistance to the contraction of muscles for motility.
3. Many animals have hydroskeletons.
a. Hydras use a fluid-filled gastrovascular cavity to support tentacles that rapidly contract.
b. Planaria easily glide over substrate with muscular contractions of body walls and cilia.
c. Roundworms have a fluid-filled pseudocoelom and move when their longitudinal muscles contract
against it.
d. Earthworms are segmented with septa dividing the coelom into compartments; circular and
longitudinal muscles contract in each segment to coordinate elongation and contraction.
e. Animals with exoskeletons or endoskeletons move selected body parts by means of muscular
hydrostats, i.e., fluid contained within certain muscle fibers assists movement of that part.
B. Exoskeletons and Endoskeletons
1. An exoskeleton is an external skeleton.
a. Molluscs have exoskeletons that are predominantly calcium carbonate (CaCO3).
b. Insects and crustacea have jointed exoskeletons composed of chitin, a strong, flexible, nitrogenous
polysaccharide.
c. The exoskeleton provides protection against damage from enemies and also keeps tissues from
drying out.
d. Although stiffness provides support for muscles, the exoskeleton is not as strong as an
endoskeleton.
e. The clam and snail exoskeletons grow with the animals; their thick nonmobile CaCO 3 shell is for
protection.
f. The chitinous exoskeleton of arthropods is jointed and moveable.
g. Arthropods must molt when their exoskeleton becomes too small; a molting animal is vulnerable
to predators.
2. Vertebrates have an endoskeleton composed of bone and cartilage that grows with the animal.
a. The endoskeleton does not limit the space available for internal organs and it can support greater
weight.
b. Soft tissues surround the endoskeleton to protect it; injuries to soft tissue are easier to repair.
c. Usually an endoskeleton has elements that protect vital internal organs.
d. The jointed exoskeleton of arthropods and endoskeletons of vertebrates allow flexibility and
helped arthropods and vertebrates colonize land.
39.2 The Human Skeletal System
1.
2.
3.
4.
Skeletons protect organs: skull (brain), vertebral column (spinal cord), and rib cage (heart and lungs).
The large, heavy leg bones support the body against the pull of gravity.
Leg and arm bones permit flexible body movement.
The flat bones of the skull, ribs, and breastbone contain red bone marrow that manufactures blood
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cells.
5. All bones store inorganic calcium and phosphorous salts.
A. Bone Growth and Renewal
1. The prenatal human skeleton is cartilaginous; cartilage structures serve as “models” for bone
construction.
a. The cartilaginous models are converted to bones when calcium salts are deposited in the matrix,
first by cartilaginous cells and later by bone-forming cells called osteoblasts.
b. Conversion of cartilaginous models to bones is called endochondral ossification.
c. Some bones (e.g., facial bones) are formed without a cartilaginous model.
2. During endochondral ossification, there is a primary ossification center at the middle of a long bone;
latter secondary centers form at the ends.
3. A cartilaginous growth plate occurs between primary and secondary ossification centers.
4. As long as the growth plate remains between the two centers, bone growth occurs.
5. The rate of growth is controlled by hormones, including growth hormone (GH) and sex hormones.
6. Eventually plates become ossified and bone stops growing; this determines adult height.
7. In adults, bone is continually being broken down and built up again.
a. Bone-absorbing cells (osteoclasts) break down bone, remove worn cells, and deposit calcium in
the blood.
b. Osteoblasts form new bone, taking calcium from the blood.
c. Osteoblasts become entrapped in the bone matrix and become osteocytes in the lacunae of osteons.
d. This continual remodeling allows bones to gradually change in thickness.
e. Osteoclasts also determine the calcium level in the blood; calcium level is important for muscle
contraction and nerve conduction and levels are controlled by the hormones PTH and calcitonin.
8. Adults need more calcium in the diet than do children to promote the work of osteoblasts.
B. Anatomy of a Long Bone
1. A long bone illustrates the principles of bone anatomy.
a. A long bone consists of a central medullary cavity surrounded by compact bone.
b. Ends are composed of spongy bone surrounded by a thin layer of compact bone and covered with
hyaline cartilage.
c. Compact bone contains many osteons (Haversian systems); bone cells in tiny chambers (lacunae)
are arranged in concentric circles around central canals.
d. Central canals contain blood vessels and nerves.
e. The lacunae are separated by a matrix that contains protein fibers of collagen and mineral
deposits.
2. Spongy bone has numerous plates and bars separated by irregular spaces.
a. Spongy bone is lighter but designed for strength; solid portions of bone follow the lines of stress.
b. Bone spaces are often filled with red bone marrow, a specialized tissue that produces blood cells.
C. The axial skeleton lies at the midline of the body and consists of the skull, vertebral column, sternum, and ribs.
1. The Skull
a. The skull is formed by the cranium and the facial bones.
b. Newborns have membranous junctions called fontanels that usually close by the age of two.
c. The bones of the cranium contain sinuses, air spaces lined with mucous membrane that reduce the
weight of skull and give a resonant sound to the voice.
d. Two mastoid sinuses drain into the middle ear; mastoiditis is an inflammation that can lead to
deafness.
e. The cranium is composed of eight bones: a frontal, two parietal, an occipital, two temporal, a
sphenoid, and an ethmoid.
f. The spinal cord passes through the foramen magnum, an opening at the base of the skull in the
occipital bone.
g. Each temporal bone has an opening that leads to the middle ear.
h. The sphenoid bone completes the sides of the skull and forms the floors and walls of the eye
sockets.
i. The ethmoid bone is in front of the sphenoid, part of the orbital wall, and a component of the nasal
septum.
j. Fourteen facial bones include: mandible, two maxillae, two palatine, two zygomatic, two
lacrimal, two nasal, and vomer.
k. The mandible or lower jaw is the only movable portion of the skull; it contains tooth sockets.
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l. The maxilla forms the upper jaw and the anterior of the hard palate; it also contains tooth sockets.
m. The palatine bones make up the posterior portion of the hard palate and the floor of the nasal
cavity.
n. The zygomatic gives us our cheekbone prominences.
o. Nasal bones form the bridge of the nose.
p. Other bones make up the nasal septum which divides the nose cavity into two regions.
q. The ears are elastic cartilage and lack bone; the nose is a mixture of bone, cartilage, and fibrous
connective tissue.
2. The Vertebral Column and Rib Cage
a. The vertebral column supports the head and trunk and protects the spinal cord and the roots of
the spinal nerves.
b. The vertebral column serves as an anchor for all of the other bones of the skeleton.
c. Seven cervical vertebrae are located in the neck.
d. Twelve thoracic vertebrae are in the thorax or chest.
e. The lumbar vertebrae are in the small of the back.
f. One sacrum is formed from five fused sacral vertebrae.
g. One coccyx is formed from four fused coccygeal vertebrae.
h. Normally, the spinal column has four normal curvatures that provide strength and resiliency in
posture.
i. Scoliosis is an abnormal sideways curvature; hunchback and swayback are also abnormal.
j. Intervertebral disks between the vertebrae act as a padding to prevent the vertebrae from grinding
against each other, and to absorb shock during running, etc.; they weaken with age.
k. Vertebral disks allow motion between vertebrae for bending forward, etc.
l. The rib cage: all twelve pairs of ribs connect directly to the thoracic vertebrae in back; seven
attach directly to the sternum.
1) Three pairs connect via cartilage to the sternum at front.
2) The two ribs totally unattached to the sternum are called “floating ribs.”
3) The rib cage protects the heart and lungs, yet is flexible enough to allow breathing.
D. The Appendicular Skeleton
1. The appendicular skeleton consists of the bones within the pectoral girdle and upper limbs and the
pelvic girdle and lower limbs.
2. The pectoral girdle and arms are specialized for flexibility; the pelvic girdle and legs are built for
strength.
3.
The components of the pectoral girdle are only loosely linked by ligaments.
a.
The clavicle (“collarbone”) connects with the sternum in front and the scapula
(“shoulderblade”) in back.
b.
The scapula connects with the clavicle; it is freely movable and held in place
only by muscles.
4. The humerus is the long bone of the upper arm; its smoothly rounded head fits into a socket of the
scapula.
5. The radius is the more lateral of the bones of the lower arm; it articulates with the humerus at the
elbow joint, a hinge joint, and the radius crosses in front of the ulna for easy twisting.
6. The ulna is the more medial of the two bones of the lower arm; its end is the prominence in your
elbow.
7.
The many hand bones increase its flexibility.
a.
The wrist has eight carpal bones which look like small pebbles.
b.
Five metacarpal bones fan out to form the framework of the palm.
c.
The phalanges are the bones of fingers and thumb.
8.
The pelvic girdle consists of two heavy, large coxal (hip) bones.
a.
The coxal bones are anchored to the sacrum; together with the sacrum they form
a hollow cavity that is wider in females than in males; it transmits weight from the vertebral
column via the sacrum to the legs.
b.
The femur is the largest bone of the body; it is limited in the amount of weight
that it can support.
c.
The tibia has a ridge called the “shin”; its end forms the inside of the ankle.
d.
The fibula is the smaller of the two bones; its end forms the outside of the ankle.
e. Seven tarsal bones are in each ankle; one receives the weight and passes it to the heel and ball of
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foot.
f.
The metatarsal bones form the arch of the foot and provide a springy base.
g.
The phalanges are the bones of the toes, which are stouter than the fingers.
E. Classification of Joints
1. Bones are joined at joints that are classified as fibrous, cartilaginous, or synovial.
2. Fibrous joints, such as those between the cranial bones, are immovable.
3. Cartilaginous joints, such as those between the vertebrae, are slightly moveable; the two hipbones are
slightly movable because they are ventrally joined by cartilage and respond to pregnancy hormones.
4. Synovial joints are freely movable.
a. Most joints are synovial joints, with the two bones separated by a cavity.
b. Ligaments are fibrous connective tissue that bind bone to bone, forming a joint capsule.
c. In a “double-jointed” individual, the ligaments are unusually loose.
d. The joint capsule is lined with a synovial membrane that produces a lubricating synovial fluid.
e. The knee represents a synovial joint.
1) Knee bones are capped by cartilage; a crescent-shaped piece of cartilage, the meniscus, is
between the bones.
2) Athletes who injure the meniscus have torn this cartilage.
3) The knee joint also contains 13 fluid-filled sacs called bursae to ease friction between the
tendons and ligaments and tendons and bones.
4) Inflammation of the bursae is bursitis; “tennis elbow” is a form of bursitis.
5) The knee and elbow are hinge joints; the shoulder and hip are ball-and-socket joints.
f. Synovial Joints
1) Synovial joints are subject to arthritis.
2) In rheumatoid arthritis, the synovial membrane becomes inflamed and thickened.
3) The joint degenerates and becomes immovable and painful.
4) This is likely caused by an autoimmune reaction.
5) In osteoarthritis from old age, the cartilage at the ends of bones disintegrates; the bones then
become rough and irregular.
F.
You Can Avoid Osteoporosis (Health Focus box)
1. Osteoporosis is a condition in which the bones are weakened due to a decrease in the skeleton
bone mass.
2. After the age of 40–50, the bone reabsorption exceeds bone formation, and the total bone mass
slowly decreases.
3. Men have a lower risk for osteoporosis because they tend to have denser bones and testosterone
levels do not begin to significantly decline until after the age of 65.
4. Consuming adequate dietary calcium throughout life is an effective measure to prevent
osteoporosis.
a.
During puberty, boys and girls are recommended to take 1,200–1,500 mg of calcium per
day.
b.
Males and females require 1,000 mg of calcium per day until the age of 65.
5. Consuming vitamin D is also recommended to prevent osteoporosis.
a.
Vitamin D helps the absorption of calcium.
b.
Although the body can synthesize vitamin D from the sun, people in colder
climates should find alternate sources of vitamin D.
6. Regular physical activity, such as walking or jogging, may also reduce the risk of osteoporosis.
7. People who may be considered high risk should consider having a bone density scan.
8. There is hormone therapy and medication prescribed to increase bone density.
39.3 The Human Muscular System
1.
2.
Skeletal muscle contraction assists homeostasis by helping maintain constant body temperature.
Skeletal muscle contraction also causes ATP breakdown, releasing heat that is distributed about the
body.
A. Macroscopic Anatomy and Physiology
1. Skeletal muscles are attached to the skeleton by tendons made of fibrous connective tissue.
2. When muscles contract, they only shorten or pull; therefore, skeletal muscles must work in
antagonistic pairs.
a. One muscle of an antagonistic pair bends the joint and brings a limb toward the body.
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b. The other one straightens the joint and extends the limb.
If a muscle is given a rapid series of stimuli, it responds to the next stimulus before completely
relaxing.
4. Muscle contraction summates until it reaches a maximal sustained contraction, called tetanus.
5. Even at rest, muscles maintain tone by some fibers contracting; this is essential to maintaining posture.
B. Microscopic Anatomy and Physiology
1. A whole skeletal muscle consists of a number of muscle fibers in bundles.
2. Each muscle fiber is a cell with some special features.
a. A plasma membrane called the sarcolemma forms a T (transverse) system.
1) Transverse (T) tubules penetrate down into the cell and contact with, but do not fuse with,
the modified endoplasmic reticulum (the sarcoplasmic reticulum).
2) Expanded portions or sacs of the sarcoplasmic reticulum are modified for Ca2+ ion storage;
this encases hundreds and sometimes thousands of myofibrils.
b. The myofibrils are contractile portions of fibers that lie parallel and run the length of the fiber.
c. A light microscope shows light and dark bands called striations.
d. An electron microscope shows that these striations of myofibrils are formed by placement of
protein filaments within sarcomeres.
e. The two protein filaments are either thick (made of myosin) or thin (made of actin).
f. A sarcomere has repeating bands of actin and myosin that occur between two Z lines in a
myofibril.
1) The I band contains only actin filaments.
2) The H zone contains only myosin filaments.
3. Sliding Filament Model
a. As a muscle fiber contracts, sarcomeres within the myofibrils shorten.
b. As a sarcomere shortens, actin filaments slide past the myosin; the I band shortens and the H zone
disappears.
c. Sliding filament model: actin filaments slide past myosin filaments because myosin filaments
have cross-bridges that pull actin filaments inward, toward their Z line.
d. The contraction process involves the sarcomere shortening although the filaments themselves
remain the same length.
e. ATP supplies the energy for muscle contraction.
f. Myosin filaments break down ATP to form cross-bridges that attach to and pull the actin filaments
toward the center of the sarcomere.
4. ATP
a. Muscle cells contain myoglobin that stores oxygen; cellular respiration does not immediately
supply all of the ATP needed.
b. Muscle fibers rely on a supply of stored creatine phosphate (phosphocreatine), a storage form of
high-energy phosphate.
c. Creatine phosphate does not directly participate in muscle contraction but regenerates ATP
rapidly: creatine—P + ADP → ATP + creatine
d. This reaction occurs in the midst of sliding filaments and is speedy.
e. When all creatine phosphate is depleted, and if O2 is in limited supply, fermentation produces a
small amount of ATP, but this results in a buildup of lactate.
f. The buildup of lactate partially accounts for muscle fatigue and represents oxygen debt.
g. Lactate is transported to the liver; 20% is completely broken down to CO2 and H2O in aerobic
respiration.
h. The ATP gained from this respiration is then used to reconvert 80% of the lactate to glucose.
i. In persons who train, the number of mitochondria increases, reducing the need for fermentation.
C. Muscle Innervation
1. Muscles are stimulated to contract by motor nerve fibers.
2. The neuromuscular junction is a region where an axon bulb is in close association with the
sarcolemma of a muscle fiber.
3. An axon bulb contains synaptic vesicles filled with the neurotransmitter acetylcholine.
4. When nerve impulses travel down a motor neuron to the axon bulb, vesicles merge with the presynaptic
membrane and acetylcholine molecules are released into the synaptic cleft.
5. Acetylcholine rapidly diffuses to and binds with receptors on the sarcolemma.
3.
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6. The sarcolemma generates impulse spreading down the T tubule system to the sarcoplasmic reticulum
where it triggers the release of Ca2+ ions out amongst the myofilaments.
7. The Ca2+ ions then initiate muscle contraction.
8. Ca2+ ions bind to troponin, which causes tropomyosin threads to shift position.
9. The change in the structure of tropomyosin exposes the myosin heads with ATP binding sites.
10. The myosin heads function as ATPase enzymes, splitting ATP into ADP and P i.
11. After attaching to actin filaments, the myosin cross-bridges bend forward and the actin filament is
pulled along.
12. While ATP and Ca2+ ions are available, cross-bridges attach; as ADP and P are released, the crossbridges change their positions and cause a power stroke as filaments pull together.
13. When another ATP molecule binds to the myosin head, the cross-bridge detaches and the cycle begins
again.
14. When a nerve impulse ceases, active transport proteins in the sarcoplasmic reticulum pump calcium
ions back into calcium storage sites and muscle relaxation occurs.
15. Without ATP, the myosis heads can’t detach from actin, nor can calcium be pumped back into the
sacroplasmic reciculum, and the muscles remain contracted, called rigor mortis.
Lecture Enrichment Ideas
Experience Base: While the culture of sports will make some concepts familiar to some
students, there is some misunderstanding of the nature of muscle and the role of creatine
phosphate. In many cases, the chemistry background of students may not be adequate to fully
understand the dynamics of muscle contraction.
1.
Ask why the exoskeleton system and its internal musculature work well for a small animal
such as an insect but would not be feasible for an organism the size of a human. Note that
humans have bone cross sections sufficient to support individuals of common height. People
who grow to over eight feet tall have severe problems with their back and leg bones
supporting the added stress of that much architecture. Note how larger animals have leg
bones with much larger cross sections, or are aquatic where the weight is buoyed. Therefore,
there is no possibility of 40-foot giants or skyscraper-sized gorillas.
2.
Discuss the relative advantages and disadvantages of an endoskeleton and an exoskeleton.
3.
Describe the different structures of human bones (long, short, flat, and irregular).
4.
Pursue why it is important that there be constant breakdown and replacement of bone tissue,
and discuss the problems encountered by astronauts and cosmonauts, who spend long periods
in weightlessness, where bone tissue deteriorates from lack of stimulation. Hibernating
animals have an ability to maintain bone tissue without this constant activity.
5.
Emphasize the importance of calcium in bone formation, muscle contraction, and blood
clotting, with emphasis on the problems of osteoporosis in older women and men.
6.
Point out the variety of skeletons in molluscs—with slugs having none, snails having a coiled
shell, clams and oysters having a bivalved shell, and octopuses having none.
7.
The protein component in bones can be demonstrated by removing the calcium salts from a
chicken bone by placing it in acetic acid for a few days prior to use. It becomes very flexible.
8.
Bone is a very active tissue requiring a blood supply; bone surgery is among the bloodiest of
operations. Surgeons may actually retrieve blood from the surgical wound and filter out bone
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fragments to re-transfuse the blood to the patient.
9.
Lecture question: How can the femur of a child be hard like an adult’s, yet the femur
gradually becomes bigger and bigger without ever completely disassembling and starting
over larger, as we would have to do if rebuilding a large brick office building out of a small
brick house. If students understand the concept of the osteoblasts, osteoclasts, and
cartilaginous disk centers, they can understand how bone can accomplish this “expansion of a
brick house.”
10.
Both hormones and physical stress affect bone growth. Lecture question: Why is a walking
cast often preferred for patient recovery. You can also relate this to astronauts losing bone
mass while in weightlessness in space due to lack of stress on bones.
11.
There is a need for both estrogen and calcium to prevent osteoporosis in postmenopausal
women, a dilemma today when estrogen replacement therapy may carry increased risk of
breast cancer.
12.
A student may ask how can we stick out our tongue or how can our heart beat, in both cases
without flexor-extensor muscles operating across a bone hinge. In these cases, we have
transverse muscle fibers that constrict the shortened muscle cells, forcing them to elongate
much as squeezing a balloon makes it lengthen. Until recently, textbooks often explained the
expansion of the heart as due to filling with blood; but it expands even if empty.
13.
Note for students that many muscle names are formed from the names of the bones at their
origin and insertion; therefore to learn the muscles, you need to know the bones and their
processes.
Critical Thinking
Question 1.
Most land animals with exoskeletons are smaller than the volume of a mouse, and most
vertebrates are larger than a mouse (marine invertebrates can be somewhat bigger). What keeps an insect with an
exoskeleton from being as big as a horse?
Answer: The major factors are limitations to gas diffusion, ability to support internal organs, and necessity to molt.
Gas must diffuse through openings in the exoskeleton and becomes stagnant a short distance down a trachea.
Anchoring organs to a larger outside suit of armor provides too little support and results in herniation during fast
movements. A hard suit of armor must be changed as you grow, and shedding a large suit leaves a large, unprotected
soft body to secrete the next shell.
Question 2.
Bone gains its rigidity from brick-hard deposits amidst living cells. Yet we cannot build a large
stone skyscraper from a small stone house without breaking the house apart. Yet our bones grow from small hard
bones to large hard bones without ever breaking apart into soft vulnerable tissue. How is that accomplished?
Answer: The bone tissue is simultaneously built up by osteoblasts and broken down by osteoclasts. Because the
process is both microscopic and piecemeal, a small hard structure can become a large hard structure without
disassembling.
Question 3.
Why will an increase in the strength of a stimulus cause an increase in the force of contraction in a
whole muscle but not in a single muscle fiber?
Answer: A single muscle fiber, when stimulated by a threshold stimulus, will contract maximally (all-or-none law).
A whole muscle, however, contains many muscle fibers, and therefore the degree of contraction is dependent on the
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total number of fibers contracting. With a greater strength of stimulus above a threshold level, more fibers will
contract until at the maximal stimulus, all the fibers are contracting.
Question 4.
If a person exercises to achieve bigger muscles, what effort must be exerted and what actually
increases, the size or the number of muscle cells?
Answer: Muscles must contract to 75% of maximum extension for exercise to be effective. Increased muscle is due
to more myofibrils within muscle fibers.
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304
CHAPTER
40
HORMONES AND ENDOCRINE SYSTEMS
This chapter has been entirely rearranged from the previous edition. The categories of hormones
are described as are the biochemical activities elicited by the individual hormones. Each glands’
hormones are described individually. Many endocrine-related disorders are described, including
a detailed discussion of diabetes mellitus. A Science Focus box discusses “Isolation of Insulin.”
Chapter Outline
40.1 Endocrine Glands
1.
2.
3.
The endocrine system functions differently from the nervous system.
An endocrine system consists of glands that coordinate body activities through hormones.
In contrast to the exocrine glands, which have ducts leading to other organs or outside the body, the
endocrine glands secrete their products into the bloodstream, which delivers them throughout the body.
4. Chemical signals are used between individuals, between body parts, and between cells.
5. Both the nervous system and the endocrine system rely on negative feedback mechanisms.
A. Hormones Are Chemical Signals
1. Chemical signals are a means of communication between cells, body parts, or individuals.
2. In general, they affect the metabolism of the cells that have receptors for them.
3. Most hormones act at a distance between body parts, traveling through the bloodstream from the gland
to the target cell.
4. Some hormones (e.g., prostaglandins, growth factors) are local hormones—they affect only
neighboring cells.
5. Pheromones are environmental signals that act at a distance between individual organisms.
a. Women may synchronize their menstrual cycles with women who live in their household, and a
particular pheromone released by men may reduce premenstrual nervousness and tension in
women.
B. The Action of Hormones
1. The Action of Peptide Hormones
a. The term peptide hormone is used to include hormones that are peptides,
proteins, glycoproteins, and modified amino acids.
Epinephrine is a peptide hormone that binds to a receptor protein in the target cell’s plasma
membrane; a relay system leads to conversion of ATP to cyclic AMP (cAMP).
c. Cyclic adenosine monophosphate (cAMP) is made from ATP; it has one phosphate group
attached to adenosine at two locations.
d. Peptide hormones are the first messenger; cAMP and calcium are the second messenger.
e. The peptide hormone does not enter the cell; the second messenger sets an enzyme cascade in
motion.
2. The Action of Steroid Hormones
a. Steroid hormones are lipids and cross cell membranes freely; they do not bind to plasma
membrane receptors.
b. Inside the cytoplasm or a nucleus, steroid hormones (e.g., estrogen, progesterone) bind to a
specific receptor.
c. The hormone-receptor complex binds to DNA, resulting in activation of genes (transcription) that
produce enzymes (translation).
d. Steroids act more slowly than peptide hormones because it takes more time to synthesize new
proteins than to activate enzymes already present in cells; however, their action lasts longer.
e. Steroid hormones are produced in the adrenal cortex, the ovaries, and the testes.
40.2 Hypothalamus and Pituitary Gland
1. The hypothalamus regulates the internal environment through the autonomic system.
a. It controls heartbeat, temperature, water balance, as well as glandular secretions of the pituitary
b.
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gland.
The pituitary gland (hypophysis) is connected to the hypothalamus by a stalklike structure.
a. It is about 1 cm in diameter and lies just below the hypothalamus.
b. It is comprised of two portions: the posterior pituitary and the anterior pituitary.
A. Posterior Pituitary
1. Neurons in the hypothalamus called neurosecretory cells produce antidiuretic hormone (ADH) and
oxytocin, which pass through axon endings in the posterior pituitary and are stored until released.
2. Antidiuretic hormone (ADH) promotes reabsorption of water from the collecting ducts in the
kidneys.
3. Nerve cells in the hypothalamus determine when the blood is too concentrated; ADH is released and the
kidneys respond by reabsorbing water.
a. As the blood becomes dilute, ADH is no longer released; this is a case of negative feedback.
b. Inability to produce ADH causes diabetes insipidus (watery urine), in which the individual
produces copious amounts of urine and a resultant loss of ions from the blood.
4. Oxytocin is also made in the hypothalamus and stored in the posterior pituitary.
a. Oxytocin stimulates uterine muscle contraction in response to uterine wall nerve impulses.
b. It also stimulates the release of milk from mammary glands.
c. This positive feedback mechanism increases intensity; such positive feedback does not maintain
homeostasis.
d. Oxytocin also may play a role in the propulsion of semen through the male reproductive tract.
B. Anterior Pituitary
1. Stimulation by the hypothalamus controls the release of anterior pituitary hormones through a portal
system consisting of two capillary systems connected by a vein.
2. The hypothalamus produces hypothalamic-releasing and hypothalamic-inhibiting hormones which
pass to the anterior pituitary by this portal system.
a. Thyroid-releasing hormones released from the hypothalamus act on cells in the anterior pituitary
to stimulate the production and secretion of a specific hormone.
b. Thyroid-inhibiting hormones produced in and released from the hypothalamus act on cells in the
anterior pituitary to inhibit the production and secretion of a specific hormone.
3. The anterior pituitary produces six different hormones.
a. Three of these anterior pituitary hormones affect other glands.
1) The thyroid-stimulating hormone (TSH) stimulates the thyroid to produce and secrete
thyroxin.
2) Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to release cortisol.
3) Gonadotropic hormones (follicle-stimulating hormone [FSH] and luteinizing hormone
[LH]) act on the gonads (ovaries and testes) to secrete sex hormones.
b. The other three hormones do not affect other glands.
1)
Prolactin (PRL) is produced in quantity only after childbirth.
2.
a) Prolactin causes the mammary glands to produce milk.
b) It also plays a role in carbohydrate and fat metabolism.
2) Melanocyte-stimulating hormone (MSH) causes skin color changes in fishes, amphibians,
and reptiles with melanophores, special skin cells.
3) Growth hormone (GH or somatotropic hormone)
40.3
a) GH promotes skeletal and muscular growth.
b) GH acts to stimulate the transport of amino acids into cells and to increase
the activity of ribosomes.
c) GH promotes fat metabolism rather than glucose metabolism.
d) Too little GH during childhood makes an individual a pituitary dwarf.
e) Too much forms a giant; life expectancy is less because GH affects blood
glucose levels and promotes diabetes mellitus.
f) The overproduction of GH in adults results in acromegaly; since long bone
growth is no longer possible, only the feet, hands, and face grow.
Other Endocrine Glands and Hormones
A. Thyroid and Parathyroid Glands
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1. The thyroid gland is in the neck and attached to the trachea just below the larynx.
2. The parathyroid glands are embedded in the posterior surface of the thyroid gland.
3. Thyroid Gland
a.
b.
The thyroid gland is the largest endocrine gland.
The two hormones produced by the many follicles of the thyroid gland both contain iodine.
1) Thyroxine (T4) contains four iodine atoms.
2) Triiodothyronine (T3) contains three iodine atoms.
c. Lack of iodine causes enlargement of the thyroid (simple goiter).
1) Simple goiter is easily prevented by supplementing iodine intake in salt.
d. Thyroid hormones increase the metabolic rate; there is no one target organ—all organs respond.
e. Congenital hypothyroidism (cretinism) occurs in individuals who have suffered from low
thyroid function since birth.
1) Affected individuals are short and stocky and have had hypothyroidism since infancy.
2) Thyroid treatment helps but unless it is begun in the first two months, mental retardation can
occur.
f. Myxedema is hypothyroidism in adults; thyroid hormones can restore normal function.
g. Hyperthyroidism (Graves disease) occurs when the thyroid gland is enlarged or overactive.
1) The eyes protrude because of edema in the eye socket tissue; this is called exophthalmic
goiter.
2) Removal or destruction of some thyroid tissue by surgery or radiation often cures the
condition.
h. The thyroid gland also produces calcitonin.
1) Calcitonin lowers the calcium level in the blood and increases deposits in the bone by
reducing osteoclasts.
2) Calcitonin is also necessary for blood clotting.
3) If blood calcium is lowered to normal, the release of calcitonin is inhibited.
4) Too low calcium levels stimulate the release of parathyroid hormone (PTH) by the
parathyroid glands.
4. Parathyroid Glands
a. Parathyroid glands produce parathyroid hormone (PTH).
1) Under the influence of PTH, the calcium level in blood increases and the phosphate level
decreases.
2) PTH stimulates the absorption of Ca2+ by activating vitamin D, the retention of Ca2+ (and
excretion of phosphate) by the kidneys and demineralization of bone by promoting the
activity of osteoclasts.
3) When the blood calcium level reaches the right level, the parathyroid glands no longer
produce PTH.
4) If PTH is not produced in response to low blood Ca 2+, tetany results because the Ca2+ plays
an important role in both nerve conduction and muscle contraction.
b. In tetany, the body shakes from continuous muscle contraction due to the increased excitability of
nerves that fire spontaneously and without rest; this condition may be due to hypoparathyroidism.
c. In hyperparathyroidism, the abnormally high blood calcium levels can cause the bones to be soft
and fragile, and the individual to be prone to kidney stones.
d. PTH and calcitonin are antagonistic hormones because their action is opposite to one another, and
both hormones work together to regulate the blood calcium level.
B. Adrenal Glands
1. Two adrenal glands sit atop the kidneys.
2. Each gland consists of two parts: an outer adrenal cortex and an inner adrenal medulla.
3. The cortex and medulla have no physiological connection between them.
4. The hypothalamus exerts control over both portions.
a. Nerve impulses travel via the brain stem to the spinal cord to sympathetic nerve fibers to the
medulla.
b. The hypothalamus uses ACTH-releasing hormone to control the anterior pituitary’s secretion of
ACTH.
5. Adrenal hormones increase during times of physical and emotional stress.
6. Both epinephrine and norepinephrine are produced by the adrenal medulla.
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7.
Both hormones bring about body changes corresponding to an emergency.
a. The blood glucose level rises and metabolic rate increases.
b. The bronchioles dilate and breathing rate increases.
c. Blood vessels to the digestive tract and skin constrict; those to the skeletal muscles dilate.
d. The cardiac muscle contracts more forcefully and the heart rate increases.
8. The adrenal cortex hormones provide a sustained response to stress.
a. The adrenal cortex secretes two types of hormones: glucocorticoids and
mineralocorticoids.
9.
1) Glucocorticoids help to regulate blood glucose levels.
2) Mineralocorticoids regulate the levels of minerals in the blood.
3) The adrenal cortex also secretes a small amount of both male and female sex hormones in
both sexes.
Glucocorticoids: Cortisol is a biologically significant glucocorticoid.
a. Cortisol promotes the breakdown of muscle protein into amino acids taken up by the liver from
the blood.
b. Cortisol breaks down fatty acids rather than carbohydrates; cortisol therefore raises blood glucose
levels.
c. Cortisol counteracts the inflammatory response; it helps medicate arthritis and bursitis.
10. Mineralocorticoids: Aldosterone is the most important of the mineralocorticoids.
a.
b.
The primary target organ is the kidney where it promotes the reabsorption of Na + and the excretion
of K+.
Mineralocorticoid secretion is controlled by the renin-angiotensin-aldosterone system.
1) Under low blood volume and sodium levels, the kidneys secrete renin.
2) The enzyme renin converts the plasma protein angiotensinogen to angiotensin
I; this becomes angiotensin II by a converting enzyme in the lungs.
3) Angiotensin II stimulates the adrenal cortex to release aldosterone.
4) Angiotensin I constricts the arterioles directly; aldosterone causes the kidneys
to absorb calcium.
5) When the blood sodium rises, water is reabsorbed as the hypothalamus
secretes ADH; blood pressure then increases to normal.
c.
Atrial natriuretic hormone (ANH) causes the excretion of sodium.
1) When the atria of the heart are stretched due to increased blood volume, cardiac cells release
ANH.
2) ANH inhibits the secretion of renin by the kidneys and the secretion of aldosterone from the
adrenal cortex.
3) When sodium is excreted, so is water; the blood volume and pressure then return to normal.
11. Malfunction of the Adrenal Cortex
a. Low levels of adrenal cortex hormones (hyposecretion) result in Addison disease.
1) When ACTH is in excess, like MSH, it can lead to the buildup of melanin and
a bronzing of the skin.
2) The lack of cortisol results in low glucose levels; a stressed person has
insufficient energy.
3) The lack of aldosterone drops blood sodium levels; a person then has low
blood pressure and dehydration.
4) Left untreated, Addison disease can be fatal.
b. High levels of adrenal cortex hormones from hypersecretion result in Cushing
syndrome.
1) Excess cortisol causes a tendency toward diabetes mellitus.
2) Muscular protein then decreases and subcutaneous fat forms an obese trunk
but normal arms and legs.
C. Pancreas
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1.
2.
The pancreas lies transversely in the abdomen between the kidneys and near the duodenum.
The pancreas is composed of two types of tissue.
a. Exocrine tissue produces and secretes digestive juices into the small intestine by way of ducts.
b. Endocrine tissues called pancreatic islets (islets of Langerhans) produce insulin and glucagon.
3. All body cells utilize glucose; therefore, its level must be closely regulated.
4. Insulin is secreted when the blood glucose level is high after eating; insulin has three actions.
a. Insulin stimulates liver, fat, and muscle cells to take up glucose.
b. Insulin stimulates the liver and muscles to store glucose as glycogen.
c. Insulin promotes buildup of fats and proteins and inhibits their use as an energy source.
5. Glucagon is secreted between meals in response to low blood glucose level.
a. Liver and adipose tissue are the main targets.
b. Adipose tissue cells break the fat into glycerol and fatty acids.
c. The liver uses glycerol and fatty acids as substrates for glucose, raising the blood glucose levels.
6. Diabetes Mellitus
a. Diabetes mellitus is a fairly common disease where the body cells do not take up or metabolize
sugar.
b. Blood glucose level becomes high enough for the kidneys to excrete glucose; therefore this is
detected by a urine test.
c. The liver is not storing glucose as glycogen and cells are not utilizing glucose for energy.
d. Since carbohydrate is not being metabolized, the body breaks down protein and fat for energy.
e. Ketones then build up in blood; the resulting reduced blood volume and acidosis can lead to coma
and death.
f. In type 1 (insulin-dependent) diabetes, the pancreas does not produce insulin.
1) A viral infection can cause cytotoxic T cells to destroy pancreatic islets.
2) This is treated with a daily administration of insulin; an overdose or lack of eating results in
hypoglycemia.
3) The brain also has constant sugar requirements; low blood sugar can result in unconsciousness.
4) An immediate intake of sugar is a simple and effective treatment.
7. Of 18 million diabetics in the U.S., most have type 2 (noninsulin-dependent) diabetes.
a. This form of diabetes usually occurs in obese and inactive individuals of any age.
b. The pancreas does produce insulin but live muscle cells do not respond to it.
c. Initially, this is a result of cells lacking the receptors for insulin.
d. Untreated, type II diabetes can have serious symptoms: blindness, kidney disease, circulatory
disorders, strokes, etc.
e. A low fat diet and regular exercise help; oral drugs can make cells more sensitive to insulin or
stimulate higher levels of insulin production by the pancreas.
D.
Isolation of Insulin (Science Focus box)
1. In 1920, physician Frederick Banting decided to try to isolate insulin.
2. He decided to try to tie off the pancreatic duct, which he knew would lead to the degeneration only of
the cells that produce digestive juices, and not of the pancreatic islets where insulin is made.
3. He and Charles Best worked and obtained pancreatic extracts that did lower the blood glucose level in
diabetic dogs.
4. A biochemist was brought in to purify the extract.
5. Insulin therapy for the first human patient began in 1922, and large-scale production of purified insulin
from pigs and cattle followed.
6. Banting, and his professor Mcleod received a Nobel Prize for their work in 1923.
7. The amino acid sequence of insulin was determined in 1953.
8. Insulin is presently synthesized by using recombinant DNA technology.
E. Testes and Ovaries
1. The testes located in the scrotum function as gonads and produce androgens (e.g., testosterone).
a. Testosterone is the male sex hormone.
b. It stimulates the development of male secondary sex characteristics: large vocal cords, pubic hair,
etc.
c. Testosterone is largely responsible for the sex drive.
d. Anabolic steroids are supplemental testosterone or similar chemicals with serious side effects.
e. Testosterone also affects sweat glands, expression of baldness genes, and other effects.
2. The ovaries, located in the pelvic cavity, produce the female sex hormones estrogens and
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progesterone.
a. Estrogens secreted at puberty stimulate the maturation of ovaries and other sexual organs.
b. Estrogen is necessary for oocyte development.
c. It is responsible for the development of female secondary sex characteristics: a layer of fat beneath
the skin, a larger pelvic girdle, etc.
d. Estrogen and progesterone are required for breast development and the regulation of the uterine
cycle.
F. Pineal Gland
1. The pineal gland, located in the brain, produces melatonin, primarily at night.
2. The pineal gland and melatonin help establish circadian rhythms, 24-hour physiological cycles.
3. The pineal gland may also be involved in human sexual development; children in whom a brain tumor
has destroyed the pineal gland experience puberty earlier.
G. Thymus Gland
1. The thymus gland is a lobular gland that lies just beneath the sternum in the upper thoracic cavity.
2. It reaches its largest size and is most active during childhood; with age, it shrinks and becomes fatty.
3. Some lymphocytes that originate in the bone marrow pass through the thymus and change into T
lymphocytes.
4. The thymus produces thymosins, which aid in the differentiation of T cells and may stimulate immune
cells.
H. Other Hormones
1. Leptin is a peptide hormone secreted by adipose tissue.
Its main function is its role in the feedback control of appetite; it can suppress appetite.
2. Erythropoietin (EPO) is a peptide hormone produced in the kidneys.
It stimulates the production of red blood cells and speeds up their maturation.
3. Local hormones such as prostaglandins are produced by certain cells and act on neighboring cells.
a. Prostaglandins are potent chemical signals which have several known functions.
b. They cause the contraction of uterine muscle, and they mediate the effects of pyrogens (chemicals
believed to affect the temperature regulatory center of the brain).
c. Certain prostaglandins reduce gastric secretions, others lower blood pressure.
d. Aspirin reduces temperature and controls pain because of its effect on prostaglandins.
Lecture Enrichment Ideas
Experience Base: Many students will have an interest in this section due to hormone
involvement in sexual development, and awareness of some of the abnormalities that result from
faulty hormone activity. Abuse of steroids and “blood doping” may be understood by many
students. It is likely, though, that there will be more than a few misconceptions concerning the
pineal gland, melatonin and circadian rhythms.
1. Describe the differences in actions of nonsteroid hormones versus steroid hormones as a
function of the kind of chemicals involved, with peptides unable to pass easily through the
plasma membrane, while lipid-based steroids easily diffuse through the plasma membrane.
2. Explain how nonsteroid and steroid hormones activate proteins present in the cytoplasm, as
opposed to stimulating the production of new hormones. This affects the length of time
required for the response and the stability of the response.
3. Discuss the mechanism of control of some kinds of endocrine functions (i.e., receptors in the
hypothalamus respond to the presence or absence of the final hormone in the sequence, thus
turning on or off the regulatory hormones that stimulate the anterior pituitary to release
stimulatory hormones).
4. Discuss how the nervous and endocrine systems interact in regulating the homeostatic
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mechanisms of the body. The innervation of the adrenal medulla and the action of the brain
as a gland serve to blur the lines between what were once considered quite different control
systems.
5. The quantity and duration of hormones, as well as the time for feedback mechanisms to
work, are important considerations. Lecture question: Does a blood recipient get a dosage of
hormones in a blood transfusion? A male who receives a blood transfusion from a female
donor does get some additional estrogen in the plasma but the quantity of blood is a small
proportion of his total blood volume, and this diluted hormone level is not sustained because
there are no ovaries producing more estrogen. His own adrenal glands would provide more
estrogens over time in his blood stream naturally.
6. Blood-borne hormones wash throughout the body and are recognized by target tissues only
as they “pass by.” Stress that there is no intent or “purposefulness” in the movement of
chemicals by physical properties. However, the close placement and related capillary
circulation of some endocrine organs to target tissues can speed the process.
7. Classes generally cover organ systems organ-by-organ as if they were distinct operators on
an assembly line. However, many organs must coordinate with others and hormones are a
central part of this coordination. Lecture question: What would happen if the gall bladder
didn’t “know” when fat was moving onward from the stomach, or the production of breast
milk was not in some way associated with the contractions of the uterus in childbirth?
8. Why is the hypothalamus and pituitary buried so deep and protected at the base of the brain?
Why are so many critical functions controlled from this one center?
9. Syntocin is a synthetic oxytocin; note how this is a very useful hormone when a physician
finds that a pregnancy is overdue and problems of delayed childbirth (e.g., pre-eclampsia)
begin to escalate.
10. Students may have seen huge laboratory rats with growth hormone irregularities. There are a
growing number of U.S. students and professors who are heavier than average, but otherwise
“normal.” What symptoms would be expected if this obesity was hormonal?
11. A network newscaster reporting immediately after the Chernobyl nuclear plant disaster
reported that citizens in northern Europe were taking iodine tablets “to prevent radioactive
iodine fallout from entering their body.” The newscaster got the reason wrong; students who
understand the use of iodine by the thyroid for synthesis of hormones, and the limited
number of receptor sites, and the excretion of surplus substances by the kidney, should be
able to identify this error.
12. Case histories about various endocrine abnormalities can be presented to a class to allow
them to recognize abnormalities and discuss the cause and treatment of the disease.
13. Embryologically, the adrenal medulla is differentiated from tissues similar to nerves. Lecture
question: Does this make sense, considering the function of the adrenal medulla hormones?
Yes, since the action is much faster than most hormones. Also, the modern perspective
presented in this text, where hormones are part of a continuum of environmental signals, puts
this in a correct light.
Lecture question: Cortisone has anti-inflammatory action and is useful for reducing deep
inflammation, as in Bell palsy, etc. Yet the dosage is given rapidly and for only a brief time.
Why would a physician not want to boost and maintain cortisone levels over a longer time?
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Students sometimes deduce wrongly that eating lots of sugar makes a normal person diabetic.
Lead students through a review of the compensating mechanisms involved while cavalierly
eating several candy bars. It is also possible to carry in a urine sample and test for sugar in urine;
although test results are visual but not easily projected on a screen, they can be reported by
eyesight. Adding sugar to a normal sample can convert the test. Lecture question: Why would
the medical lab want a patient to abstain from eating eight or more hours before a blood sample
is taken?
Critical Thinking
Question 1. If cortisol is good at reducing inflammation, especially of deep tissues, why isn’t
it a commonly used over-the-counter long-term drug for the many inflammation problems people
have every day?
Answer: Cortisol is far more than an anti-inflammatory medication. It promotes hydrolysis of muscle proteins to
amino acids that enter the blood. It increases blood glucose levels and shifts metabolism to fatty acids rather than
carbohydrates. Therefore, dosages used for reducing inflammation are carefully monitored and usually of short
duration.
Question 2.
Not only does the adrenal cortex secrete some estrogens, the normal fat tissue also produces some
estrogen. Why might a female Olympic athlete cease having a menstrual cycle during strenuous training before a
major event?
Answer: The hypothalamus, pituitary, and uterus are responding to the overall threshold level of estrogen in the
system. This is provided by a base level from the adrenal cortex and fat tissue, with the cycling level of estrogen
from the ovaries providing an additional level that is detected by these feedback tissues. When fat is dramatically
reduced, the total level of estrogens—now coming only from the ovaries—is too low and never cycles to high
enough concentrations to trigger the tissues involved in the menstrual cycle.
Question 3.
Biological controls are promoted as a replacement for pesticides because insects eventually
develop strains that are resistant. Hormone traps are one form of biological control where the sex pheromone is used
to lure the males (usually) to a trap that removes them from the mating population. There is natural variation in the
male response to pheromones. Is this biological control immune from the resistance problem that pesticides faced?
Answer: No. Individuals that are less-attracted will avoid the traps and be the only ones to mate; therefore, this
lower attractiveness will soon predominate and be a strain “resistant” to pheromone trapping. A pheromone may
have many components; only one is usually purified and used in traps.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
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CHAPTER
41
REPRODUCTIVE SYSTEMS
This chapter presents a complete study of animal reproduction, with emphasis on the anatomy
and physiology of human reproduction. Sections on birth control and infertility precede an
extensive description of the sexually-transmitted diseases. A Heath Focus box focuses on
“Preimplantation Genetic Diagnosis,” and another Health Focus box discusses “Preventing
Transmission of STDs.”
Chapter Outline
41.1 How Animals Reproduce
There are two patterns of reproduction:
1. Asexual—only one parent is involved and the offspring are identical to the parent.
2. Sexual—two parents are involved and the offspring are genetically unique.
A. Asexual Reproduction
1. Sponges produce asexual gemmules that develop into new individuals.
2. Hydra reproduce asexually by budding; a new individual arises as an outgrowth (bud) of a parent.
3. Obelia undergoes an alternation of generations: an asexual colonial stage and a sexual medusa stage.
4. Flatworms can constrict into two halves, each half becoming a new individual.
5. Fragmentation followed by regeneration is seen among annelids, sponges, and echinoderms.
6. Parthenogenesis is found among some insects, worms, fish, lizards, and some other animals; an
unfertilized egg develops into a complete individual.
7. In honeybees, the queen can fertilize or not fertilize the eggs, producing diploid female workers (if
fertilized) or haploid male drones (if unfertilized).
B. Sexual Reproduction
1. In sexual reproduction, the egg of one parent is fertilized by the sperm of the other; most animals are
dioecious (having separate sexes).
2. Hermaphroditic organisms possess both male and female organs.
a. Earthworms undergo cross-fertilization.
b. Tapeworms are capable of self-fertilization.
c. Sequential hermaphroditism, or sex reversal, involves the changing of sex; a male wrass (a reef
fish) has a harem but if the male dies, the largest female becomes a male.
3. Gonads are organs specialized to produce gametes.
a. Sponges are an exception since their collar cells give rise to sperm and eggs.
b. Hydras produce only temporary gonads in the fall when sexual reproduction occurs.
c. Animals in other phyla have permanent gonads.
4. There are two types of gonads: testes produce sperm and ovaries produce eggs.
5. Eggs and sperm cells derive from germ cells that specialize early for this development.
6. Other cells in the gonads support and nourish the developing gametes or produce hormones for
reproduction.
7. Accessory organs form ducts and storage areas that aid in bringing gametes together.
8. Sexually-reproducing animals have various methods to ensure that the gametes unite.
a. Aquatic animals that practice external fertilization must synchronize egg release.
b. The lunar cycle is one trigger that cues animals by tides.
c. Hundreds of thousands of palolo worms rise to the surface to release eggs during a 2–4 hour
period.
9. Copulation is sexual union to facilitate the reception of sperm by a female.
a. The penis is a male copulatory organ typical of terrestrial males; it deposits sperm into the
female’s vagina.
b. Birds lack a penis or vagina; they transfer sperm from cloaca to cloaca.
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c.
Female damselflie curls her abdomen forward to receive sperm previously deposited in a pouch by
the male, and copulation doesn’t occur.
C. Life History Strategies
1. Many aquatic animals use external fertilization; eggs and sperm join outside the body in the water.
2. Terrestrial animals tend to practice internal fertilization; eggs and sperm join inside the female’s body.
3. Both types of animals are usually oviparous; they deposit eggs in the external environment.
4. Insect eggs are produced in ovaries; they mature and increase in size as a result of the accumulation of
yolk.
a. Yolk is stored food to be used by the developing embryo.
b. To prevent insect eggs from drying out, their eggshell has several layers of protein or wax.
c. In insects, small holes are left at one end to allow for the entry of sperm.
5. Some insects have a special organ to store sperm so the eggs can be fertilized later.
6. A larval stage is often quite different in appearance and way of life from the adult form.
a. The larva is able to seek its own food to sustain itself until it becomes an adult.
b. Metamorphosis is a major change in form that some animals undergo during development.
c. Incomplete metamorphosis lacks a pupal stage and the nymphs look more like adults.
d. Larval aquatic forms can utilize a different food source than the adults.
e. The bilaterally symmetrical sea star larvae attach to a substrate and become radially symmetrical
adults.
f. The free-swimming barnacle larvae metamorphose into sessile adults with calcareous plates.
g. The crayfish lacks a larval stage; eggs hatch into tiny juveniles with the same form as the adults.
7. Reptiles and birds provide their eggs with plentiful yolk; there is no larval stage.
a. Complete development takes place within a shell containing extraembryonic membranes.
b. The chorion is the outermost membrane that lies next to the shell and functions in gas exchange.
c. The amnion forms a water-filled sac around the embryo ensuring that it will not dry out.
d. A yolk sac holds yolk which nourishes the embryo.
e. The allantois holds nitrogen waste products.
f. A shelled egg frees an animal from any need to reproduce in water and also helps it live
completely on land.
8. Birds tend their eggs.
a. Newly hatched birds have to be fed before they develop to where they can seek food on their own.
b. Parent bird’s reproductive behaviors involve complex hormone and neural regulation.
9. In oysters and sea horses; the eggs remain inside the body until they hatch fully-developed.
10. Garter snakes, water snakes, and pit vipers also retain eggs until they hatch and give birth to live
young.
11. Mammals are viviparous, producing living young.
a. The nutrients needed for development are constantly supplied by the mother.
b. Viviparity represents the ultimate in caring for the zygote and the embryo.
c. The evolution of viviparity can be seen in the primitive mammals.
1) The exceptions are the duckbill platypus and the spiny anteater, which are egg-laying
mammals.
2) Marsupials give birth to immature offspring that finish developing within a pouch.
3) In all other mammals, development occurs in a placenta.
12. The placenta is a complex organ comprised of maternal and embryonic tissues.
a. A placenta exchanges O2, CO2, nutrients, wastes, etc., between the fetal and maternal circulations.
b. Evolution allowed embryos to exchange materials with the mother; this made the shell
unnecessary.
41.2 Male Reproductive System
1. Paired testes are suspended in the scrotal sacs of the scrotum.
2. The testes begin development in the abdominal cavity but descend into the scrotal sac during
development.
3. If the testes do not descend, without surgery or hormonal therapy, sterility results.
4. The lower temperature of the scrotum is vital to normal sperm production.
5. Sperm produced in the testes mature within the epididymides.
a. These are tightly coiled tubules outside of the testes in which the sperm undergo maturation.
b. The maturation time in the epididymis is required for the sperm to develop the ability to swim to
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the egg.
Once sperm have matured, they are propelled into the vasa deferentia by muscular contractions.
Sperm are stored in both the epididymides and the vasa deferentia.
When a male is sexually aroused, the sperm enter the urethra, part of which extends through the penis.
A vasectomy is a surgical form of birth control in which the vasa deferentia are severed or blocked and
sperm cannot complete their journey down the male reproductive tract.
A. Orgasm in Males
1. The penis is a cylindrical copulatory organ used to introduce spermatozoa into the female vagina.
a. The penis has a long shaft and an enlarged tip called the glans penis.
b. The glans penis is normally covered by a layer of skin called the foreskin; circumcision is the
surgical removal of the foreskin soon after birth.
c. Three columns of spongy, erectile tissue extend down the penile shaft.
d. During sexual arousal, nervous reflexes cause an increase in the arterial blood flow to the penis.
e. Increased blood flow fills and distends the erectile tissue, and the penis stiffens and increases in
size.
f. These changes cause an erection; failure to achieve an erection is called erectile dysfunction.
2. Semen (seminal fluid) is thick, whitish fluid that contains sperm and glandular secretions.
a. The seminal fluid is formed by the seminal vesicles, prostate gland, and bulbourethral glands.
b. The seminal vesicles lie at the base of the urinary bladder.
1) Each joins a vasa deferensia to form an ejaculatory duct that enters the urethra.
2) They secrete into the ejaculatory duct a thick fluid containing nutrients for use by the sperm.
c. The prostate gland is located just below the urinary bladder and surrounds the upper portion of the
urethra.
1) It secretes a milky, slightly alkaline solution that promotes sperm motility and viability.
2) In older men, the prostate gland may become enlarged and constrict the urethra.
3) Prostate cancer is also common in older men.
d. The bulbourethral glands are located below the prostate gland and on either side of the urethra;
they release mucus secretions that provide lubrication.
3. The urethra also conducts urine from the bladder during urination.
4. Ejaculation results in the expulsion of semen; this is achieved at the peak of sexual arousal.
5. The first phase of ejaculation is emission.
a. Nerve impulses from the spine trigger the epididymides and vasa deferentia to contract.
b. Subsequent motility causes the sperm to enter the ejaculatory duct; seminal vesicles, the prostate
gland, and the bulbourethral glands release their secretions.
c. A small amount of secretion from the bulbourethral glands may leak from the end of penis; it
functions to clean the urethra of acid but it may contain sperm.
6. The second phase of ejaculation is expulsion.
a. Rhythmical contractions at the base of the penis and within the urethral wall expel the semen in
spurts.
b. Rhythmical contractions are a release from myotonia, or muscle tenseness, an important sexual
response.
7. An erection lasts for a limited time and the penis generally returns to a flaccid state following
ejaculation.
8. A refractory period follows during which stimulation does not bring about an erection.
9. Orgasm is the physiological and psychological sensations that occur at the climax of sexual
stimulation.
B. The Testes
1. A longitudinal section shows compartments called lobules, each of which contains one to three
seminiferous tubules.
a. Altogether, seminiferous tubules have a combined length of about 250 meters.
b. In a microscopic cross section, tubules show cells undergoing spermatogenesis, a process of
meiosis.
c. The sustentacular (Sertoli) cells support, nourish, and regulate spermatogenic cells.
2. Mature sperm (spermatozoa) have three parts.
a. The sperm head contains a nucleus covered by an acrosome.
1) An acrosome is a caplike covering over the anterior end of nucleus; it stores enzymes to
penetrate the egg.
6.
7.
8.
9.
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2) A human egg is surrounded by several layers of cells and thick membrane; the enzymes allow
the sperm to penetrate.
b. The middle piece contains mitochondria wrapped around microtubules of the flagellum; the
mitochondria provide the energy for movement.
c. The tail also contains microtubules as components of a flagellum; its movement propels sperm.
3. The ejaculate of a normal human male contains several hundred million sperm.
4. Fewer than 100 ever reach the vicinity of an egg; and only one sperm normally enters an egg.
C. Hormonal Regulation in Males
1. The hypothalamus has ultimate control of the testes’ sexual function through secreting of
gonadotropic-releasing hormone (GnRH) that stimulates the pituitary to produce gonadotropic
hormones.
2. There are two gonadotropic hormones: follicle-stimulating hormone (FSH) and luteinizing hormone
(LH) found in both males and females.
3. In males, FSH stimulates spermatogenesis in the seminiferous tubules.
4. In males, luteinizing hormone (LH) is also called interstitial cell-stimulating hormone (ICSH); it
stimulates testosterone secretion by interstitial cells of the testes.
5. The seminiferous tubules also release the hormone inhibin.
6. The hypothalamus-pituitary-testis systems are involved in a negative feedback relationship that
maintain a fairly constant production of sperm and testosterone.
7. Functions of Testosterone
a. Testosterone is the main sex hormone in males.
b. Testosterone is essential for the development of male secondary sex characteristics that develop
at puberty, and for the maturation of sperm.
1) It causes the tallness, longer legs, and broader shoulders of males.
2) Testosterone causes the larynx and vocal cords to enlarge, thus causing a deeper voice.
3) It is responsible for greater muscle strength of males; some athletes take supplemental
anabolic steroids (that are natural or synthetic testosterone).
4) Testosterone causes males to develop hair on the face, chest, and back.
5) Testosterone is also involved in triggering baldness if baldness genes are present.
41.3 Female Reproductive System
1. The female reproductive system includes: ovaries, oviducts, uterus, and vagina.
2. The ovaries produce a secondary oocyte each month; the ovaries are located in the pelvic cavity.
3. The oviducts (uterine tubes, fallopian tubes) extend from the ovaries to the uterus.
a. The oviducts are not attached to the ovaries.
b. Fingerlike projections called fimbriae sweep over the ovaries and waft in the egg when it erupts.
c. This is the normal site for fertilization; the embryo is slowly moved by ciliary movement toward
the uterus.
d. Tubal ligation is a surgery in which oviducts are either burned or clipped shut so that sperm cannot
reach the oocyte.
4. The uterus is a hollow, thick-walled muscular organ the size and shape of an inverted pear.
a. An embryo completes development by embedding itself in uterine lining, the endometrium.
b. The narrow end of the uterus is the cervix.
c. A small opening at the cervix of the uterus leads to the vaginal canal.
5. The vagina is a tube at a 45o angle with the small of the back.
a. Its mucosal lining lies in folds and it can extend, as necessary in childbirth.
b. It receives the penis during copulation and also serves as the birth canal.
A. Orgasm in Females
1. The external genitalia of women are known collectively as the vulva.
a. The mons pubis, labia minora, and labia majora are to the side of the vaginal and urethral
openings.
b. At the front juncture of the labia minora is the clitoris.
1) This is homologous to the penis in males.
2) The clitoris has a short shaft of erectile tissue and is capped by a pea-shaped glans.
3) It contains many sensory receptors that allow it to function as a sexually sensitive organ.
c. Orgasm involves the release of neuromuscular tension in the muscles of the genital area, vagina,
and uterus.
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B. The Ovaries
1. The ovaries alternate in producing one oocyte each month.
2. The ovaries produce the egg (ovum) and the female sex hormones, estrogens and progesterone,
during the ovarian cycle.
3. The Ovarian Cycle
a. In a longitudinal section, an ovary shows many cellular follicles, each containing an oocyte (egg).
b. As a follicle matures during the ovarian cycle, it develops from a primary follicle to a secondary
follicle to a vesicular (Graafian) follicle.
c. As oogenesis is occurring, a secondary follicle contains a secondary oocyte pushed to one side of
fluid-filled cavity.
d. The vesicular follicle fills with fluid until the follicle wall balloons out on the surface and bursts,
releasing a secondary oocyte surrounded by a zona pellucida and follicular cells.
e. Ovulation is the rupture of the vesicular follicle with the discharge of the secondary oocyte into
the oviduct.
f.
The secondary oocyte completes a second meiotic cell division when fertilization occurs.
g. Meanwhile, the follicle develops into the corpus luteum; if pregnancy does not occur, the corpus
luteum begins to degenerate in 10 days.
4. Phases of the Ovarian Cycle
a. The ovarian cycle is under the control of gonadotropic hormones: follicle-stimulating hormone
(FSH) and luteinizing hormone (LH).
b. The gonadotropic hormones are not present constantly but are secreted at different rates during the
cycle.
c. During the follicular phase, FSH promotes the development of a follicle that secretes estrogens.
d. As the estrogen level in the blood rises, it exerts feedback control over the anterior pituitary
secretion of FSH; the follicular phase comes to an end.
e. Estrogen levels in the blood rise, causing the hypothalamus to secret more GnRH; this causes a
surge in LH secretion.
f. The LH surge then triggers ovulation.
g. The luteal phase is the second half of the ovarian cycle following ovulation.
1) LH promotes the development of the corpus luteum, which secretes large amounts of
progesterone.
2) As the blood level of progesterone rises, negative feedback to the anterior pituitary’s secretion
of LH causes the corpus luteum to degenerate.
3) As the luteal phase ends, menstruation occurs.
C. The Uterine Cycle
1. Estrogens and progesterone affect the endometrium of the uterus to cause a cycle of events known as
the uterine cycle.
2. An average 28-day uterine cycle is divided into four sections.
a. During days 1–5, low levels of estrogen and progesterone in the body cause menstruation.
b. During days 6–13, an increased production of estrogens by an ovarian follicle causes the
endometrium to thicken and become vascular and glandular (proliferative phase).
c. Ovulation usually occurs on day 14 of the 28-day cycle.
d. Days 15–28 see increased production of progesterone by the corpus luteum that causes the
endometrium to double in thickness; uterine glands mature, producing a thick mucoid secretion;
this is the secretory phase.
1) The endometrium is now prepared to receive a developing embryo.
2) If no pregnancy occurs, the progesterone and estrogen levels decline and the corpus luteum
degenerates.
3) With low levels of progesterone, the uterine lining also begins to degenerate.
3. Menstruation is the periodic shedding of tissue and blood from the endometrium; this lining
disintegrates and the blood vessels rupture.
a. A flow of blood and tissues passes out through the vagina.
b. The enzyme fibrinolysin prevents the blood from clotting.
c. The first menstrual period, menarche, typically occurs between the ages of 11 and 12.
d. If menarche does not occur by age 16, amenorrhea exists.
1) Primary amenorrhea is usually caused by nonfunctional ovaries or developmental
abnormalities.
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2) Secondary amenorrhea may be caused by weight loss or excessive exercise.
Menopause, when menstruation ceases because the ovaries are no longer functioning, usually
occurs between the ages of 45 and 55.
1) Menopause is not complete until menstruation is absent for a year.
D. Fertilization and Pregnancy
1. If fertilization occurs, the embryo begins development as it travels down the oviduct to the uterus.
2. The embryo becomes embedded in the endometrium several days following fertilization.
3. The placenta develops from both maternal and embryonic tissues.
a. The placenta functions to exchange gases and nutrients between the fetal and maternal circulation.
b. There is normally no mixing of the blood between the maternal and fetal circulations.
4. Initially, the placenta produces human chorionic gonadotropin (HCG) which maintains the corpus
luteum.
5. The corpus luteum is maintained by the HCG until the placenta produces its own progesterone and
estrogens.
6. The progesterone and estrogens have two effects at this stage.
a. They shut down the anterior pituitary so that no new follicles mature.
b. They maintain the lining of the uterus so the corpus luteum is not needed.
7. There is no menstruation during pregnancy.
E. Estrogen and Progesterone
1. Estrogens maintain the normal development of the related organs and are responsible for the secondary
sex characteristics of females.
2. There is less body and facial hair, and more fat beneath the skin provides a more rounded appearance.
3. The pelvic girdle enlarges and the pelvic cavity is larger; therefore, women have wider hips.
4. Both estrogen and progesterone are required for breast development.
F. The Female Breast
1. The female breast contains 15–24 lobules, each with a mammary duct.
2. The mammary duct begins at the nipple and divides into numerous ducts which end in alveoli (blind
sacs).
3. The hormone prolactin is needed for lactation (milk production) to begin.
4. Production of prolactin is suppressed by the feedback inhibition that estrogens and progesterone have
on the anterior pituitary during pregnancy; therefore, it takes a couple of days after delivery of a baby
for milk production to begin.
5. The breasts produce a watery, yellowish white fluid (colostrum) similar to milk but containing more
protein and less fat, and it is rich in IgA antibodies providing some immunity to a newborn.
6. Breast cancer is the most common form of cancer in females; women should have regular breast
checks and mammograms when recommended.
e.
41.4 Control of Reproduction
A. Birth Control Methods
 The most reliable method of birth control is abstinence; it has the advantage of preventing transmission of a
sexually transmitted disease.
1. Contraceptive Injections
a. Contraceptive injections are now being developed—possibilities include vaccination against HCG
or sperm.
2. Morning-after Pills
a. These regimens either prevent fertilization or stop a fertilized egg from implanting.
b. Preven is a kit of four synthetic progesterone pills; the medication makes it difficult for the
embryo to implant in the endometrium; it is considered 85% effective.
c. Mifepristone, also known as RU-486, causes the loss of an implanted embryo.
1) It blocks the progesterone receptors of the endometrial cells.
2) Without functioning receptors for progesterone, the uterine lining sloughs off, carrying the
embryo with it.
3) Taken in conjunction with a prostaglandin to induce uterine contractions, it is 95% effective.
B.
Reproductive Technologies
 Infertility is the inability of a couple to achieve pregnancy after one year of regular, unprotected intercourse.
1. Sometimes the causes of infertility may be corrected by medication intervention so that couples can
have children.
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a.
It is possible for women to take fertility drugs; these treatments may cause multiple ovulations and
multiple births.
2. If unable to have children, many couples choose to adopt children.
3. Assisted reproductive technologies (ART) have been developed to increase the chance of pregnancy.
a. Sperm and/or oocytes are retrieved from the testes and ovaries, and fertilization takes place in a
clinical or laboratory setting.
4. Artificial Insemination by Donor (AID)
1. A sperm sample is injected into the vagina.
2. If the husband’s sperm count is low, many samples can be combined.
3. Artificial insemination from a donor is necessary when the husband lacks viable sperm.
4. Intrauterine insemination can be coordinated with drugs used to stimulate the ovaries.
5. With artificial insemination, sperm can be sorted into those that are X-bearing (producing a girl) or
Y-bearing (producing a boy).
5. In Vitro Fertilization (IVF)
1. In IVF, conception occurs in laboratory glassware.
2. Ultrasound machines spot maturing follicles and a laparoscope is used to harvest the eggs using a
needle.
3. When sperm and egg are combined in glassware, they can be transferred to the uterus after 2–4
days.
4. While in glassware, the new embryos can be tested for genetic diseases.
6. Gamete Intrafallopian Transfer (GIFT)
1. A gamete is a sex cell, either a sperm or an egg.
2. Due to the low success rate of IVF (15–20%), GIFT immediately places the sperm and egg in the
oviduct.
3. A variation is to fertilize the eggs in the laboratory and then place the zygotes in the oviducts.
7. Surrogate Mothers
1. Women can be contracted and paid to have babies; they are then surrogate mothers.
2. The sperm and/or egg can be contributed by the contracting parents.
8. Intracytoplasmic Sperm Injection (ICSI)
1. A single sperm is injected into an egg.
2. This is used when a man has severe infertility problems.
C. Preimplantation Genetic Diagnosis (Health Focus box)
1. Parents may want to determine whether their offspring will be free of a genetic disorder.
2. This testing can be done by determining the genotype of the embryo.
3. Following in vitro fertilization (IVF), only embryos that will not have the genetic disorders of interest
are placed in the uterus to continue developing.
4. It is possible to test the oocyte if the disorder of concern is recessive.
a. By testing the polar body for the recessive mutated allele, those embryos are rejected, and only the
oocyte that has the normal dominant alleles are placed in the uterus for development.
41.5 Sexually Transmitted Diseases
1.
2.
3.
4.
5.
A. AIDS
1.
2.
3.
4.
5.
Sexually transmitted diseases (STDs) are caused by organisms ranging from viruses to arthropods.
Humans cannot develop lasting immunity to any STDs; therefore, prompt medical treatment should be
received when exposed to an STD.
To prevent STDs, a condom can be used.
It is difficult to cure STDs caused by viruses; treatment is available for AIDS and genital herpes.
STDs caused by bacteria (e.g., gonorrhea, chlamydia, and syphilis) are treatable with antibiotics.
Acquired immunodeficiency syndrome (AIDS) is caused by the human immunodeficiency virus
(HIV).
HIV attacks the helper T cells that stimulate the activity of B lymphocytes to produce antibodies.
After an HIV infection begins, helper T cells decline in number and a person becomes more
susceptible to infections.
Symptoms
AIDS has three stages of infection called category A, B, and C
a. The category A stage may last about a year.
1) An individual is asymptomatic but can pass on the infection.
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B.
C.
D.
E.
F.
2) Immediately after infection but before testing positive, a large number of infectious viruses
are in the blood.
3) After testing positive, a person may remain well for as long as he or she can maintain
sufficient helper T cells (above 500 mm3).
b. The category B stage may last six to eight years.
1)
Lymph nodes swell.
2)
There is weight loss, night sweats, fatigue, fever, and diarrhea.
3)
Infections such as thrush and herpes reoccur.
c. The category C stage is full-blown AIDS.
1) Nervous disorders and opportunistic diseases (e.g., an unusual type of pneumonia or skin
cancer) occur.
2) Without intensive medical treatment, an AIDS patient usually dies by about 7–9 years after
infection.
3) A recent combination therapy of several drugs allows AIDS patients in the United States to
live longer.
6. Transmission
a. AIDS is transmitted by sexual contact with an infected person (vaginal or rectal intercourse and
oral/genital contact).
b. Needle sharing among intravenous drug users is a high-risk behavior.
c. Transfusions of blood or clotting factors are now a rare mode of transmission and can be screened.
d. The largest increases in AIDS cases now involve heterosexual contact and intravenous drug use.
e. Women now account for 19% of all newly diagnosed cases of AIDS.
7. Treatment
a. Even though there is no cure for AIDS, a treatment called highly active antiretroviral therapy
(HAART) is usually able to stop HIV replication.
b. HAART uses a combination of drugs that interfere with the life cycle of HIV.
c. Entry inhibitors stop HIV from entering a cell.
d. Reverse transcriptase inhibitors (e.g., AZT) interfere with the enzyme reverse transcriptase.
e. Integrase inhibitors prevent HIV from inserting its own genetic material into that of the host.
f. Protease inhibitors prevent protease from processing newly created polypeptides.
Genital Warts
1. Genital warts are caused by the human papillomaviruses (HPVs).
2. Many carriers are asymptomatic or they have minimal symptoms.
3. If visible warts are removed, they may recur.
4. HPVs are now associated with cancer of the cervix as well as tumors of the vulva, vagina, anus, and
penis.
5. Some researchers believe viruses are involved in 90–95% of all cases of cancer of the cervix.
Genital Herpes
1. Genital herpes is caused by the herpes simplex virus.
2. Type 1causes cold sores and fever blisters; type 2 more often causes genital herpes.
3. Individuals infected with this type of virus can be asymptomatic carriers.
4. Symptoms include painful ulcers on the genitals, fever, painful urination, and swollen lymph nodes.
5. Exposure to herpes in the birth canal can cause neurological disorders and even death in a newborn;
birth by cesarean section avoids this possibility.
Hepatitis
1. Hepatitis A is usually acquired from sewage-contaminated drinking water but is also an STD
contracted by oral/anal contact.
2. Hepatitis B is spread in the same manner as AIDS but is more infectious; a vaccine is available.
3. Hepatitis C is called post-transfusion hepatitis, but it can be transmitted through sexual contact.
4. Hepatitis infections infect the liver and can lead to liver failure, liver cancer, and death.
Chlamydia
1. Chlamydia is named for the bacterium that causes it: Chlamydia trachomatis.
2. New chlamydial infections have increased faster than any other STD.
3. It also causes cervical ulcerations which increase the risk of acquiring AIDS.
4. It also causes pelvic inflammatory disease (PID).
5. If a baby is exposed at birth, inflammation of the eyes or pneumonia can result.
Gonorrhea
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1.
2.
3.
4.
5.
6.
Gonorrhea is caused by the bacterium Neisseria gonorrhoeae.
Male diagnosis is not difficult: typical symptoms include urination pain and a thick, greenish yellow
discharge.
In males and females, latent infections lead to pelvic inflammatory disease (PID); the vasa deferentia
or the oviducts become infected and inflamed.
As these tubes heal, they may become partially blocked, resulting in sterility or infertility.
If a baby is exposed at birth, an eye infection can lead to blindness; therefore all newborns are given
eye drops.
Gonorrhea proctitis is an infection of the anus; gonorrhea can infect the mouth, throat, tonsils, heart,
and joints.
Previously easily cured by antibiotics, nearly 40% of modern strains are now antibiotic resistant.
7.
G. Syphilis
1. Syphilis is caused by the bacterium Treponema pallidum.
2. This disease has three stages typically separated by latent periods.
a. The primary stage involves the appearance of a hard chancre (ulcerated sore).
b. The second stage involves the appearance of a rash all over the body, including the palms and feet.
c. The third stage involves neurological and cardiac disorders.
1) An infected individual may become mentally retarded, blind, walk with a shuffle, or become
insane.
2) Large destructive ulcers (gummas) develop on the skin or within internal organs.
3. Syphilitic bacteria can cross the placenta, causing birth defects or stillbirth.
4. Unlike the other STDs discussed, there is a blood test to diagnose syphilis.
5. Tracing sexual partners is very important in controlling syphilis.
H. Two Other Infections
1. Bacterial vaginosis is caused by Gardnerella vaginalis, Trichomonas vaginalis (a flagellated
protozoa), or Candida albicans (a yeast).
2. The protozoan infection causes a frothy, foul-smelling discharge with itching.
3. Trichomoniasis is most often transmitted through sexual intercourse.
4. The Candida yeast infection causes a white, curdy discharge with itching.
5. Candida albicans is a normally-occurring organism in the vagina; yeast infections can result from
taking birth-control pill or antibiotics.
I. Preventing Transmission of STDs (Health Focus box)
1. Sexual activities transmit STDs
a. Ways to prevent the transmission of STDs include:
1)
Abstain from sexual intercourse.
2) Refrain from multiple sex partners or having relations with someone who has multiple sex
partners.
3) Remember that the highest rate of increase of AIDS is in heterosexuals.
4) Be aware that having relations with an intravenous drug user is risky.
5) Uncircumcised males are more likely to become infected with an STD than a circumcised
male.
6) Avoid anal-rectal intercourse.
2. Unsafe sexual practices transmit STDs
a. To reduce the risk of transmitting or contracting an STD:
1)
Always use a latex condom during sexual intercourse.
2) Avoid fellatio (kissing and inserting of the penis into the partner’s mouth) and cunnilingus
(kissing and insertion of the tongue into the vagina).
3) Be cautious about the use of alcohol or any drug that may prevent you from being able to
control your behavior.
3. Drug use transmits Hepatitis and HIV
a. Stop or do not start the habit of injecting drugs into your veins.
b. Always use a new sterile needle for injection or one that has been cleaned in bleach.
Lecture Enrichment Ideas
Experience Base: Student experiences in this subject may be varied but this is generally a high
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interest topic. Usually a science teacher strives to provide as rich a multi-sensory and meaningful
lecture as possible, including video of the actual anatomy as well as hands-on lab dissections,
etc. However, students in some communities may have cultural reservations about viewing
graphic nudity on CD, videos, or slides. More abstract line illustrations in slide or overhead
transparencies may be more politically correct, if less effective. An instructor must make
decisions appropriate to the maturity of his/her audience. This may be more dependent upon the
classroom rapport of the instructor than on university level rules and academic freedom.
1. Discuss the process of internalization of reproduction as a requirement for life on land, and
examine the shelled eggs of reptiles and birds, the egg cases of insects, and the internal
development of the fetus in mammals. Discuss how each is adapted to prevent desiccation.
2. Female students may have some experience with male anatomy because of babysitting and
diapering infants. This provides a legitimate experience base for explaining the reflexive
action of erection, the nature of scrotal movements for temperature regulation, and other
physiology outside of any sexual context.
3. When the sperm enters the egg, the mitochondria midpiece and parts of the tail are now
known to also enter. However, the sperm mitochondria are small in number and worn out,
and the egg cell marks them for destruction. Thus, all of a person’s mitochondria are still
inherited from his/her mother, with some rare exceptions.
4. Prior conventional wisdom led to statements that impotence (failure to achieve erection) was
nearly always a psychological problem. The widespread effectiveness of Viagra has altered
this viewpoint by revealing that much impotence can be biochemically cured. Note that
Viagra is not an aphrodisiac, but merely inhibits an enzyme that breaks down cGMP; this in
turn allows fuller expansion of the erectile tissue. The nervous stimulation must still be
provided.
5. Hernias result whenever organs protrude through a body wall (stomach across the diaphragm,
intestines along the femoral artery passageway, or at the umbilical region, etc.). Query why
humans have more problems with inguinal hernias. An overhead of how the intestines are
loosely anchored by mesentery from the back may help students understand why we are not
fully adapted to our recent upright posture. Students may not completely understand the
transfer of weight when we lift; standing on a scale while lifting a chair and tracing how the
weight is transferred through our frame may help them understand the force on the viscera.
6. There has been a substantial decline in both sperm counts and the quality of human sperm
over the last half century; this correlates with male anatomical abnormalities in aquatic
animals as well. Query as to why this might be.
7. Twinning is more common in the tropics where the life span is shorter; less common in
northern Europe and Asia where the lifespan has been longer going back into prehistoric
times. Lecture question: How does the variation in rates of twinning show an evolutionary
basis? (Simply, selection is for leaving the most offspring and where lifespans are short, there
is more pressure to bear more children earlier.)
8. Once the hormone feedback system of the uterine cycle has been described, pose the lecture
question: What changes are needed to maintain the lining and prevent monthly ovulation
during pregnancy. Which chemicals did we need to duplicate in the birth control pill to
simulate pregnancy?
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9. Note that more women have breast reduction surgery than breast enlargement. The size
difference is primarily a difference in fat tissue and is unrelated to adequacy in nursing. Note
that breast reduction surgery is quite common in men nowadays.
10. The use of the IUD has decreased greatly due to worry over liability; the IUD irritates the
endometrium and its presence may be a risk if a pregnancy occurs.
11. There is a subtle but important difference between a contraceptive, which prevents
conception, and birth control, which controls viable births. There is likewise a slight
distinction between fertilization, where the sperm crosses the ovum membrane, and
conception where the sperm and egg nuclei fuse.
12. In the last two decades, a strain of gonorrhea that can produce an enzyme penicillinase has
become common near port military bases, having originated at Pacific bases where local
prostitutes were regularly treated with penicillin. Lecture question: How did this “natural
selection” of a resistant strain occur? Weekly use of penicillin for prostitutes, to keep them
and their clients free of infection, provided the constant natural selection pressure to culture a
resistant strain.
Critical Thinking
Question 1.
In earlier history, males volunteered to be castrated to serve as eunuchs in the Chinese palace, and
young boy singers likewise preserved their high voice to sing “castrato” in Italian opera. If this operation was
conducted before puberty, what physiological and body changes would occur, and would ejaculation be possible?
Answer: Removal of the testes would remove the overriding source of testosterone and some estrogen would be
present from the adrenals and from fat tissue. Primary sexual characteristics of a pre-puberty boy would remain.
However, the voice box would not grow (preventing the deepening of voice) and body proportions (width of
shoulders and hips, and layering of fat) would be somewhat more feminine. Sex drive would be reduced without
testosterone but the physiological mechanism of erection remains, although any ejaculate would be free of sperm.
Question 2.
Many male and female reproductive organs are equivalent tissues, developmentally from the same
origin. From your knowledge of the tissues of the scrotum, penis, clitoris and labia, which are likely “equivalent”?
Answer: Because they both contain erectile tissue, the penis and clitoris are equivalent. The scrotum and the labia
are also equivalent (the central scar running up the center of the scrotum [the raphe] is the fusion of the two side
tissue).
Question 3.
The percentage of cases of infertility has gone up recently. Although environmental factors may
be involved, a portion of these are cases where the husband has normal healthy sperm and the wife has normal
ovaries producing eggs. What relationship might this have to the epidemic of sexually transmitted diseases?
Answer: Several of the STDs, particularly gonorrhea and chlamydia, lead to pelvic inflammatory disease and scar
tissue formation in the oviducts. This would then block the sperm from meeting the egg or block the much larger
fertilized egg from passing down the oviduct to the uterus.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
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CHAPTER
42
ANIMAL DEVELOPMENT
This chapter explores the topic of animal development. Beginning with a study of the stages of
early development, the discussion moves into developmental processes, and then into human
embryonic and fetal development. The chapter concludes with a discussion of the process of
birth. A Health Focus box discusses “Preventing and Testing for Birth Defects.”
Chapter Outline
42.1 Early Developmental Stages
1.
Fertilization requires that sperm and egg interact to form a zygote.
a. A human sperm cell has three parts.
1) The head contains a haploid nucleus covered by a caplike acrosome
containing enzymes, allowing the sperm to penetrate the egg.
2) A middle piece contains ATP-producing mitochondria.
3) The tail is a flagellum that allows the sperm to swim.
2. The plasma membrane of the egg is surrounded by the zona pellucida.
a. The zona pellucida is surrounded by a few layers of adhering follicular cells, collectively called
the corona radiata.
b. These cells nourished the egg when it was in a follicle of the ovary.
3. Fertilization involves the following steps.
a. Several sperm penetrate the corona radiata and several sperm attempt to penetrate the zona
pellucida.
b. One sperm enters the egg and their nuclei eventually fuse.
c. After the sperm head binds tightly to the zona pellucida, the acrosome enzymes digest and form a
pathway for the sperm through the zona pellucida.
d. The head, middle piece, and usually the tail enters the egg.
e. Prevention of polyspermy depends on changes in the egg plasma membrane when the sperm
touches the egg and depolarizes the egg plasma membrane; this is called Fast block.”
f. Vesicles in the egg called cortical granules secrete enzymes that turn the zona pellucida, forming
an impenetrable fertilization membrane; this is called “Slow block.”
g. As soon as plasma membranes of the sperm and egg fuse, the zona pellucida lifts away from the
surface of the egg, forming a moat that prevents entrance of any other sperm.
h. The diploid zygote forms when a nuclear envelope surrounds the sperm and egg chromosomes.
A. Embryonic Development
1. Cellular Stages of Development
a. Development is all of the changes that occur during the life cycle of an organism.
b. An organism is an embryo during the first stages of development.
c. After fertilization, a zygote undergoes cleavage, cell division without growth.
d. DNA replication and mitosis occur repeatedly, and the cells get smaller with each division.
e. In the lancelet, the cell divisions are equal in the resulting morula.
f. A cavity called the blastocoel develops forming a hollow ball called the blastula.
2. Tissue Stages of Development
a. The tissue stages of development are early gastrula and late gastrula.
b. The early gastrula stage begins with the invagination of certain cells into the blastocoel to form
two of the three primary germ layers.
c. The outer layer of cells becomes ectoderm; ectoderm gives rise to the epidermis of the skin, the
epithelial lining of the mouth and rectum, and the nervous system.
d. The inner layer of cells becomes the endoderm that gives rise to the epithelial lining of the
digestive tract and the respiratory tract, associated glands of the digestive and respiratory system,
and the lining of the urinary bladder; a pore created by invagination is the blastopore.
e. The late gastrula has, in addition to ectoderm and endoderm, a middle layer of cells called the
mesoderm.
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3.
1) The outpocketings grow and fuse, forming a two layered mesoderm.
2) The space between them is the coelom that contains the body organs.
3) The mesoderm gives rise to the skeleton, the dermis of the skin, the skeletal system, the
muscular system, the excretory system, the reproductive system (including most epithelial
linings), and the outer layers of respiratory and digestive systems.
f. These germ layers then develop into those future organs.
Organ Stages of Development
a. The newly formed mesoderm cells along the main axis coalesce to form a dorsal notochord; it
persists in lancelets but is replaced in frogs, chicks, and humans by the vertebral column.
b. The nervous system develops from the midline ectoderm located just above the notochord.
1) At first, the cells on the dorsal surface of the embryo thicken, forming the neural plate.
2) Then neural folds develop on either side of a neural groove which becomes the neural tube
when the folds fuse.
3) At this point the embryo is called a neurula.
4) Later, the anterior end of the neural tube develops into the brain; the rest becomes the spinal
cord.
c. Midline mesoderm cells that did not contribute to the formation of the notochord now become two
longitudinal masses of tissue.
1) The two tissue masses become blocked off into somites.
2) The somites give rise to segmental muscles in all chordates; in vertebrates the somites also
form the vertebral bones.
42.2 Developmental Processes

Development requires growth, differentiation, and morphogenesis.
1. Cellular differentiation occurs when cells become specialized in their structure and function.
2. Morphogenesis produces a change in the shape and form of a body part; this includes both early cell
movement and later pattern formation.
A. Cellular Differentiation
1. Each body cell contains a full set of chromosomes; therefore differentiation is not due to parceled out
genes.
2. Cells in the adult body are totipotent; each contains all of the instructions to form any specialized cell.
3. Since only muscle cells produce myosin, only red blood cells produce hemoglobin, and only skin cells
produce keratin, there must be differential gene expression.
4. Two mechanisms—cytoplasmic segregation and induction—seem especially important.
5. Cytoplasmic Segregation
a. Differentiation begins long before we can recognize specialized cell types.
b. Eggs contain substances called maternal determinants that influence the course of development.
c. Cytoplasmic segregation parcels out the maternal determinants as mitosis occurs and determines
how the various cells of morula develop.
d. Early experiments showed the cytoplasm of a frog egg is not uniform in content.
e. After the first cleavage of a frog embryo, only a daughter cell that receives a portion of the gray
crescent develops into a complete embryo.
f. Hans Spemann (Nobel Prize in 1935) found particular chemical signals within the gray crescent
turn on the genes that control development.
6. Induction and Frog Experiments
a. As development proceeds, differentiation involves signals from neighboring cells.
b. Induction is the ability of one tissue to influence the development of another tissue.
c. Cell migration occurs during gastrulation; one set of cells can influence the migratory path of
another set.
d. Spemann showed that the dorsal lip of a blastopore (primary organizer) was necessary for
development.
1) The cells closest to the primary organizer become endoderm, those farthest away become
ectoderm, and the intermediate cells became mesoderm.
2) A molecular concentration gradient likely acts as a signal to induce germ layer differentiation.
e. Spemann and Hilde Mangold worked on the dorsal side of the embryo where the notochord and
the nervous system develop.
1) The presumptive notochord tissue induces the formation of the nervous system when placed
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beneath belly ectoderm.
2) Warren Lewis found that a developing lens induces the optic vesicle to form the optic cup in a
frog embryo.
7. Induction in Caenorhabditis elegans
a. Caenorhabditis elegans is a transparent worm, 1mm long and easily raised in Petri dishes or liquid
media.
b. It is hermaphroditic and self-fertile; therefore induced recessive mutations appear in the next
generation.
c. Its entire genome has been sequenced.
d. Worm development takes only three days and an adult worm contains only 959 cells.
e. The destiny of each cell can be followed in specialization and fate maps drawn.
f. The anchor cell receives the most inducers to become the inner vulva; neighboring cells receive
less and become the outer vulva.
g. Work with C. elegans shows that induction requires transcriptional regulation of genes in a
particular sequence.
B. Morphogenesis
1. Morphogenesis in Drosophila melanogaster (the fruit fly)
a. The first event in successful development is the establishment of the anterior/posterior axes.
b. In Drosophila eggs, there is a greater concentration of bicoid protein at one end.
1) Bicoid means “two-tailed”; a maternal mutation causes the egg to lack a head and it has two
tails.
2) By cloning the bicoid gene and using it as a probe, mRNA was found in a gradient from
anterior to posterior.
3) Proteins that influence morphogenesis are morphogens.
4) The bicoid gradient switches on the expression of segmentation genes; a gradient has a range
of effects.
c. The Segmentation Pattern
1) The bicoid gradient begins a cascade of segmentation genes.
2) By exposing flies to mutagens and then mapping mutated genes, they located segmentation
abnormalities.
3) The first genes activated are gap genes; if mutated, they cause missing blocks of segments.
4) The pair-rule genes ensure 14 segments; a mutation reduces this to half.
5) Segment-polarity genes cause each segment to have an anterior and posterior half.
6) Morphogen gradients turn on genes because they are transcription factors that regulate which
genes are active in which parts of the embryo in what order.
d. Homeotic Genes
1) Homeotic genes control pattern formation in animal morphogenesis.
2) In normal fruit fly development, homeotic genes are activated after the segmentation genes.
3) Homeotic genes have been found in many organisms; they all contain the same sequence of
nucleic acids called a homeobox.
4) Because homeotic genes contain a homeobox in mammals, they are called Hox genes.
5) Each homeobox has a homeodomain, a sequence of sixty amino acids.
6) A homeodomain protein produced by one homeotic gene binds to and turns on the next
homeotic gene, and this orderly process determines the overall pattern of the embryo.
7) Homeoboxes are derived from an original nucleic acid sequence that has been conserved
because of its importance in regulation of animal development.
e. Apoptosis
1) Apotosis (programmed cell death) is important in morphogenesis.
2) When a cell-death signal is received, an inhibiting protein becomes inactive, allowing a celldeath cascade to proceed.
42.3 Humans Embryonic and Fetal Development
1.
2.
Development covers events from conception (fertilization followed by implantation) to birth
(parturition).
a. The time of birth is calculated by adding 280 days to the start of last menstruation.
b. Only about 5% of babies arrive on the forecasted date due to so many variables.
Human development is divided into embryonic and fetal development.
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a.
The embryonic period, during months 1 and 2 of pregnancy, is when the major organs are
formed.
b. Fetal development is during months 3–9, during which organ systems are refined.
3. Development can also be divided into trimesters.
a. First trimester: embryonic and early fetal development occurs.
b. Second trimester: development of organs and organ systems; the fetus is distinctly human at the
end of the second trimester.
c. Third trimester: the fetus grows rapidly and the organ systems become functional.
4. Extraembryonic membranes
a. Evolution of extraembryonic membranes in reptiles made development on land possible.
1)
If an embryo develops in water, the water supplies the oxygen and takes away the wastes.
2)
The surrounding water prevents desiccation and provides a protective cushion.
3)
For an embryo on land, these functions are performed by extraembryonic membranes.
b. Chick extraembryonic membranes develop from extensions of germ layers, which spread over
yolk.
1)
The chorion lies next to the shell and carries on gas exchange.
2)
The amnion contains protective amniotic fluid that bathes a developing embryo.
3)
The allantois collects nitrogenous wastes.
4)
A yolk sac surrounds the remaining yolk that provides nourishment.
c. Humans also have these membranes; their function is modified for internal development.
1) The chorion develops into the fetal half of the placenta.
2) A yolk sac is the first site of blood cell formation.
3) Allantoic blood vessels become the umbilical blood vessels.
4) The amnion surrounds the embryo and cushions it with amniotic fluid.
d. Therefore, all chordate animals develop in water, either in bodies of water or within the
amniotic fluid.
A. Embryonic Development
1. The First Week
a. Fertilization occurs in the upper third of the oviduct; cleavage begins as the embryo moves down
this tube to the uterus.
b. By the time the embryo reaches the uterus on the third day, it is a morula.
c. By the fifth day, the morula is transformed into a blastocyst.
1) A blastocyst is a hollow ball of cells, resulting from cleavage.
2) The trophoblast is an outer single layer of cells, which later gives rise to the chorion.
3) The inner cell mass is the mass of cells from which the embryo, and eventually the fetus, will
develop.
2. The Second Week
a. At end of the first week, an embryo begins the process of implantation in the wall of the uterus.
b. The trophoblast secretes enzymes to digest away some of the tissue and blood vessels of the
uterine wall.
c. The trophoblast begins to secrete human chorionic gonadotropin causing the corpus luteum to
be maintained.
d. As the week progresses, the inner cell mass detaches itself from the trophoblast, and two more
extraembryonic membranes form: the yolk sac and amnion.
e. The yolk sac forms below the embryonic disk; with no nutritive function in humans, it is the site
of blood cell formation.
f. As in chick development, a human amnion and its cavity are where the embryo (and then the
fetus) develops.
g. In humans, amniotic fluid insulates against any thermal changes; it also cushions and protects the
fetus from trauma.
h. Gastrulation occurs during this week resulting in the inner cell mass flattening into an embryonic
disk.
1) The embryonic disk is composed of two cell layers: the ectoderm above and the endoderm
below.
2) Once an embryonic disk elongates to form the primitive streak (similar to that found in birds),
a third germ layer, the mesoderm, forms by invagination of the cells along the streak.
i. The trophoblast is reinforced by mesoderm and becomes the chorion.
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3.
The Third Week
a. The nervous system is the first organ system to become visually evident.
1) It appears as a thickening along the entire dorsal length of the embryo; invagination occurs as
the neural folds appear.
2) When the neural folds meet at the midline, the neural tube is formed.
3) After the notochord is replaced by the vertebral column, the nerve cord is called the spinal
cord.
b. The development of the heart begins in the third week and continues into the fourth.
1) The right and left heart tubes fuse; the heart begins pumping blood, although the chambers are
not fully formed.
2) The veins enter this largely tubular heart posteriorly, and the arteries exit anteriorly.
3) Later the heart twists so that all of the major vessels are located anteriorly.
4. The Fourth and Fifth Weeks
a. A bridge of mesoderm (the body stalk) connects the caudal (tail) end of the embryo with the
chorion, which has projections called chorionic villi.
b. The fourth extra embryonic membrane (the allantois) is contained in this stalk; its blood vessels
become the umbilical blood vessels.
c. The head and tail then lift up, and the body stalk moves anteriorly by constriction.
d. Once this process is complete, the umbilical cord is fully formed.
e. Limb buds appear from which the arms and legs will later develop.
f. The head enlarges and the sense organs become more prominent.
g. Rudiments of the eyes, ears, and nose are evident.
5. The Sixth Through Eighth Weeks
a. The developing human becomes more humanlike in appearance.
b. As the brain develops, the head achieves its normal relationship with the body as a neck region
develops.
c. The nervous system is developed well enough to permit reflex actions (e.g., the startle response to
touch).
d. At the eighth week, the embryo is about 38 mm long and weighs no more than an aspirin tablet; all
organs are established.
B. The Structure and Function of the Placenta
1. Providing gas, nutrient and waste exchange, the placenta begins formation once the embryo is fully
implanted.
2. Chorionic villi are treelike extensions of the chorion.
a. Chorionic villi project into the maternal tissues.
b. Later, the chorionic villi disappear in all areas except where the placenta develops.
3. By the tenth week, the placenta is fully formed and has already begun to produce progesterone and
estrogen.
a. Due to the negative feedback control by the hypothalamus and anterior pituitary, no new follicles
mature.
b. They maintain the lining of uterus and there is no menstruation during pregnancy.
4. The chorionic villi are surrounded by maternal blood sinuses; the maternal and fetal blood do not mix.
5. Exchange of molecules between the fetal and maternal blood takes place across the walls of the
chorionic villi.
6. CO2 and wastes move across from the fetus; O2 and nutrients flow from the maternal side.
7. The umbilical cord stretches between the placenta and the fetus.
8. Umbilical arteries transport CO2 and other waste molecules to the placenta for disposal; the umbilical
vein transports O2 and nutrient molecules from the placenta to the rest of the fetal circulatory system.
9. Harmful chemicals can cross the placenta.
a. This is of particular concern during the embryonic period, when various structures are first
forming.
b. Each organ has a sensitive period during which a substance can alter the normal development.
c. A pregnant woman who takes thalidomide tranquilizer between days 27 and 40 is likely to have an
infant born with deformed limbs; after this time period, the infant is born normal.
C. Fetal Development and Birth
1. Fetal development (months 3–9) involves an extreme increase in size; the weight multiplies 600 times.
2. The genitalia appear in the third month and gender can be identified anatomically.
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3.
4.
5.
A fetus soon acquires hair, eyebrows, eyelashes, and nails.
Fine, downy hair (lanugo) covers the limbs and trunk; it later disappears.
The skin grows so fast that it wrinkles; a waxy vernix caseosa protects the skin from the watery
amniotic fluid.
6. A fetus at first only flexes its limbs and nods its head; later it moves its limbs vigorously; a mother
feels movements from the fourth month onward.
7. After 16 weeks, a fetal heartbeat is heard through a stethoscope.
8. A fetus born at 24 weeks may survive; the lungs are still immature and often cannot capture O 2
adequately.
D. Stages of Birth
1. When the fetal brain matures, the hypothalamus causes the pituitary to stimulate the adrenal cortex so
that androgens are released.
2. The placenta uses androgens as precursors for estrogens that stimulate the production of prostaglandin
and oxytocin.
3. The hormones estrogen, prostaglandin, and oxytocin all cause the uterus to contract and expel the
fetus.
4. The process of birth (parturition) has three stages: dilation of the cervix, birth of the baby, and
expulsion of the placenta.
E. Preventing and Testing for Birth Defects (Health Focus box)
1. One way to prevent birth defects is by consuming a nutritious diet and avoid potentially harmful
substances, radiation, and pathogens.
a. For example, women of childbearing age are encouraged to consume adequate amounts of the
vitamin folic acid in order to prevent neural tube defects such as spina bifida and anencephaly in
their children.
b. The CDC recommends that women of childbearing age consume at least 400 micrograms of folic
acid every day through vitamin supplements and fortified foods.
2. To reduce the risk of fetal alcohol syndrome (FAS) it is strongly recommended not to consume alcohol
while pregnant.
a. It is also recommended not to smoke cigarettes or consume illegal drugs in order to reduce the risk
of birth defects.
3. Some medications can cause birth defects in newborns.
a. In the late 1950s and early 1960s, thalidomide was prescribed as a sleeping pill.
1) Thalidomide use resulted in thousands of children were born with flipper-like structures
instead of limbs.
2) Thalidomide was promptly taken off the market.
b. Between 1938–1971 DES (diethylstilbestrol) was prescribed to prevent premature labor and
miscarriage.
1) Daughters of women who took DES while pregnant have an increased risk of vaginal and
cervical cancer.
2) Sons of women who took DES while pregnant have an increased frequency of noncancerous
cysts of the epididymis.
4. Viruses such as rubella or HIV cause infections in newborns; most newborns die.
5. X-rays can hinder cell division and damage fetal DNA.
a. If an X-ray is unavoidable, notify the X-ray technician so that the fetus can be protected as much
as possible.
Lecture Enrichment Ideas
Experience Base: Developmental biology is also rapidly expanding, perhaps second only to new
breakthroughs in understanding the brain and nervous system. Much of this chapter will be
critical to understanding future controversies concerning the use of fetal tissues, definition of
when personhood begins (not a science question, but the correct statement of the questions
“When does life begin?”). However, childbirth remains hidden from most students’ experiences
and the complexities of developmental biology research likewise will require using media.
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1. Describe how cellular differentiation occurs as different genes are turned on and off, and how
different proteins and enzymatic products are produced in the different types of cells; review
the differences in various kinds of tissues and discuss how dissimilar cells could come from a
single zygote.
2. Illustrate how the cleavage process can go through many rounds of cell division with no cell
growth, starting with the egg as the largest single cell. Describe how some differentiation
may begin with the first cell division.
3. Examine why you could consider the yolk of an egg to be a single cell but why that really is
not a reasonable conclusion, since the majority of the yolk is simply nutrient material used by
the cellular part of the yolk that goes through cell division to form an embryonic plate that
will develop into a chick. Non-rural students usually wrongly envision a chicken egg as one
cell, assuming it develops by cleavage of the whole egg under the shell; most of the egg is
stored food, and a fertilized egg already has 60,000 cells developing on the surface of the
yolk when the egg is finally laid. Bring in a fertilized egg, or use illustrations, to show the
microscopic development on the surface of the yolk.
4. Emphasize how a chick embryo has requirements other than the nutrients in the yolk and
how they are dealt with—such as oxygen requirements (enters the shell) and removal of
wastes (accumulate as uric acid in one of the extraembryonic membranes and are discarded
with the eggshell).
5. Discuss how amniotic fluid is used to examine a fetus for genetic defects, since cells of the
fetus are shed from the skin and from the respiratory and urinary tracts into the fluid. Also
discuss how chorionic villi can be sampled in prenatal diagnosis.
6.
It is true that a person inherits all of his or her mitochondria from his or her mother. It was originally thought
that the process of fertilization excludes the mitochondria by leaving the midpiece and tail of the sperm outside
the egg membrane. We now know that the midpiece does enter the egg but the few (approximately 100)
mitochondria are worn out and tagged with ubiquitin for destruction. The much larger numbers of mitochondria
from the egg provide all the cellular respiration for the zygote and subsequent cells.
7. Because variations are not mentioned, it is often assumed that all males in the animal
kingdom produce sperm with tails. However, some sperm lack tails, as is the case with the
roundworm Ascaris, and its amoeboid sperm carries in the mitochondria during fertilization.
8. Students may also ask in this section how identical and fraternal twins develop. They may
also be interested in learning that the rate of twinning varies from high in tropical countries
with short life expectancies (over evolutionary time) to low in Asia and northern Europe with
long life expectancies.
9. Developmental biology is a complex topic requiring understanding of rather abstract
experimental regimes, knowledge of cell membranes and communication chemistry, and
experience seeing variability in complex embryonic systems.
10. Note the critical role experimental animals played, both historically and currently, in
developmental biology. This includes amphibian eggs for their transparent development, fruit
flies for the genetic connection and manipulation, and the roundworm Caenorhabditis
elegans for its discrete cell number and full genome just described.
11. The homeobox concept is not simple; graphics aid in visualizing the sequences involved in
pattern development and how they were preserved across long periods of evolution.
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12. Getting from one fertilized egg cell to the many trillions of cells in an adult may seem
impossible, especially if combined with the fact that most cells divide only 50 times!
However, cell division is exponential growth at the power of two, and a brief episode of
doubling will achieve very high numbers in a short time.
13. There are exceptions to continual replication of cells. Nerve and muscle cells do not continue
many replications after birth. Stem cells in bone marrow, skin, and intestine do continue to
replicate more than other cells. And the “germ line” leading through the sperm and egg of
each individual who reproduces is part of an “immortal” cell lineage. It is a peculiar but
correct biological perspective to consider individual organisms as temporary “support units”
nurturing an immortal germ line. This is also related to the nonscientific question posed of
“when does human life begin?” Should a student pose this question in class, you can refer
back to Virchow’s proposal that all cells come from pre-existing cells, and the work of
Pasteur relative to spontaneous generation, which is the biologically definitive answer since
the cell lines of advanced organisms continue through sperm and egg, always continuous,
always living. The question of when, during development, “personhood” is assigned is a
sociological and not a biological question. Students may be aware of the various policies the
U.S. has had regarding funding of human fetal research; their attitudes will influence future
policies.
14. There is a general wrong impression among people that all of the fertilized egg becomes the
baby. Experiments with mammal eggs (mouse) indicate that only three of the cells in a 64cell stage embryo become the fetus while 61 become the embryo’s contribution to the
placenta. Lecture question: Why is such a lopsided distribution made so early? The need for
the placenta is immediate although short-lived compared to the fetus.
15. On very rare occasions, there is evidence that one person is the result of two fused independently fertilized
embryos. The embryos would normally have implanted separately and become fraternal twins, but when the cell
masses are too close, they fuse before implanting, and develop into one fetus. The individual is born normal but
is a mosaic of cells, some regions expressing features from one zygote, some from the other. The fact that they
develop into one normal person reflects the ability of the cells to communicate with each other as they develop.
16. For demonstration in the classroom, secure a placenta from a local hospital. Fetal stages can be observed from
preserved pigs and chickens. The pig uterus is dramatically different in its design for nurturing a litter.
Critical Thinking
Question 1.
It is possible to intermingle the embryos (at the 8–64 cell stage) of three mice; they cluster and
become one mouse (called a “chimera”) with different parts showing the features of its six different parents! What
does this indicate relative to the independence of one individual cell as the smallest unit of life?
Answer: If each fertilized egg remained an independent set of cells after it divided, they could not respond to each
other to develop into one mixed organism. This chimera shows that the embryos were chemically sensitive to the
presence of other cells and began developing in an organized manner responding to each other.
Question 2.
In Eastern Europe, a mother with type O blood gave birth to her biological child with type AB
blood. Since this appears impossible, a check of her parents revealed that she was a chimera, a fusion of two
different sibling embryos that then developed as one individual but with two tissue types. If her husband had type B
blood, what are her two tissue types and how does the science of embryology now place a caveat on the
mathematical pronouncements of genetics in paternity cases?
Answer: Her blood stem cell line developed from the type O zygote, while her reproductive organs developed from
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the type A sibling zygote; she is a mosaic of these two. Since a chimera is a very remote possibility, but nevertheless
possible, it becomes more difficult to determine paternity by genetics without also examining the grandparents.
Question 3. When researchers use various pure-bred strains of mice (brown, white, black, etc.)
to form
chimeral mice with many parents, they cannot get more than three features to occur in the
offspring mouse when they mix embryos at the 64-cell stage. After many attempts, they have
concluded that only three cells of this 64-cell stage become the fetal mouse and the other 61
become placenta. Why would the distribution of mouse-versus-placenta cells not be 50-50 or
even less for the placenta?
Answer: The placenta has a very critical role to play and must develop early; by the time of birth, its job is done.
The placenta must therefore develop more rapidly and committing 61 or 64 cells is how the mammal embryo gets
this head start.
Question 4. What triggers breast milk production and why couldn’t it just be tied to begin at
the end of a 9-month pregnancy cycle?
Answer: High levels of estrogen and progesterone during pregnancy suppress the anterior pituitary production of
prolactin. After birth occurs, the drop in these hormones allows the pituitary to produce prolactin and this triggers
milk production. If milk production was merely triggered by a clock mechanism at the end of a 9-month pregnancy,
there would be no milk produced for a premature infant or too early production for a long-term pregnancy; this
feedback-hormone system synchronizes birth and milk production.
Technology from McGraw-Hill
Please consult your Course Integration Guide for technology correlations for this chapter.
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PART
VIII
BEHAVIOR AND ECOLOGY
Behavior precedes ecology because behavior underpins many of the interactions of organisms within ecosystems.
The behavior chapter has an experimental approach that provides meaningful examples. The ecology chapters
address traditional ecology and environmental concerns. Some instructors begin the biology course with this topic.
43 Animal Behavior
44
45
46
47
Population Ecology
Community and Ecosystem Ecology
Major Ecosystems of the Biosphere
Conservation of Biodiversity
CHAPTER
43
BEHAVIORAL ECOLOGY
The genetic basis of behavior is discussed in the context of development of behavioral patterns, environmental
influences, and evolutionary adaptiveness. Topics discussed include “nature vs. nurture,” conditioning, adaptive
mating behaviors, sociobiology, and animal communication. Two Science Focus boxes explore “Do Animals Have
Emotions,” and “Sexual Selection in Male Bowerbirds.”
Chapter Outline
43.1 Inheritance Influences Behavior
1.
2.
Behavior is any action that can be observed and described.
The nature versus nurture question asks to what extent both our genes (nature) and environmental
influences (nurture) affect behavior.
A. Experiments That Suggest Behavior Has a Genetic Basis
1. Several species of lovebirds differ in the way the build nests.
2. Hypothesis: if the behavior for obtaining and carrying nesting material is inherited, then hybrids might
show intermediate behavior.
a. When two species of lovebirds were mated, the hybrid birds had difficulty carrying nesting
material.
b. These studies supported the hypothesis that behavior has a genetic basis.
3. Experiments with the garter snake (Thamnophis elegans) were conducted to
determine if food preference have a genetic basis.
a.
b.
c.
d.
4.
Inland populations are more aquatic and feed on frogs and fish; they refused to feed on slugs.
Coastal populations are more terrestrial and feed on slugs.
The hybrid newborn garter snakes have an intermediate acceptance of slugs.
Work with smell receptors and tongue flicks showed that physiological differences underlie the
behavior.
Experiments with Humans
a. Twin studies in the humans have been used to probe the nature-versus-nurture question.
b. The studies with twins support the hypothesis that at least certain types of behavior are primarily
influenced by nature.
B. Experiments That Show Behavior Has a Genetic Basis
1.
Behavior of the marine snail Aplysia shows endocrine involvement in behavior.
a. Following copulation, the slug extrudes long strings of eggs and uses its head movements to attach
the eggs to rocks.
b. Scientists isolated an egg-laying hormone (ELH) that causes the animal to lay eggs even if it has
not mated.
c. ELH is a small protein of 36 amino acids that excites the reproductive tract and causes egg
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2.
43.2
expulsion.
d. Recombinant DNA studies isolated the ELH gene and showed it controls the egg-laying behavior.
Maternal behavior in mice activated by gene fosB.
a. When mothers first inspect their newborn, various sensory information travel to the hypothalamus.
b. The sensory information causes fosB alleles to be activated and product a protein.
c. This protein changes the neural circuitry within the hypothalamus, which manifests itself in good
behavior.
d. Mice that lack good maternal behavior also lack fosB alleles.
The Environment Influences Behavior
1.
2.
3.
Fixed action patterns (FAPs) were believed to be behaviors that were always performed the same
way, and they were elicited by a sign stimulus.
Many behaviors formerly thought to be fixed action patterns are found to have developed after
practice.
Learning is defined as a durable change in behavior brought about by experience.
4. Deer grazing on the side of a busy highway, oblivious to traffic, is an example of
habituation.
A. Instinct and Learning
1. Laughing gull chicks beg food from parents by pecking at the parents’ beaks;
however, their accuracy improves with practice.
2. The chicks first peck at any beak model; later they only peck at models resembling
the parents.
3. This interaction between chicks and parents appears to be a FAP.
B. Imprinting
1. Imprinting, another form of learning, involves a sensitive period.
a. Chicks, ducklings, and goslings follow the first moving object they see after
hatching (usually their mother).
b. A sensitive period is the only period during which a particular behavior such as
imprinting, develops.
C. Social Interactions and Learning
1. Song Learning
a. Song learning in birds involves a sensitive period when an animal is primed to learn; songs heard
outside this period have no effect.
b. When provided with an adult tutor, birds learned other species’ songs, indicating that social
interactions assist in learning.
D. Associative Learning
1. A change in behavior that involves an association between two events is termed associative learning.
2. Classical Conditioning
a. In classical conditioning, the paired simultaneous presentation of two different types of stimuli
causes an animal to form an association between them.
b. This suggests that an organism can be trained (conditioned) to associate any response with any
stimulus.
c. Unconditioned responses are those that occur naturally; conditioned responses are those that are
learned.
3. Operant conditioning
a. In operant conditioning, a stimulus-response connection is strengthened.
b. This resulted from reinforcing a particular behavior.
c. F. Skinner was famous for his studies in operant conditioning, always rewarding animals for the
proper response.
E. Orientation and Migratory Behavior
1. Migration is long-distance travel from one location to another.
a. Loggerhead sea turtles hatch on a Florida beach and then travel across the Atlantic Ocean to the
Mediterranean.
b. Monarch butterflies fly from North America to Mexico so they can continue breeding.
2. Migration requires orientation, which is the ability to travel in a particular direction, such as south in
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F.
the winter and north in the spring.
a. Many birds use the sun during the day or the stars at night to orientate themselves.
3. Experienced birds can navigate, or be able to change their direction in response to other environmental
clues that tell them they are currently headed in the wrong direction.
Cognitive Learning
Animals also learn through imitation and insight.
Insight learning occurs when an animal suddenly solves a problem without any prior
experience with the problem.
G. Do Animals Have Emotions? (Science Focus box)
1. Scientists believe they have sufficient data to suggest that at least other vertebrates
and/or mammals have feelings, including fear, joy, embarrassment, jealousy, anger,
love, sadness, and grief.
2. Iguanas tend to stay where it is warm.
a. Warmth rises the iguanas’ body temperature and increase in heart rate.
b. These biological responses are those associated with emotions in humans.
3. Researchers have found a high level of dopamine in the brain when rats play, and the
dopamine level increases even when rats anticipate the opportunity to play.
4. Studying animal emotions may best be left to field research; laboratory animals may
be too stressed to provide convincing data on emotions.
43.3 Animal Communication
1. Some animals are largely solitary and join with a member of the opposite sex only to reproduce.
2. Others pair, bond, and cooperate in raising offspring.
3. Society members are organized in a cooperative manner extending beyond sexual or parental behavior.
A. Communicative Behavior
1. Communication is an action by a sender that influences the behavior of a receiver.
2. When the sender and receiver are members of the same species, signals will benefit both the sender
and the receiver.
3. Chemical Communication
a. These signals are chemicals (e.g., pheromones, urine, and feces) and have the advantage of
working both night and day.
b. A pheromone is a chemical released to cause a predictable reaction of another member of the
same species.
c. Female moths attract males with tail gland pheromones; cats mark territory with urine, etc., and
antelope mark twigs with eye gland secretions.
4. Auditory Communication
a. Auditory (sound) communication has advantages.
1) It is faster than chemical communication.
2) It is effective both night and day.
3) It can be modified by loudness, pattern, duration, and repetition.
b. Male crickets have calls for reproduction.
c. Birds have various songs for distress, courting, and marking territories.
d. Whale songs have six basic themes for sexual and group identification.
e. Only humans can produce many different sounds and assemble them in many different ways.
f. Nonhuman primates are limited to about 40 distinct vocalizations with limited meaning.
g. Chimpanzees using artificial language cannot advance beyond the level of a 2-year-old child.
h. Chimps appear incapable of using language to reason or of using grammar.
5. Visual Communication
a. Visual signals are most often used by species that are active during the day.
b. Contests between males make use of threat postures and may prevent fighting.
c. Defense and courtship displays are exaggerated and always performed in the same way so the
meaning is clear.
d. Visual communication allows animals to signal others of their intentions without the need to
provide any auditory or chemical messages.
6. Tactile Communication
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a. Tactile communication occurs when one animal touches another.
b. Gull chicks peck at the parent’s beak in order to induce the parent to feed them.
c. A male leopard nuzzles the female’s neck to calm her and to stimulate her
willingness to mate.
d. Honeybees use tactile communication to impart information about the
environment.
43.4 Behaviors That Increase Fitness
1. Behavioral Ecology assumes that behavior is subject to natural selection.
A. Territoriality
1. A territory is an animal’s home range where they can be found during the course of the day, and will
defend for their exclusive use.
2. Territoriality involves the type of behavior needed to defend a particular territory.
3. Gibbons maintain territories by loud singing.
4. Defending territories exerts a lot of energy; the defender may also be wounded.
5.
The optimal foraging model states that it is adaptive for foraging behavior to be as energetically
efficient as possible.
a. For example, shore crabs eat intermediate-sized mussels because the net energy gain was more
than if they ate larger-sized mussel
B. Reproductive Strategies
1. Some animals, such as gibbons, are monogamous; they pair bond, and both male and female help with
the rearing of the young.
2. Most other primates are polygamous; males monopolize multiple females.
3. A limited number of primates are polyanthrus.
a. Tamarins live together in groups of one or more families in which one female mates with more
than one male.
4. Sexual selection refers to adaptive changes in males and females that lead to an increased ability to
secure a mate.
3) In males, this may result in an increased ability to compete with other males for a mate.
4) Females may select a mate with the best fitness (ability to produce surviving offspring), thereby
increasing her own fitness.
C. Sexual Selection in Male Bowerbirds (Science Focus box)
1. Male bowerbirds create elaborate nests (or bower) to attract female bowerbirds during mating season.
2. When a female bowerbird approaches the male bowebird’s bower, the male begins his courtship
display of fluffing his feathers, and flapping his wings to the beat of a call.
3. If the female enters the bower and crouches, the two bowerbirds mate.
4. The male bowerbirds’ displays are highly intense and aggressive and it scares off some females.
5. A study in Australia looked at the ability of male bowerbirds responding to female bowerbird signals
by giving their highest-intensity courtship displays for females who crouched the fastest and are least
likely to be startled.
6. The study found that males who modulate their displays more effectively in response to female signals
are more successful in courting females.
7. Male responsiveness to female signals may be an important part of successful courtship in many
species.
D. Societies
1.
2.
3.
4.
The principles of evolutionary biology can be applied to the study of social behavior in animals.
It is based on a reproductive cost-benefit analysis of the value of living in a society.
There are both benefits and costs to living in a social group.
Only if the benefits, in terms of reproductive success, outweigh the disadvantages will societies evolve.
a. Advantages to living in a social group might include help to avoid predators, to raise young, and to
find food.
1) A group of impalas has more eyes to see approaching predators, etc.
2) Many fish moving rapidly in a school can distract a predator.
3) The trumpet manucode (a bird) pair bonds for life; both sexes are needed to raise the young.
4) Weaver birds form giant colonies to protect them from predators.
5) Primate members signal the group when they find a bountiful fruit tree.
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6) Lions working together can capture larger prey, such as a zebra or buffalo.
There are also disadvantages to living in a social group.
1) Disagreements occur between members over the best feeding places and resting sites.
2) Among red deer, subordinate females are at a disadvantage in producing more prolific sons.
3) Primate grooming may be necessary to keep them healthy since parasites spread easily in
groups.
E. Altruism Versus Self-Interest
b.
1. Altruism is defined as behavior that has the potential to decrease the lifetime
reproductive success of the altruist, while benefitting the reproductive success of
another member of the society.
2. Fitness, judged by reproductive success, may explain altruistic behavior.
3. Genes can be passed directly from parents to offspring.
4. Genes are also passed by relatives; an individual helping a relative survive and
reproduce passes on the individual’s genes.
5. Direct selection is adaptation to the environment due to the reproductive success of
an individual.
6. Indirect selection, also called kin selection, involves adaptation to the environment
due to the reproductive success of an individual’s relatives.
7. Thus, the inclusive fitness of an individual includes personal reproductive success
plus that of its close relatives.
8.
In social insects, altruism is extreme and is explained on the basis that it helps reproducing siblings
survive.
a. Only the queen among an army ant colony reproduces; the three castes of ants
have given up reproducing.
b. In social bees and wasps, the queen is diploid but the drone is haploid; therefore
sterile female workers are 75% related to their nest mates but would be only 50%
related to their own offspring if they reproduced; thus, it is an advantage to care
for the queen and her offspring.
9. Male chimpanzees in Africa did not interfere with each other’s matings because they shared genes.
10. Reciprocal Altruism
a. In some species, bird offspring from one clutch of eggs may stay at the nest to help parents rear
and feed the next batch of offspring.
b. For Florida scrub jays, the number of fledglings produced by an adult pair doubled when they had
helpers.
c. Mammal offspring also help their parents; African jackals raise 1.4 pups alone; with helpers they
raise 3.6.
d. The above are examples of reciprocal altruism: an animal helps or cooperates
with another animal with no immediate benefit, but the animal that was helped
will repay the debt at a later time.
Lecture Enrichment Ideas
Experience Base: Students today generally have reduced experiences personally viewing the
behavior of outdoor wildlife; observations of pets, domestic, and zoo animals are unlikely to
provide meaning to the many concepts covered in this chapter. Still photos likewise provide a
limited understanding of the classic concepts described here.
1. Discuss why behavior can be assumed to be adaptive and evolving, as exemplified by the
behavior of black-crowned gulls in removing eggshells or by the nest-building behavior of
wood-hoopoes.
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2. Darwin was unexpectedly ahead of his time in extending his ideas on natural selection from
morphology to behavior. Today, we take behavior as an evolutionary phenomenon as logical,
but reading from his early work can give a sense of his insightfulness.
3. Discuss how imprinting behavior would be useful in both offspring and parents in species
such as ducks and geese, where there are numerous offspring that are mobile upon hatching.
Should we expect that kind of imprinting in a herd mammal (such as a wild horse or
antelope) or a human?
4. Discuss why it would be advantageous to a female mouse to abort a pregnancy when her
mate in the pregnancy is displaced by a new male.
5. Examples of other work with Aplysia may give students an understanding of why this snail is
useful as a “lab rat” for research work that tries to associate behavior with nerve physiology.
While it is possible to explain the properties of a building by examining its bricks, it is not
always possible to predict the possible emergent properties by studying a brick alone—this is
the problem with predicting behaviors from studying nerves and neurons alone.
6. Lecture question: A cuckoo lays an egg in another bird’s nest; its larger offspring present a
larger gape and are thus fed more food by the adoptive parents. Why can’t the host birds
evolve a defense against this cuckoo chick behavior?
7. Darwin is often stereotyped as merely the author of the natural selection mechanism of
evolution. However, sexual selection is a concept originating with Charles Darwin (and he
wrote books on the earthworm’s role in soil formation and emotions and facial expressions).
Discussions on reproductive strategies and parental investment may interest some students in
further readings in sociobiology.
8. Note that Darwin’s theory of natural selection is often reduced incorrectly to “survival of the
fittest” in an image of a dog-eat-dog world, all competition and no cooperation. It is more
correctly generalized as reproduction of the fittest, and that allows more room for
cooperation and sociality if the behavior improves survival of genes common to those who
“sacrifice” or cooperate.
9. If a zoo is located nearby, take a field trip and determine the various forms of communication
and behavior among different animals.
10. Discuss the difference between genuine altruism in humans, where individuals have gone to
war and died on behalf of unrelated compatriots, versus animal “altruism” that appears to
always be tied to securing some advantage for the individual’s genes in “calculated
selfishness.”
11. Ask students to relate to whom they have helped in the past: hungry relatives or strangers.
Most human societies are set up so we give favor to near relatives. Why might this be
genetically selected?
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12. Discuss the controversies inherent in sociobiology, the theory that human behavior can be
explained based on physiology and evolution. Some scientists are reluctant to consider that
antisocial behavior may be an inborn part of human nature. Asking for opinions will
probably generate a lengthy discussion.
13. Discu