CHAPTER 4
A Tour of the Cell
1. Cells are the smallest entity that exhibits all the characteristics of life.
2. Cells are as fundamental to biology as atoms are to chemistry.
3. An understanding of cell structure and function is essential to understanding most
human disease.
Biology and Society: Drugs That Target Bacterial Cells
1. Explain how antibiotics specifically target bacteria while minimally harming the human
host.
The Microscopic World of Cells
2. Compare the following pairs of terms, noting the most significant differences: light
microscopes versus electron microscopes, scanning electron microscopes versus
transmission electron microscopes, magnification versus resolution, prokaryotic cells
versus eukaryotic cells, plant cells versus animal cells.
Membrane Structure
3. Describe the structure of the plasma membrane and other membranes of the cell.
Explain why this structure is called a fluid mosaic.
4. Explain how MRSA bacteria disable human immune cells.
5. Compare the structures and functions of a plant cell wall and the extracellular matrix of
an animal cell.
The Nucleus and Ribosomes: Genetic Control of the Cell
6. Explain how the genetic information in the nucleus is used to direct the production of
proteins in the cytoplasm.
The Endomembrane System: Manufacturing and Distributing Cellular
Products
7. Compare the structures and functions of the following components of the
endomembrane system: rough endoplasmic reticulum, smooth endoplasmic reticulum,
Golgi apparatus, lysosomes, and vacuoles.
Chloroplasts and Mitochondria: Energy Conversion
8. Compare the structure and function of chloroplasts and mitochondria. Describe the
adaptive advantages of extensive folds in the grana of chloroplasts and the inner
membrane of mitochondria.
The Cytoskeleton: Cell Shape and Movement
9. Describe the functions of the cytoskeleton. Compare the structures and functions of cilia
and flagella.
Evolution Connection: The Evolution of Antibiotic Resistance
10. Explain how and why antibiotic-resistant bacteria have evolved.
Key Terms
cell junctions
cell theory
central vacuole
chloroplast
chromatin
chromosome
cilia
cristae
cytoplasm
cytoskeleton
cytosol
electron microscope (EM)
endomembrane system
endoplasmic reticulum (ER)
eukaryotic cell
extracellular matrix
flagella
fluid mosaic
food vacuoles
gene
Golgi apparatus
grana
light microscope (LM)
lysosomal storage disease
lysosome
magnification
microtubule
mitochondria
nuclear envelope
nucleolus
nucleus
organelles
phospholipid
phospholipid bilayer
plasma membrane
prokaryotic cell
resolving power
ribosome
rough ER
scanning electron microscope (SEM)
smooth ER
stroma
transmission electron microscope (TEM)
transport vesicles
vacuole
Word Roots
chloro = green (chloroplast: the green organelle of photosynthesis)
chromo = color (chromosome: a threadlike, darkly staining structure packaging DNA in the
nucleus)
cili = small hair (cilium: a short, hair like cellular appendage with a microtubule core)
cyto = cell (cytoplasm: cell region between the nucleus and the plasma membrane)
endo = inner (endomembrane system: an internal system of membranous organelles)
eu = true (eukaryotic: cell type with a membrane-enclosed nucleus and other organelles)
extra = outside (extracellular: the substance around animal cells)
flagell = whip (flagellum: a long, whiplike cellular appendage that moves cells)
micro = small (microtubule: microscopic tubular filaments contributing to the cytoskeleton)
plasm = molded (plasma membrane: the thin layer that sets a cell apart from its
surroundings)
pro = before (prokaryotic: the first cells, lacking a membrane-enclosed nucleus and other
organelles)
reticul = network (endoplasmic reticulum: membranous network where proteins are
produced)
trans = across (transport vesicles: membranous spheres that move materials across a cell)
vacu = empty (vacuole: sac that buds from the ER, Golgi apparatus, or plasma membrane)
Student Media
Activities
Metric System Review
Prokaryotic Cell Structure and Function
Comparing Cells
Build an Animal Cell and a Plant Cell
Membrane Structure
Role of the Nucleus and Ribosomes in Protein Synthesis
The Endomembrane System
Build a Chloroplast and a Mitochondrion
Cilia and Flagella
Review: Animal Cell Structure and Function
Review: Plant Cell Structure and Function
BioFlix
Tour of an Animal Cell
Tour of a Plant Cell
BLAST Animations
Animal Cell Overview
Plant Cell Overview
Vesicle Transport along Microtubules
Vacuole
Mitochondrion
Plant Cell Wall
MP3 Tutors
Cell Organelles
Process of Science
What Is the Size and Scale of Our World?
Videos
Discovery Channel Video: Cells
Prokaryotic Flagella
Euglena
Cytoplasmic Streaming
Chlamydomonas
Paramecium Cilia
Paramecium Vacuole
Relevant Current Issues in Biology Articles
Current Issues in Biology, volume 6 (ISBN 0-321-59849-0)
Your Cells Are My Cells
Relevant Songs to Play in Class
“Tainted Love,” Soft Cell
“Golgi Apparatus,” Phish
Chapter Guide to Teaching Resources
The Microscopic World of Cells
Student Misconceptions and Concerns
1. Students typically cannot distinguish between resolution and magnification. However,
pixels and resolution of digital images can help “clarify” the distinction. Consider printing
the same image at high and low resolution or enlarging the same image at two different
levels of resolution. Teaching Tip 2 below suggests another related exercise.
2. Students frequently equate the functions of mitochondria and chloroplasts as alternative
ways to acquire usable energy. This often leads to the conclusion that animal cells have
mitochondria but not chloroplasts and that plant cells have chloroplasts but not
mitochondria. Plant cells have both.
Teaching Tips
1. Here is a chance to challenge students to identify technology that has extended our
senses. Chemical probes can identify what we cannot taste, listening devices detect what we
do not normally hear, night vision and ultraviolet (UV) cameras detect or magnify
wavelengths beyond our vision, and so on. Students could be assigned the task of preparing
a short report on one of these technologies.
2.Here is a way to demonstrate resolving power in the classroom. Use a marker and your
classroom marker board to make several pairs of dots separated by shorter and shorter
distances. Start out with two dots clearly separated apart—perhaps by 4–5 cm—and end
with a pair of dots that touch. Label them a, b, c, and so on. Ask your students to indicate
the letters of the pairs of points that they can distinguish as separate; this is the definition of
resolution for their eyes. (They need not state their answers publicly, to avoid
embarrassment.)
3.Most biology laboratories have two types of microscopes for student use: a dissection (or
stereo-) microscope and a “compound” light microscope using microscope slides. The way
these scopes function parallels the workings of EMs. Dissection microscopes are like an
SEM—both rely on a beam reflected off a surface. As you explain this to your class, hold up
an object, identify a light source in the room, and explain that our eyes see most images
when our eyes detect light that has reflected off the surface of an object. Compound light
microscopes are like TEMs, in which a beam is transmitted through a thin sheet of material.
If you have an overhead or other strong light source, hold up a piece of paper between your
eye and the light source. You will see the internal detail of the paper as light is transmitted
through the paper to your eye—the way a compound light microscope or TEM works!
4.Even in college, students still struggle with the metric system. When discussing the scale
of life, consider bringing a meterstick to class. The relationship between a meter and a
millimeter is the same as a millimeter is to a micrometer. Each is a difference of 1,000.
5.This is a place where a visual image comparing a prokaryotic and eukaryotic cell can be
very helpful. These cells are strikingly different in size and composition. A visual reference
point instead of just abstract ideas and traits will be a continual reminder for your students
during your discussion of these cells.
6.Students might wrongly conclude that prokaryotes are typically one-tenth the volume of
eukaryotic cells. A difference in diameter by a factor of ten translates into a much greater
difference in volume. Students might be challenged to recall enough geometry to calculate
the difference in the volume of two cells with diameters that differ by a factor of ten.
7.Germs—here is a term that we learn early in our lives but that is rarely well defined.
Students may appreciate a biological explanation. The general use of germs is a reference to
anything that causes disease. This may be a good time to sort the major disease-causing
agents into three categories: (1) bacteria (prokaryotes), (2) viruses (not yet addressed), and
(3) single-celled and multicellular eukaryotes (athlete’s foot is a fungal infection; malaria is
caused by a unicellular eukaryote).
8.Some instructors have reported great success by challenging their students to make
analogies to the functions of the many organelles discussed. Students may wish to construct
one inclusive analogy between a society or factory (used in the text) and a cell or to
construct separate analogies for each organelle. As with any analogy, it is important to list
the similarities and differences/exceptions.
9. This might be a good time to discuss the evolution of antibiotic resistance. Teaching tips
and ideas for related lessons can be found at
www.pbs.org/wgbh/evolution/educators/lessons/lesson6/act1.html.
Membrane Structure
Student Misconceptions and Concerns
1.Students often think of the function of cell membranes as mainly containment, like that of
a plastic bag. Consider relating the functions of membranes to our human skin. (For
example, both membranes and our skin detect stimuli, engage in gas exchange, and serve as
sites of excretion and absorption.)
Teaching Tips
1. The hydrophobic and hydrophilic ends of a phospholipid molecule naturally create a
lipid bilayer. The hydrophobic edges of the layer will seal to other such edges, eventually
wrapping a sheet into a sphere that can enclose water (a simple cell). Further, because of
these hydrophobic properties, lipid bilayers are naturally self-healing. That all of this
organization naturally emerges from the properties of phospholipids is worth sharing with
your students.
2. You might wish to share a very simple analogy that seems to work with some students.
A cell membrane is a little like a peanut butter and jelly sandwich with jellybeans poked
into it. The bread represents the hydrophilic portions of the bilayer (and bread does indeed
quickly absorb water). The peanut butter and jelly represent the hydrophobic regions (and
peanut butter, containing plenty of oil, is generally hydrophobic). The jellybeans stuck into
the sandwich represent proteins variously embedded partially into or completely through the
membrane. Transport proteins would be like the jellybeans that poke completely through the
sandwich. Analogies are rarely perfect. Challenge your students to critique this analogy to
find exceptions. (For example, this analogy does not include a model of the carbohydrates
on the cell surface.)
The Nucleus and Ribosomes: Genetic Control of the Cell
Student Misconceptions and Concerns
1. Noting the main flow of genetic information from DNA to RNA to protein on the board
will provide a useful reference for students when explaining these processes. As a review,
have students note where new molecules of DNA, RNA, and proteins are produced in a cell.
2. Consider challenging your students to explain how we can have four main types of
organic molecules functioning in specific roles in our cells, yet DNA and RNA only
specifically dictate the generation of proteins (and more copies of DNA and RNA). How is
the production of specific types of carbohydrates and lipids in cells controlled? (Answer:
primarily by the specific properties of enzymes.)
Teaching Tips
1. Some of your more knowledgeable students may like to guess the exceptions to 46
chromosomes per human cell. These exceptions include gametes, some of the cells that
produce them, and red blood cells in non-fetal mammals.
2.If you wish to continue the text’s factory analogy, nuclear pores might be said to function
most like the doors to the boss’s office.
3. If you want to challenge your students further, ask them to consider the adaptive
advantage of using mRNA to direct the production of proteins instead of using DNA
directly. Some biologists suggest that DNA is better protected in the nucleus and that
mRNA, exposed to more damaging cross-reactions in the cytosol, is the temporary
“working copy” of the genetic material. In some ways, this is similar to making a working
photocopy of an important document, keeping the original copy safely stored away.
The Endomembrane System: Manufacturing and Distributing
Cellular Products
Student Misconceptions and Concerns
1. Students might have trouble connecting the diverse functions to the organelles. The
pathway of secretory proteins is a good process to use to introduce the primary organelle
functions. The movement of information and products extends generally from the central
nucleus to the interconnected rough ER, to the more peripherally located Golgi, and finally
to the outer plasma membrane. Introducing the steps of this process with the central to
peripheral flow may help students better see the interrelationships and recall the sequence.
2. Conceptually, some students seem to benefit from the well-developed factory-like-a-cell
analogy developed in the text. The use of this analogy in lecture might help to anchor these
relationships.
Teaching Tips
1. The endoplasmic reticulum is continuous with the outer nuclear membrane. This
explains why the ER is usually found close to the nucleus.
2.Some people think the Golgi apparatus looks like a stack of pita bread.
3. If you continue the factory analogy, the addition of a molecular tag is like adding
address labels in the shipping department of a factory.
Chloroplasts and Mitochondria: Energy Conversion
Student Misconceptions and Concerns
1. Students often mistakenly think that chloroplasts are a substitute for mitochondria in
plant cells. They may falsely think that cells either have mitochondria or they have
chloroplasts. You might wish to emphasize the presence and significance of mitochondria
and chloroplasts in plant cells.
2. The evidence that mitochondria and chloroplasts evolved from free-living prokaryotes is
further supported by the small prokaryote size of these organelles. Mitochondria and
chloroplasts are therefore helpful in comparing the general size of eukaryotic and
prokaryotic cells.
Teaching Tips
1. ATP functions in cells much like money functions in modern societies. Each holds
value that can be generated in one place and “spent” in another. This analogy has been very
helpful for many students.
2.Mitochondria and chloroplasts are each wrapped by multiple membranes. In both
organelles, the innermost membranes are the sites of greatest molecular activity and the
outer membranes have fewer significant functions. These outer membranes best correspond
to the plasma membrane of the eukaryotic cells that originally wrapped the free-living
prokaryotes during endocytosis.
3. Mitochondria and chloroplasts are not cellular structures that are synthesized in a cell
like ribosomes and lysosomes. Instead, mitochondria only come from other mitochondria
and chloroplasts only come from other chloroplasts. This is further evidence of the
independent evolution of these organelles from free-living ancestral forms.
The Cytoskeleton: Cell Shape and Movement
Student Misconceptions and Concerns
1. Students often regard the fluid of the cytoplasm as little more than a watery fluid, which
suspends the organelles. The diverse functions of thin, thick, and intermediate filaments are
rarely appreciated before college.
2.Students often think that the cilia on the cells lining our trachea function like a comb,
removing debris from the air. Except in cases of disease or damage, these respiratory cilia
are instead covered by mucus. Cilia lining our trachea do not reach the air to “comb” it.
Instead, these cilia sweep the dirty mucus up our respiratory tracts. (See also Teaching Tip 2
below.)
3. The dynamic, weblike structure of the cytoskeleton is very different from the skeletons
that students may already know. Their dynamic structures (see Teaching Tip 2 below) are
quite unlike any human designs. Students have much to gain from vivid illustrations of
cytoskeletal diversity. Consider sharing some impressive images from a Google image
search or other resources.
Teaching Tips
1. Students might enjoy this brief class activity. Have everyone in the class clear their
throats at the same time. Wait a few seconds. Have them notice that after clearing, they
swallowed. The mucus that trapped debris is swept up the trachea by cilia. When we clear
our throats, this dirty mucus is disposed of down our esophagus and among the strong acids
of our stomach!
2. Analogies between the infrastructure of human buildings and the cytoskeleton are
limited by the dynamic nature of the cytoskeleton. Few human structures have their
structural framework routinely constructed, deconstructed, and then reformed in a new
configuration on a regular basis. (Tents are often constructed, deconstructed, and then
reformed repeatedly but typically rely upon the same basic design.) Thus, caution is
especially warranted in such analogies.
Answers to End-of-Chapter Questions
The Process of Science
11. Suggested answer: In the cell membrane, phospholipids are facing either the aqueous
environment outside the cell or the aqueous environment inside the cell. Since the polar
“heads” of the phospholipids are hydrophilic, they orient toward water. Therefore, two
layers are necessary. The membranes that enclose oil droplets face only an aqueous
environment on the exterior of the droplet. This means that one layer of phospholipids is
sufficient. The polar heads point outward, whereas the hydrophobic fatty acids point inward
toward the oil.
12. Suggested answer: Upon initial inspection, one would expect to see cells containing
indigestible substances accumulated in the lysosomes. The symptoms of lysosomal storage
diseases can be diverse, depending on the specific digestive enzyme that is not present or
functioning correctly. A wide battery of blood tests and a physical exam might reveal some
of these specific symptoms. In addition, some of the disease symptoms might also exist in
one of the parents of the child.
Biology and Society
13. Some issues and questions to consider: Were the cells Moore’s property, a gift, or just
surplus? Was Moore asked to donate the cells? Was he informed about how the cells might
be used? Is it important to ask permission or inform the patient in such a case? How much
did the researchers modify the cells? What did they have to do to them to sell the product?
Do the researchers and the university have a right to make money from Moore’s cells? Is the
fact that they saved Moore’s life a factor here? Does Moore have the right to sell his cells?
Would Moore have been able to sell the cells without the researchers’ help?
Additional Critical Thinking Questions
The Process of Science
1. You are comparing two cells. One cell is very small, and the other cell is huge. You are
asked to determine which cell will be more successful based solely on size. What would
your answer be? Give a specific explanation for your answer.
Suggested answer: The smaller cell would be expected to be more successful. It will
have a greater surface-to-volume ratio. This makes it easier for the cell to acquire
nutrients, remove wastes, and communicate faster between the nucleus and cytoplasm.
Larger cells do have a large surface area; however, they also have a larger volume and
much greater distances for materials to diffuse.
2.Several lines of evidence support the hypothesis that mitochondria and chloroplasts
evolved from parasitic prokaryotic cells living in the cytoplasm of primitive eukaryotic
cells. What types of evidence would you look for to support or refute this hypothesis?
Suggested answer: Prokaryotes, like eukaryotes, contain DNA. One would expect,
therefore, to find DNA in mitochondria and chloroplasts. Such DNA is found. You
would expect that genes found within mitochondrial or chloroplast DNA are more
similar to prokaryotic genes than they are to equivalent eukaryotic genes. Prokaryotes
carry out protein synthesis, as do mitochondria and chloroplasts, which have their own
ribosomes. One curious feature of mitochondria and chloroplasts is their double
membrane. Although prokaryotes do not have a double membrane, you might imagine
how the envelopment of a prokaryote by a eukaryotic cell would wrap a second “outer
membrane” around the cell. Thus, the inner membrane would represent the original
prokaryotic membrane, and the outer membrane would represent the remains of the
eukaryotic membrane.
3.The inside of lysosomes is acidic with a pH (5.0) significantly lower than the slightly
basic pH of the cytoplasm. As you might expect, the hydrolytic enzymes stored inside the
lysosome work best at this low pH. Can you think of a hypothesis to explain why such a pH
difference was important in the evolution of the cell? How would you test your hypothesis?
Suggested answer: Lysosomes are dangerous organelles because of their large arsenal of
enzymes capable of digesting virtually any macromolecule produced by the cell. Were a
lysosome to leak, the potential damage to the cell would be great. It is inevitable that a
few lysosomes rupture, either because of accident or because of age. Because of the pH
difference, leaked enzymes would be unable to work in the pH of the cytoplasm, and no
damage would be done to the cell. To test your hypothesis, you could isolate enzymes
from lysosomes and measure their activity at several different controlled pHs. If your
hypothesis were correct, you would expect that the activity of the enzymes at the pH of
the cytoplasm would be very low.
4. You have isolated a new prokaryotic organism, and now you want to determine whether
it is photosynthetic. It is too small to visualize with the microscopes you have available.
Design an experiment to determine whether the new cell actively uses photosynthesis.
Suggested answer: You could provide this new cell with the ingredients needed for
photosynthesis and see what happens. For example, you could provide the cells with
water in which the oxygen has been radioactively labeled. If the cells photosynthesize,
those water molecules will be split, and radioactive oxygen gas will be released. You
could also provide the cells with radioactively labeled carbon dioxide. If the cells
perform photosynthesis, the radioactivity should be found in glucose in the cells.
Biology and Society
5. Panspermia is an old idea that living cells did not evolve on Earth, but instead came to
Earth from space. The Greek philosopher Anaxagoras was the first to consider the
possibility that living seeds arrived on Earth from another world. Several famous scientists
have also held this view. The Swedish chemist Svante Arrhenius, a Nobel Prize winner,
published a book in 1907 in which he argued that living spores escaped from the upper
atmosphere of living planets and traveled to other planets through space. The astronomer
Fred Hoyle, famous for the steady-state universe theory, argued that life originated in
comets, which seeded the Earth on impact. Frances Crick, codiscoverer of the structure of
DNA, argued in a 1981 book that life was sent to Earth on rocket ships by intelligent beings
elsewhere in the universe, a process he termed directed pangenesis. What objections to this
theory would its supporters have to consider? Do you consider this idea a reasonable
explanation of how living cells evolved? How would such an idea change your view of your
position in the universe?
Some issues and questions to consider: Could living cells or spores survive the long
journey through space? Would they be damaged by the intense radiation and extreme
temperatures found in space? What force could eject cells from their home planet?
Would the force of such an ejection damage the cells? Could cells survive passage
through the earth’s atmosphere? Assuming cells survived this journey inside a protected
environment (such as the center of a large comet), how would they exit this
environment upon arrival? Could they survive the environment of their new home? If
there are supportive data for each of these concerns, does the hypothesis merely push
back the question of how life evolved? If it did not evolve on Earth, then how did it
evolve in some other corner of the universe? If cells did evolve elsewhere in the
universe, the most important conclusion would be that indeed we are not alone in the
universe. Perhaps our universe is teeming with life.
6. Explain why animal cells would be unable to exist without the presence of plant cells. Is
this relationship reciprocal?
Some issues and questions to consider: What things do plants produce via
photosynthesis that animal cells require? Do animals produce anything that plants need?
What would happen to animals if plant cells all died? What would happen to plant cells
if animal cells all died?