16 PROKARYOTES: THE FIRST
SINGLE-CELLED CREATURES
CHAPTER OUTLINE
Origin of the First Cells (p. 342)
16.1
Origin of Life (p. 342; Figs. 16.1, 16.2)
A. There are at least three possibilities for the origin of life on this planet: extraterrestrial origin,
special creation, and evolution.
1. Only evolution can be tested scientifically.
B. Forming Life’s Building Blocks
1. Some scientists believe that the first organic molecules formed spontaneously from
building blocks subjected to lightning and UV radiation.
2. Miller and Urey reconstructed the oxygen-free early atmosphere, and conducted
experiments that confirmed these beliefs.
3. Other scientists favor a bubble model for the formation of early organic molecules.
16.2 How Cells Arose (p. 344; Fig. 16.3)
A. Scientists now suspect that the first macromolecules were not proteins but RNA molecules.
B. The First Cells
1. Most scientists believe that the first cells aggregated spontaneously as microspheres that
eventually were able to incorporate molecules and energy.
2. It took millions of years for the first cell to develop.
Prokaryotes (p. 346)
16.3
The Simplest Organisms (p. 346; Figs. 16.4, 16.5)
A. According to the fossil record, prokaryotes have been on earth for 2.5 billion years, 1 billion
of which they were the only organisms.
B. Today, prokaryotes are still the simplest and most abundant organisms on earth.
C. Bacteria play a key role in cycling minerals within ecosystems and are responsible for a host
of diseases.
D. The Structure of a Prokaryote
1. Prokaryotes are tiny cells that are simple in structure and lack organelles and a nucleus.
2. Bacteria and archaea are prokaryotes.
3. They possess a cell wall outside their plasma membrane; some species of bacteria also
have an outer membrane component to the cell wall.
4. These bacteria are said to be gram-negative because the outer membrane prevents the cell
wall from taking up a type of stain called gram stain; they are also more resistant to
antibiotics.
5. Gram-positive bacteria lack this extra membrane layer, and thus take up gram stain and
are also more susceptible to antibiotics.
6. Many kinds of bacteria have threadlike strands of protein called flagella that function in
cell movement; shorter extensions called pili attach the bacterial cells firmly to surfaces
or other cells.
7. A number of bacteria can produce thick-walled endospores that allow them to survive for
extended periods (even centuries) during times of harsh environmental conditions.
E. How Prokaryotes Reproduce
1. Prokaryotes reproduce by binary fission, which is a simple method of increasing in size
and then pulling apart into two cells.
2. Sometimes plasmids can be passed from one cell to another by conjugation.
3. Bacteria can also obtain genetic information through transformation (taking up DNA
from the environment) or from bacterial viruses.
16.4 Comparing Prokaryotes to Eukaryotes (p. 348; Table 16.1)
A. Prokaryotes differ from eukaryotes in a number of notable ways.
1. Prokaryotes lack the internal compartmentalization of eukaryotes, are considerably
smaller than eukaryotes, and are unicellular only.
2. The prokaryotic chromosome is a single circle of DNA, prokaryotic cell division and
flagella are simple, and metabolism is diverse in prokaryotes.
B. Prokaryotic Metabolism
1. Many prokaryotes are autotrophs and obtain their carbon from inorganic CO 2.
2. Autotrophs that obtain their energy from sunlight are photoautotrophs; those that obtain
energy from inorganic compounds are chemoautotrophs.
3. Other prokaryotes are heterotrophs and obtain at least some of their carbon from organic
molecules.
4. Heterotrophs that obtain their energy from sunlight are photoheterotrophs; those that
harvest energy from organic molecules are chemoheterotrophs.
5. Most bacteria, including decomposers and pathogenic bacteria, are chemoheterotrophs.
16.5 Importance of Prokaryotes (p. 349; Fig. 16.6)
A. Prokaryotes and the Environment
1. Prokaryotes were responsible for creating the properties of the atmosphere and soil for
over 2 billion years.
2. Prokaryotes make many important contributions to the world’s ecosystems, including
playing key roles in cycling carbon and nitrogen.
B. Bacteria and Genetic Engineering
1. Genetic engineering may be able to produce “improved” strains of bacteria, capable of
attacking insect pests in nature to serving as a biological control agent.
2. Bacteria can also be genetically modified to produce therapeutic proteins, and oildegrading bacteria can be used to clean up oil spills.
C. Bacteria, Disease, and Bioterrorism
1. Some bacteria cause major diseases in plants and animals.
16.6 Prokaryotic Lifestyles (p. 350; Figs. 16.7, 16.8; Table 16.2)
A. Archaea
1. Today’s archaea are mostly methanogens, producing methane in anaerobic environments.
2. Methanogens live in swamps, marshes, and in the guts of cattle and other herbivores.
3. Other archaea include thermoacidophiles, which live in hot, acidic springs.
B. Bacteria
1. Most prokaryotes are bacteria, including the cyanobacteria that are important
photosynthetic organisms.
2. The cyanobacteria possess special structures called heterocysts in which nitrogen fixation
occurs.
3. Other members of the bacteria are chemoautotrophs that obtain energy from inorganic
molecules, while most others are heterotrophs, including decomposers that use decaying
organic matter as a food source.
4. Many types of bacteria cause diseases in humans.
5. Tuberculosis is an important disease caused by a bacterium, with new drug-resistance
strains now emerging.
Viruses (p. 352)
16.7 The Structure of Viruses (p. 352; Fig. 16.9)
A. Viruses are not living organisms, according to the traits possessed by living creatures.
B. Viruses are parasitic segments of DNA or RNA that invade a host cell and use the host's
genetic machinery to turn out more copies of viral particles.
C. The true nature of viruses was discovered in 1935, when Wendell Stanley prepared purified
tobacco mosaic virus and discovered that viruses are not cellular, but chemical in nature.
D. Bacterial viruses are called bacteriophages and have complex shapes.
E. Most viruses have a capsid or protein sheath around their core of RNA or DNA.
16.8 How Bacteriophages Enter Cells (p. 353; Figs. 16.10, 16.11)
A. Bacteriophages are viruses that infect bacterial cells, and are diverse structurally and
functionally.
B. The Lytic Cycle
1. The virus contacts the host cell and injects its DNA into the host cytoplasm.
2. The T-series viruses and other viruses such as lambda phages are all virulent viruses,
multiplying within cells and lysing them.
3. When a virus kills the infected cell in which it is replicating, the viral reproductive cycle
is called a lytic cycle.
C. The Lysogenic Cycle
1. Many bacteriophages do not immediately kill the host bacterium but instead incorporate
their DNA into the host cell’s genome, a process called lysogeny.
2. While the virus resides inside the host, it is called a prophage.
3. At a later time, the prophage may exit the genome and initiate viral replication.
4. This type of reproductive cycle is called a lysogenic cycle.
5. During the integrated portion of a lysogenic cycle, virus genes are expressed; the
introduction of foreign DNA into the host’s genome is called transformation.
D. Gene Conversion and Cholera
1. An important example of cell transformation is in the cholera-causing bacterium, Vibrio
cholerae.
2. This bacteria usually exists in a harmless form, but is transformed into the virulent form
by a bacteriophage.
16.9
How Animal Viruses Enter Cells (p. 356; Figs. 16.12, 16.13)
A. A diversity of viruses occurs among animals.
B. Attachment
1. HIV recognizes a specific cell-surface marker on macrophages in the human body.
2. A marker on the outside of the HIV virus, called gp120, precisely fits a cell-surface
marker called CD4 on macrophages.
C. Entry into Macrophages
1. T lymphocytes also possess CD4 markers, although it’s the macrophages that are infected
initially.
2. When T lymphocytes are infected, the immune system fails, leading to AIDS.
3. HIV gains entry first into macrophages because when gp120 binds to CD4, these proteins
undergo a conformational change into a new form that fits a CCR5 coreceptor molecule
on macrophages.
4. After the conformational change, the CCR5 receptor passes the gp120-CD4 complex
through the macrophage membrane by endocytosis.
D. Replication
1. Once inside the host macrophage cell, HIV sheds its coat, its RNA is “read” by reverse
transcriptase, DNA is synthesized, and the host’s cell machinery is directed to produce
more HIV particles.
2. New viruses are released from the cell by exocytosis without harming the host cell.
E. Starting AIDS: Entry into T Cells
1. HIV has a long latent period in which it is replicating in macrophages, but the individual
is experiencing no symptoms.
2. During this time, HIV alters the gp120 gene and causes it to change its coreceptor
allegiance to one that binds with CXCR4 on the surface of T lymphocytes.
3. Soon the body’s T lymphocytes become infected, and the immune system fails, leading to
AIDS.
16.10 Disease Viruses (p. 358; Table 16.3)
A. The Origin of Viral Diseases
1. Viruses that arise in one species may pass to another, causing a new disease.
2. New pathogens arising this way are called emerging viruses.
3. Emerging viruses constitute a considerable threat when air travel is readily available, and
a virus can quickly be spread worldwide.
4. Influenza is an important human disease caused by the influenza virus, which can
recombine in animal reservoirs to create new virus strains unrecognizable by human
immune defenses.
5. AIDS (HIV) is derived from the simian immunodeficiency virus, and originated in
Central Africa where monkeys, chimpanzees, and humans coexist.
6. Ebola viruses also arose in Central Africa and attack human connective tissues.
7. Hantavirus arose suddenly in the southwestern U.S., and produces a highly fatal
hemorrhagic infection.
8. SARS, severe acute respiratory syndrome, originated from a virus that infects the Chinese
horseshoe bat.
9. West Nile Virus is a mosquito-borne virus that produces a respiratory infection and is
carried by birds.
B. The Continuing Threat of Influenza: Bird Flu
1. The influenza virus has been one of the most lethal viruses in human history.
2. Flu viruses have two kinds of surface proteins, H and N, each having their own subtypes.
3. Different flu virus strains have different combinations of H and N subtypes.
4. New strains of influenza virus arise in the Far East where intermediate hosts (ducks,
chickens, pigs) live in close proximity to humans.
5. Viruses can recombine within some animals to produce novel combinations of H and N
proteins, often causing worldwide epidemics.
KEY TERMS
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Miller-Urey experiment (p. 343)
microspheres (p. 344)
prokaryotes (p. 346)
gram-positive bacteria (p. 347) These lack an outer membrane and stain purple.
gram-negative bacteria (p. 347) These possess an outer membrane and do not stain purple; gramnegative bacteria are also more resistant to antibiotics.
conjugation (p. 347) Bacteria exchange plasmids through conjugation bridges.
binary fission (p. 347)
autotroph (p. 348) Certain bacteria are photoautotrophs or chemoautotrophs.
heterotroph (p. 348) Certain bacteria are photoheterotrophs or chemoheterotrophs.
archaea (p. 350) “Ancient bacteria,” many of which live in extreme environments.
bacteria (p. 350) Members of a large, diverse kingdom of prokaryotes; the most abundant organisms
on earth.
cyanobacteria (p. 350) A type of bacteria that contributed oxygen to the early atmosphere and that can
fix nitrogen.
nitrogen fixation (p. 350) Certain bacteria are capable of converting atmospheric nitrogen into a form
that can be used by other organisms.
biofilms (p. 350)
viruses (p. 352)
bacteriophages (p. 353) Complex viruses that attack bacterial cells.
lysogeny (p. 353)
gene conversion (p. 354)
macrophages (p. 356)
HIV (p. 356) The RNA virus that leads to AIDS.
LECTURE SUGGESTIONS AND ENRICHMENT TIPS
1.
2.
3.
Students do not often fully understand the biological importance of the bacteria as a group. Be sure to
emphasize their abundance and ecological importance.
Bacteria Contribute to Our Food. Lactobacillus spp. and Lactococcus lactis are two bacteria used to
ferment milk. When used on skim milk, buttermilk is the result. Add these bacteria to cream, and sour
cream is produced. Lactobacillus acidophilus is the bacteria in “acidophilus” milk that many people
drink to help relieve intestinal distress, like diarrhea and constipation. Some evidence suggests that this
bacterium is beneficial in warding off colon cancer. Yogurt is made using two kinds of bacteria, one
species of Lactobacillus and one of Streptococcus. One is responsible for the production of acid to
curdle the milk and turn it into yogurt; the other imparts the pleasant aroma one associates with yogurt.
The cyanobacterium Spirulina is used directly as a food source in areas of Africa and can also be found
in health food stores in the United States.
Bacterial Plate Colonies. Purchase a few Petri plates containing prepared growth agar and set up a
demonstration with student participation. (Carolina Biological Supply or Ward's Biology are both good
sources for bacteriological media at reasonable expense; see the Appendix for telephone and/or fax
numbers.) Have students streak the plates using their fingers, material from their combs or hairbrushes,
or by swabbing the tops of their soda pop cans. After a couple of days of incubation, the appearance of
many and diverse bacterial colonies will impress students about the volume of bacteria surrounding
them.
CRITICAL-THINKING QUESTIONS
1.
2.
Might it be possible to employ bacterial metabolism to provide a source of useable energy? Explain.
Devise a hypothetical experiment that would show whether eating the cooked meat (muscle), rather
than brains, of cattle infected with mad cow disease could transmit the infectious prions to mammals,
such as humans. Hint: white lab mice are the best place to start.