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19 THE FIRST SINGLE-CELLED CREATURES
EXTENDED LECTURE OUTLINE
LEARNING OBJECTIVES
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Discuss the length of time bacteria were the dominant organisms living on earth.
Describe the structure of a bacterial cell.
Differentiate between gram-positive and gram-negative bacteria.
Understand how bacteria reproduce.
Know how bacteria, as prokaryotes, differ from eukaryotes in structure, size, and function.
List the major types of bacterial metabolism.
Discuss the ecological and medical importance of the bacteria.
Describe the differences between the two major groups of bacteria, the archaebacteria and the
eubacteria.
Explain what a virus is and how it is structured.
Discuss how bacteriophages and viruses invade living cells.
Explain how HIV gains entry into cells of the human immune system.
List a few of the important disease-causing bacteria and viruses.
Describe prions and viroids and comment on the diseases they cause.
Origin of the First Cells (p. 342)
19.1
19.2
Origin of Life (p. 342; Figs. 19.1, 19.2)
A. There are 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. The first organic molecules are believed to have 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. Recent findings of even older fossils, however, have refuted the findings of their
experiments.
4. Currently, a bubble model for the formation of early organic molecules is being
examined.
How Cells Arose (p. 344; Fig. 19.3)
A. Scientists now suspect that the first macromolecules were not proteins but RNA molecules.
B. Not everyone accepts the notion that life evolved spontaneously on this planet, citing the
second law of thermodynamics for evidence.
C. The First Cells
1. Most scientists believe that the first cells aggregated spontaneously as microdrops that
eventually were able to incorporate molecules and energy.
2. It took millions of years for the first cell to develop.
Prokaryotes (p. 346)
19.3
The Simplest Organisms (p. 346; Figs. 19.4, 19.5, 19.6)
A. According to the fossil record, prokaryotes have been on earth for 3.5 billion years, 2 billion
of which they existed here alone.
B. Now prokaryotes are still the simplest most abundant organisms on earth.
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19.4
19.5
19.6
C. Bacteria play a key role in cycling minerals within ecosystems, contributed an abundance of
oxygen to the early atmosphere, and are responsible for a host of diseases.
D. The Structure of a Prokaryote
1. Prokaryotes are tiny-celled organisms that are simple in structure and lack organelles or a
nucleus.
2. Bacteria and archaebacteria are prokaryotes.
3. They possess a cell wall outside their plasma membrane made of polysaccharides bound
together by molecules of protein.
4. Some species of bacteria also have another outer membrane made up of
mucopolysaccharide.
5. These bacteria are said to be gram-negative because the mucopolysaccharide prevents the
cell wall from taking up a type of stain called gram stain; they are also more resistant to
antibiotics.
6. Gram-positive bacteria lack this extra mucopolysaccharide layer, and thus take up gram
stain and are also more susceptible to antibiotics.
7. Threadlike strands of protein function like flagella in bacteria; shorter proteinaceous
extensions called pili attach the bacterial cells firmly to a substratum.
8. 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.
Comparing Prokaryotes to Eukaryotes (p. 348; Table 19.1)
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.
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.
Importance of Prokaryotes (p. 349; Fig. 19.7)
A. Prokaryotes and the Environment
1. Prokaryotes make many important contributions to the world’s ecosystems, including
playing key roles in cycling carbon and nitrogen.
2. Photosynthetic bacteria were responsible for creating the properties of the atmosphere
and soil over billions of years.
3. Bacteria are now very important in many industrial processes, from producing the acetic
acid in vinegar to the production of antibiotics.
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.
C. Bacteria, Disease, and Bioterrorism
1. Some bacteria cause major diseases in plants and animals.
Prokaryotic Lifestyles (p. 350; Figs. 19.8, 19.9; Table 19.2)
A. Archae
1. The living archae are mostly methanogens, using methane and being poisoned by oxygen
gas.
2. Methanogens live in swamps, marshes, and in the guts of cattle and other herbivores.
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3. Other archaebacteria include thermoacidophiles, living in hot sulfur springs.
B. Bacteria
1. Most bacteria are eubacteria, including the cyanobacteria that are important
photosynthetic organisms.
2. The cyanobacteria possess special structures called heterocysts that house the enzymes
needed for nitrogen fixation or the conversion of atmospheric nitrogen to usable form.
3. Other members of the eubacteria are chemoautotrophs that obtain energy from inorganic
molecules, while still others are heterotrophs that use decaying organic matter as a food
source.
4. The eubacteria make up the disease-causing bacteria.
5. Tuberculosis is an important disease caused by a bacterium, with new drug-resistance
strains now emerging.
Viruses (p. 352)
19.7
19.8
19.9
The Discovery of Viruses (p. 352; Fig. 19.10)
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 1933, when Wendell Stanley prepared purified
tobacco mosaic virus and discovered later 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.
F. Plant and animal viruses are simpler in form and are either helical or isometric.
How Bacteriophages Enter Cells (p. 353; Figs. 19.11, 19.12)
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 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. Most 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.
D. Lysogenic Conversion
1. 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.
E. Transforming the Cholera-Causing Bacterium
1. An important example of cell transformation is in the cholera-causing bacterium, Virbrio
cholerae.
2. This bacteria usually exists in a harmless form, but is transformed into the virulent form
by a bacteriophage.
How Animal Viruses Enter Cells (p. 356; Figs. 19.13, 19.14)
A. Viruses invade cells in a number of ways.
B. Plant cells are invaded through tiny cracks in their cell walls.
C. Attachment
1. The HIV viruses recognize 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.
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19.10
D. 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. How HIV gains entry into macrophages is gp120 binds to CD4, and these proteins
undergo a conformational change into a new form that fits the CCR5 coreceptor
molecule.
4. After the conformational change, the CCR5 receptor passes the gp120-CD4 complex
through the membrane by endocytosis.
E. Replication
1. Once inside the host cell, HIV sheds its coat and 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.
F. Starting AIDS: Entry into T Cells
1. HIV has a long latent period in which it is replicating 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
death.
Disease Viruses (p. 358; Fig. 19.15; Table 19.3)
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 a rapidly mutating virus, this is why
new strains of influenza surface annually.
5. New strains of influenza virus arise in the far east where intermediate hosts (ducks,
chickens, pigs) live in close proximity to humans.
6. AIDS (HIV) is derived from the simian immunodeficiency virus, and originated in
Central Africa where monkeys, chimpanzees, and humans coexist.
7. Ebola viruses also arose in Central Africa, and attack human connective tissues and are
up to 90% fatal.
8. Hantavirus arose suddenly in the southwestern U.S., and produces a highly fatal
hemorrhagic infection.
9. SARS, severe acute respiratory syndrome, originated from a virus that infects the Chinese
horseshoe bat.
10. West Nile Virus is a mosquito-borne virus that produces an encephalitis and is carried by
birds.
B. Emerging Viruses
1. Newly emerging viruses, such as Ebola, attack human connective tissue and are very
lethal.
2. Emerging viruses are those that arise in one animal and are passed to another.
KEY TERMS
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bacteria (p. 350)
gram-positive (p. 347) These lack an outer lipopolysaccharide membrane and stain purple.
gram-negative (p. 347) These possess an outer membrane and do not stain purple; gram-negative
bacteria are also more resistant to antibiotics.
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conjugation (p. 347) Bacteria exchange plasmids through conjugation bridges.
autotroph (p. 348) Certain bacteria are photoautotrophs or chemoautotrophs.
heterotroph (p. 348) Certain bacteria are photoheterotrophs or chemoheterotrophs.
archae (p. 350) “Ancient bacteria” that survive only in extreme environments today.
cyanobacteria (p. 350) A type of eubacteria that contributed oxygen to the early atmosphere and
changed the course of evolution.
nitrogen fixation (p. 350) Certain bacteria are capable of converting atmospheric nitrogen into a form
that can be used by other organisms.
bacteriophages (p. 353) Complex viruses that attack bacterial cells.
HIV (p. 356) The RNA virus that leads to AIDS.
emerging virus (p. 358) New viruses originate in other organisms and are highly pathogenic in the
new host. Ebola virus and HIV in humans are two examples.
prion (p. 355) An infectious, proteinaceous particle.
mad cow disease (p. 355) A brain disease in cattle likely caused by a prion.
LECTURE SUGGESTIONS AND ENRICHMENT TIPS
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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
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Scientists are fairly certain that eukaryotes arose as a result of endosymbiosis of several bacterial forms
that eventually became organelles. Is it feasible today to assemble a eukaryote in the lab using an
aerobe as a mitochondrion and a cyanobacterium as a chloroplast? Explain.
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.
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