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Viruses
Lesson Overview
20.2 Prokaryotes
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Classifying Prokaryotes
The smallest and most abundant
microorganisms on Earth are
prokaryotes—unicellular
organisms that lack a nucleus.
Prokaryotes DNA is located in the
cytoplasm.
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Bacteria
Bacteria live almost everywhere
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Bacteria
Bacteria are usually surrounded
by a cell wall that protects the
cell from injury and determines
its shape.
The cell walls of bacteria
contain peptidoglycan.
Some bacteria, such as E. coli,
have a second membrane
outside the peptidoglycan wall
that makes the cell especially
resistant to damage.
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Bacteria
Some also have flagella that
they use for movement, or pili,
which in E. coli serve mainly to
anchor the bacterium to a
surface or to other bacteria.
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Archaea
Under a microscope, archaea look very similar to bacteria.
The walls of archaea lack peptidoglycan, and their membranes
contain different lipids.
The DNA sequences of key archaea genes are more like those
of eukaryotes than those of bacteria.
Based on these observations, scientists have concluded that
archaea and eukaryotes are related more closely to each other
than to bacteria.
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Archaea
Many archaea live in extremely harsh environments.
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Size, Shape, and Movement
Prokaryotes are much smaller than
eukaryotic cells.
Rod-shaped prokaryotes are called
bacilli.
Spherical prokaryotes are called
cocci.
Spiral and corkscrew-shaped
prokaryotes are called spirilla.
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Size, Shape, and Movement
Prokaryotes can also be distinguished by whether they move
and how they move.
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Nutrition and Metabolism
Energy is released from these fuel molecules during cellular
respiration, fermentation, or both.
Prokaryotes vary in the ways they obtain energy and the ways
they release it.
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Nutrition and Metabolism: Energy
Capture
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Nutrition and Metabolism: Energy
Release
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Growth, Reproduction, and
Recombination
When a prokaryote has grown
so that it has nearly doubled in
size, it replicates its DNA and
divides in half, producing two
identical cells. This type of
asexual reproduction is known
as binary fission.
During favorable conditions
some bacteria can reproduce in
20 minutes.
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Growth, Reproduction, and
Recombination
When growth conditions become
unfavorable, many prokaryotic
cells form an endospore—a thick
internal wall that encloses the DNA
and a portion of the cytoplasm.
Endospores can remain dormant
for months or even years.
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Mutation
Mutations are one of the main ways prokaryotes evolve.
Mutations are random changes in DNA that occur in all
organisms.
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Conjugation
Many prokaryotes
exchange genetic
information by a process
called conjugation.
During conjugation, a
hollow bridge forms
between two bacterial cells,
and genetic material,
usually in the form of a
plasmid, moves from one
cell to the other.
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Decomposers
By decomposing dead organisms, prokaryotes, supply raw
materials and thus help to maintain equilibrium in the
environment.
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Producers
Photosynthetic prokaryotes are among the most important
producers on the planet.
Food chains everywhere are dependent upon prokaryotes as
producers of food and biomass.
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Nitrogen Fixers
All organisms need nitrogen to make proteins and other
molecules.
The process of nitrogen fixation converts nitrogen gas into
ammonia (NH3). Ammonia can then be converted to nitrates that
plants use, or attached to amino acids that all organisms use.
Nitrogen-fixing bacteria and archaea provide 90 percent of the
nitrogen used by other organisms.
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Human Uses of Prokaryotes
Prokaryotes, especially bacteria, are used in the production of a
wide variety of foods and other commercial products.
Some bacteria can digest petroleum and remove human-made
waste products and poisons from water.
Other bacteria are used to synthesize drugs and chemicals
through the techniques of genetic engineering.