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20.2 Prokaryotes
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Classifying Prokaryotes
Prokaryotes are divided into two very
distinct groups: Bacteria and Archaea.
These groups are very different from
each other; therefore, biologists now
consider each group of prokaryotes as a
separate domain. Eukaryotes are the
third domain.
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Bacteria
The larger of the
two domains of
prokaryotes is the
Bacteria.
Bacteria live almost
everywhere— fresh
water, salt water, on
land, on and within
bodies of humans and
other eukaryotes.
Escherichia coli, a
typical bacterium that
lives in human
intestines, is shown.
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Bacteria
Bacteria are usually
surrounded by a cell
wall.
Some bacteria, such as
E. coli, have a second
membrane outside the
peptidoglycan wall that
makes the cell
especially resistant.
In addition, some
prokaryotes 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. Both are equally small, lack nuclei,
and have cell walls, but there are important
differences.
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.
One group of archaea produce
methane gas and live in
environments with little or no
oxygen, such as thick mud and the
digestive tracts of animals.
Other archaea live in extremely
salty environments, such as Utah’s
Great Salt Lake, or in hot springs
where temperatures approach the
boiling point of water.
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Size, Shape, and Movement
Prokaryotes range in size from 1 to 5
micrometers, making them much smaller than
most eukaryotic cells. Prokaryotes come in a
variety of shapes.
Rod-shaped prokaryotes are called bacilli.
Spherical prokaryotes are called cocci.
Spiral and corkscrew-shaped prokaryotes are
called spirilla.
bacterial communication
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Nutrition and Metabolism:
Energy Capture
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Growth, Reproduction, and
Recombination
Reproduction: binary fission asexual form of
reproduction
When conditions are favorable, prokaryotes can
grow and divide as often as once every 20
minutes!
When growth conditions become unfavorable,
many prokaryotic cells form an endospore.
Endospores can remain dormant for months or
even years.
The bacterium Bacillus anthracis, which causes the
disease anthrax, is one such bacterium.
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Conjugation
conjugation: exchange genetic information
bridge forms between two bacterial cells,
genetic material, usually in the form of a
plasmid, moves from one cell to the other.
This transfer of genetic information
increases genetic diversity
Many plasmids carry genes that enable
bacteria to survive in new environments or
to resist antibiotics that might otherwise
prove fatal.
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Decomposers
Bacteria called actinomycetes are
present in soil and in rotting plant
material such as fallen logs, where
they decompose complex organic
molecules into simpler molecules.
Bacterial decomposers are also
essential to industrial sewage
treatment, helping to produce purified
water and chemicals that can be used
as fertilizers.
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Producers
Cyanobacteria in the genus
Anabaena form filamentous
chains in ponds and other
aquatic environments, where
they perform photosynthesis.
Photosynthetic prokaryotes are
among the most important
producers on the planet.
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Nitrogen Fixers
Some plants have symbiotic
relationships with nitrogen-fixing
prokaryotes.
The bacterium Rhizobium grows in
nodules, or knobs, on the roots of
legume plants such as soybean.
The Rhizobium bacteria within these
nodules convert nitrogen in the air into
the nitrogen compounds essential for
plant growth.
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Human Uses of Prokaryotes
Prokaryotes, esp. bacteria, are used in
the production of many foods and
commercial products.
Yogurt is produced by the bacterium
Lactobacillus.
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.
Some adapted to extreme
environments may be a rich source of
heat-stable enzymes that can be used
in medicine, food production, and
industrial chemistry.
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BIOREMEDIATION
Bioremediation: use biological organisms
to solve an environmental problem
To clean up oil spills, bacteria are
introduced to the area of the spill
where they break down the
hydrocarbons of the oil into carbon
dioxide
Toxic metals, such as mercury , can be
converted into nontoxic forms by
bacteria