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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 molecules. and other The process of nitrogen fixation converts nitrogen gas into ammonia (NH 3 ). 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 nitrogen used by other organisms.

percent of the

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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.