Lesson Overview 20.2 Prokaryotes (2)

5/20/2018 Lesson Overview 20.2 Prokaryotes (2)

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Lesson Overview Viruses

Lesson Overview20.2 Prokaryotes


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THINK ABOUT IT

Imagine living all your life as a member of what you believe is the

only family on your street. Then, one morning, you open the front

door and discover houses and neighbors all around you.

Where did all the people come from? What if the answer turned

out to be that they had always been thereyou just hadnt seenthem? How would your view of the world change?


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THINK ABOUT IT

When the microscope was first invented, we humans had just such a

shock. Far from being alone, we share every corner of our world with

microorganisms. Even a seemingly clean toothbrush contains a film

of bacteria on its bristles!


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Classifying Prokaryotes

How are prokaryotes classified?


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How are prokaryotes classified?

Prokaryotes are classified as Bacteria or Archaeatwo of the three

domains of life.


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The smallest and most abundant

microorganisms on Earth are

prokaryotesunicellular organisms

that lack a nucleus.

Prokaryotes have DNA, like all othercells, but their DNA is not found in a

membrane-bound nuclear envelope

as it is in eukaryotes. Prokaryote

DNA is located in the cytoplasm.

A bacterium such as E. coli has the

basic structure typical of most

prokaryotes.


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Recently, biologists have divided prokaryotes 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 include a wide range of

organisms with lifestyles so different

that biologists do not agree exactlyhow many phyla are needed to

classify this group.


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Bacteria

Bacteria live almost everywherein

fresh water, in salt water, on land,

and on and within the bodies of

humans and other eukaryotes.

Escherichia coli, a typical bacteriumthat lives in human intestines, is

shown.


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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

peptidoglycana polymer of sugarsand amino acids that surrounds the

cell membrane.

Some bacteria, such as E. coli,

have a second membrane outsidethe peptidoglycan wall that makes

the cell especially resistant to

damage.


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Bacteria

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 archaeaand 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 Utahs

Great Salt Lake, or in hot springs where temperatures approach the

boiling point of water.


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Structure and Function

How do prokaryotes vary in their structure and function?


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Structure and Function

How do prokaryotes vary in their structure and function?

Prokaryotes vary in their size and shape, in the way they move, and in the

way they obtain and release energy.


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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.


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Size, Shape, and Movement

Prokaryotes can also be distinguished by whether they move and how

they move.

Some prokaryotes do not move at all. Others are propelled by flagella.

Some glide slowly along a layer of slimelike material they secrete.


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Nutrition and Metabolism

Prokaryotes need a supply of chemical energy, which they store in the

form of fuel molecules such as sugars.

Energy is released from these fuel molecules during cellular respiration,

fermentation, or both.

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Nutrition and Metabolism

Prokaryotes vary in the ways they obtain energy and the ways they

release it.

Looking at the two tables on the following slides, notice that some

species are able to change their method of energy capture or release

depending on the conditions of their environment.

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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

RecombinationWhen 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 reproduction is known as binary fission.

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Recombination

Because binary fission does not involve the exchange or

recombination of genetic information, it is an asexual form of

reproduction.

When conditions are favorable, prokaryotes can grow and divide atastonishing ratessome as often as once every 20 minutes!

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Recombination

When growth conditions become unfavorable, many prokaryotic

cells form an endosporea 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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Recombination

The ability to form endospores makes it possible for some

prokaryotes to survive very harsh conditions. The bacterium Bacillus

anthracis, which causes the disease anthrax, is one such bacterium.

Lesson O er ie Viruses


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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.

In prokaryotes, mutations are inherited by daughter cells produced by

binary fission.



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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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Conjugation

Many plasmids carry genes that enable bacteria to survive in new

environments or to resist antibiotics that might otherwise prove fatal.

This transfer of genetic information increases genetic diversity in

populations of prokaryotes.



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The Importance of Prokaryotes

What roles do prokaryotes play in the living world?



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The Importance of Prokaryotes

What roles do prokaryotes play in the living world?

Prokaryotes are essential in maintaining every aspect of the ecological

balance of the living world. In addition, some species have specific uses in

human industry.



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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.



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Decomposers

By decomposing dead organisms, prokaryotes, supply raw materials

and thus help to maintain equilibrium in the environment.

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 genusAnabaena form filamentous chains in ponds

and other aquatic environments, where they perform photosynthesis.



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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.

Nitrogen gas (N2) makes up 80 percent of Earths atmosphere, but only

a few kinds of organismsall of them prokaryotescan convert N2into

useful forms.

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 nitrogenused by other organisms.



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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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Nitrogen Fixers

The Rhizobium bacteria often live symbiotically within nodules attached

to roots of legumes, such as clover, where they convert atmospheric

nitrogen into a form that is useable by plants.



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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.

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.

Bacteria and archaea adapted to extreme environments may be a rich

source of heat-stable enzymes that can be used in medicine, food

production, and industrial chemistry.

Lesson Overview 20.2 Prokaryotes (2)

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