Lesson Overview

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Lesson Overview

• 20.2 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 seen them? 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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Classifying Prokaryotes

• How are prokaryotes classified?
• Prokaryotes are classified as Bacteria or
  Archaeatwo of the three
• domains of life.

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Classifying Prokaryotes

• The smallest and most abundant microorganisms on
  Earth are prokaryotesunicellular organisms that
  lack a nucleus.
• Prokaryotes have DNA, like all other cells, 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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Classifying Prokaryotes

• 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 exactly how 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 bacterium that 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 sugars and amino acids
  that surrounds the cell membrane.
• 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

• 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 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 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 reproduction is known as binary
  fission.

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Growth, Reproduction, and 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 at astonishing ratessome as
  often as once every 20 minutes!

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Growth, Reproduction, and 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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Growth, Reproduction, and 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.

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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 genus Anabaena 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 N2 into 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 nitrogen used 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.