Prokaryotes 20-2
Federoff
Classifying Prokaryotes
– The smallest and most
abundant microorganisms
on Earth are
prokaryotes—unicellular
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.
Classifying Prokaryotes
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.
Bacteria
– Bacteria live almost
everywhere—in fresh
water, in salt water, on
land, and on and within
the bodies of humans
and other eukaryotes.
Bacteria
– Bacteria are usually
surrounded by a cell
wall made from
peptidoglycan that
protects the cell from
injury and determines
its shape.
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.
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.
Archaea
– Many archaea live in extremely harsh
environments.
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.
Size, Shape, and Movement
– Rod-shaped prokaryotes
are called bacilli.
– Spherical prokaryotes are
called cocci.
– Spiral and corkscrewshaped prokaryotes are
called spirilla.
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.
Nutrition and Metabolism
– Energy is released from these fuel molecules
during cellular respiration, fermentation, or
both.
Nutrition and Metabolism:
Energy Capture
Nutrition and Metabolism:
Energy Release
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.
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 rates—some as often as once
every 20 minutes!
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.
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.
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.
Conjugation
– This transfer of genetic information
increases genetic diversity in populations of
prokaryotes.
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.
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.
Nitrogen Fixers
– All organisms need nitrogen to make proteins and other
molecules.
– Nitrogen gas (N2) makes up 80 percent of Earth’s
atmosphere, but only a few kinds of organisms—all of them
prokaryotes—can 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.
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.
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 humanmade 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.