PROKARYOTES: THE FIRST
SINGLE-CELLED CREATURES
CHAPTER 16
ORIGIN OF LIFE
• The earth’s environment when life originated
2.5 billion years ago was very different than
it is today.
• There was little or no oxygen in the atmosphere.
• The atmosphere was full of hydrogen-rich gases,
such as SH2, NH3, and CH4.
• High energy electrons would have been freely
created by energy from the sun (UV radiation) or
electrical energy in lightning.
ORIGIN OF LIFE
• Stanley Miller and Harold Urey recreated an
oxygen-free atmosphere similar to the early
earth in the laboratory.
• When subjected to levels of lightning and UV
radiation, many organic building blocks formed
spontaneously.
• The researchers concluded that life may have
evolved in a “primordial soup” of biological
molecules.
ORIGIN OF LIFE
• The bubble model of Louis Lerman suggests
an answer to the problems of the primordial
soup model.
• Bubbles produced by volcanic eruptions under
the sea provide various gases, and the gases
could react in bubbles.
• The bubble surface would protect the gases from
breakdown by UV radiation.
• At the surface of the ocean, the bubbles could
pop, and the molecules could briefly be subject
to energy sources.
• More complex molecules could have formed and
then fell back into the sea to again be enclosed
in bubbles and re-enter the cycle.
A CHEMICAL PROCESS INVOLVING BUBBLES
MAY HAVE PRECEDED THE ORIGIN OF LIFE
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
3
3
When the bubbles
44 Bombarded by the sun's ultraviolet
persisted long enough
to rise to the surface,
they popped, releasing
their contents to the air.
radiation, lightning, and other energy
sources, the simple organic molecules
released from the bubbles reacted to
form more complex organic molecules.
22
The gases, concentrated
inside the bubbles, reacted
to produce simple organic
molecules.
55
11
Volcanoes erupted
under the sea, releasing
gases enclosed in bubbles.
The more complex
organic molecules fell
back into the sea in
raindrops. There, they
could again be enclosed
in bubbles and begin the
process again.
HOW CELLS AROSE
• RNA was probably the first macromolecule
to form.
• When combined with high energy phosphate
groups, RNA nucleotides form polynucleotide
chains.
• When folded up, these RNA macromolecules
could have been capable of catalyzing the
formation of the first proteins.
ORIGIN OF LIFE
• The first cells probably formed
spontaneously as aggregates of
macromolecules.
• For example, shaking together oil and water
produces tiny bubbles called microspheres.
• Similar microspheres might have represented the
first step in the evolution of cellular organization.
• Those microspheres better able to incorporate
molecules and energy would have tended to
persist longer than others.
HOW CELLS AROSE
• Because RNA molecules can behave as
enzymes, catalyzing there own assembling,
perhaps they could have served as early
hereditary molecules as well.
A CLOCK OF BIOLOGICAL TIME
Invasion of land
by animals
Humans appear
Invasion of
land by plants
Oldest
multicellular
organisms
4 BILLION YEARS
Oldest known rocks
Midnight
1 BILLION
YEARS
Morning
Afternoon
Oldest
definite
fossils of
eukaryotes
Noon
2 BILLION YEARS
Appearance of
oxygen in atmosphere
3 BILLION
YEARS
Oldest definite fossils
of prokaryotes
THE SIMPLEST ORGANISMS
• Prokaryotes have been plentiful on earth for
over 2.5 billion years.
• Prokaryotes today are the simplest and most
abundant form of life on earth.
THE SIMPLEST ORGANISMS
• Prokaryotes occupy an important place in
the web of life on earth.
• They play a key role in cycling minerals within the
earth’s ecosystems.
• Photosynthetic bacteria were largely responsible
for introducing oxygen into the earth’s
atmosphere.
• Bacteria are responsible for some of the most
deadly animal and plant diseases.
11
THE SIMPLEST ORGANISMS
• Prokaryotes are small and simply organized.
• They are single-celled and lack a nucleus.
• Their single circle of DNA is not confined by a
nuclear membrane.
• Both bacteria and archaea are prokaryotes.
PROKARYOTES COME IN MANY SHAPES
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Spherical
Spirally-coiled
Streptococcus
Spirillum
Flagellated
Filamentous
Stalked
Helicobacter pylori
Cyanobacteria
Gliding bacteria
Rod-shaped
E.coli
(E. coli): © Getty RF; (Streptococcus): © S. Lowry/University Ulster/Getty Images; (Spirillum): © John D. Cunningham/Visuals Unlimited; (Helicobacter): © CAMR/A. Barry
Dowsett/Photo Researchers, Inc.; (Cyanobacteria): © Dr. James Richardson/Visuals Unlimited, Inc.; (Gliding): © Phototake, Inc.
PROKARYOTE STRUCTURE
• A cell wall surrounds the prokaryotic cell’s
plasma membrane.
• The plasma membrane of bacteria is
encased within a cell wall of peptidoglycan.
• In some bacteria, the peptidoglycan layer is thin
and covered over by an outer membrane of
lipopolysaccharide.
• Bacteria who have this layer are gramnegative.
• Bacteria who lack this layer are gram-positive.
PROKARYOTE STRUCTURE
• Outside the cell wall and membrane, many
bacteria have a gelatinous layer called a
capsule.
• Many kinds of bacteria have long,
threadlike outgrowths, called flagella, that
are used in swimming.
• Some bacteria also possess shorter
outgrowths, called pili (singular, pilus) that
help the cell to attach to surfaces or other
cells.
PROKARYOTE STRUCTURE
• Under harsh conditions, some bacteria form
thick-walled endospores around their DNA
and some cytoplasm.
• These endospores are highly resistant to
environmental stress and can be dormant for
centuries before germinating a new active
bacterium.
• For example, Clostridium botulinum can persist in
cans and bottles that have not been heated to
high enough temperature to kill the spores.
THE SIMPLEST ORGANISMS
• Prokaryotes reproduce by binary fission.
• The cell increases in size and divides in two.
• Some bacteria can exchange genetic
information by passing plasmids from one
cell to another.
• This process is called conjugation.
• A conjugation bridge forms between a donor cell
and a recipient cell.
BACTERIAL CONJUGATION
1
Donor Recipient
cell
cell
2
Plasmid
Bacterial
chromosome
Conjugation
bridge
3
4
5
THE SIMPLEST ORGANISMS
• In addition to conjugation, bacteria can
also transfer or obtain genetic material by:
• Taking up DNA from the environment
(transformation),
or
• Infection by bacterial viruses.
TABLE 16.1
PROKARYOTES COMPARED
TO EUKARYOTES
Feature
COMPARING PROKARYOTES
TO EUKARYOTES
• Unlike eukaryotes, prokaryotes:
• Lack internal
compartmentalization.
• Are very small and are unicellular
only.
• Have a single circle of DNA.
• Have simple cell division and
simple flagella.
• Have diverse metabolism.
Example
Internal compartmentalization.
Unlike eukaryotic cells, prokaryotic
cells contain no internal
compartments, no internal
membrane system, and no cell
nucleus.
Prokaryotic
cell
Cell size. Most prokaryotic cells
are only about 1 micrometer in
diameter, whereas most eukaryotic
cells are well over 10 times that
size
Prokaryotic cell
Eukaryotic cell
Unicellularity. All prokaryotes are
fundamentally single celled. Even
though some may adhere together
in a matrix or form fi laments,
their cytoplasms are not directly
interconnected, and their activities
are not integrated and coordinated,
as is the case in multicellular
eukaryotes.
Unicellular bacteria
Chromosomes. Prokaryotes do
not possess chromosomes in which
proteins are complexed with the
DNA, as eukaryotes do. Instead,
their DNA exists as a single circle in
the cytoplasm.
Cell division. Cell division in
prokaryotes takes place by binary
fission (see chapter 8). The cells
simply pinch in two. In eukaryotes,
microtubules pull chromosomes
to opposite poles during the cell
division process, called mitosis.
Prokaryotic
chromosome
Eukaryotic
chromosomes
Binary fission
in prokaryotes
Mitosis in
eukaryotes
Flagella. Prokaryotic flagella
are simple, composed of a single
fiber of protein that is spun like a
propeller. Eukaryotic flagella are
more complex structures, with a
9 + 2 arrangement of microtubules,
that whip back and forth rather
than rotate.
Simple bacterial
flagellum
Metabolic diversity. Prokaryotes
possess many metabolic abilities
that eukaryotes do not: Prokaryotes
perform several different kinds
of anaerobic and aerobic
photosynthesis; prokaryotes can
obtain their energy from oxidizing
inorganic compounds (so-called
chemoautotrophs); and prokaryotes
can fix atmospheric nitrogen.
Chemoautotrophs
COMPARING PROKARYOTES TO
EUKARYOTES
• Prokaryotes are far more metabolically
diverse than eukaryotes.
• Prokaryotes have evolved many more ways than
eukaryotes to acquire the carbon atoms and
energy necessary for growth and reproduction.
• Many are autotrophs, organisms that obtain their
carbon from inorganic CO2.
• Others are heterotrophs, organisms that obtain at
least some of their carbon from organic
molecules.
COMPARING PROKARYOTES TO
EUKARYOTES
• Photoautotrophs use
the energy of
sunlight to build
organic molecules
from CO2.
• Cyanobacteria use
chlorophyll a as a
pigment.
• Other bacteria use
bacteriochlorophyll.
COMPARING PROKARYOTES TO
EUKARYOTES
• Chemoautotrophs
obtain their energy
by oxidizing
inorganic
substances.
• Different types of
these prokaryotes use
substances including
ammonia, sulfur,
hydrogen gas, and
hydrogen sulfide.
COMPARING PROKARYOTES TO
EUKARYOTES
• Photoheterotrophs
use sunlight for
energy but obtain
carbon from
organic molecules
produced by other
organisms.
• Purple nonsulfur
bacteria are an
example.
COMPARING PROKARYOTES TO
EUKARYOTES
• Chemoheterotrophs are
the most common
prokaryote and obtain
both carbon atoms and
energy from organic
molecules.
• These types of prokaryotes
include decomposers and
most pathogens.
IMPORTANCE OF PROKARYOTES
• Prokaryotes affect our lives today in many
important ways.
• Responsible for creating the properties of earth’s
atmosphere and soil; play key ecological roles.
• Can be genetically modified to help with pestcontrol in crops and to produce therapeutic
proteins.
• Oil-degrading bacteria are used to clean up spills.
• Cause major diseases in plants and animals.
• May be used in bioterrorism.
USING BACTERIA TO CLEAN UP OIL
SPILLS
PROKARYOTIC LIFESTYLES
• Many of the archaea that survive today are
methanogens.
• They use H2 gas to reduce CO2 to CH4
• They are strict anaerobes and are found in
swamps and marshes where other microbes have
consumed all of the oxygen.
• Other archaea are extremophiles, which live
in unusually harsh environments.
• For example, thermoacidophiles favor hot, acidic
springs.
THERMOACIDOPHILES LIVE IN HOT
SPRINGS
29
KINGDOM BACTERIA
• Most prokaryotes
are members of the
Kingdom Bacteria.
• Cyanobacteria are
among the most
prominent of the
photosynthetic
bacteria.
• Other forms of bacteria are nonphotosynthetic.
• Most bacteria are unicellular but some form layers
on the surface of a substrate.
• This layer of cells is called a biofilm.
THE STRUCTURE OF VIRUSES
• Viruses do not satisfy all of the criteria for
being considered “alive” because they
possess only a portion of the properties of
living organisms.
• Viruses are literally segments of DNA (or
sometimes RNA) wrapped in a protein coat.
• They cannot reproduce on their own.
THE STRUCTURE OF VIRUSES
• Viruses are extremely
small, with most
detectable only through
the use of an electron
microscope.
• Most viruses form a protein
sheath, or capsid, around
a nucleic acid core.
• Many viruses form a
membranelike envelope
around the capsid.
RNA
Proteins
(b) Tobacco mosaic virus (TMV)
THE STRUCTURE OF BACTERIAL, PLANT,
AND ANIMAL VIRUSES
DNA
Head
Capsid
(proteins heath)
RNA
Envelope
protein
Neck
Whiskers
Envelope
Tail
Proteins
Enzyme
Base
plate
RNA
Tail fiber
(a) Bacteriophage
Capsid
(b) Tobacco mosaic virus (TMV)
(c) Human immunodeficiency virus (HIV)
HOW BACTERIOPHAGES ENTER
PROKARYOTIC CELLS
DNA
• Bacteriophages are
viruses that infect
bacteria.
• They are structurally and
functionally diverse.
• When the virus kills the
infected host in which it is
replicating, this is called a
lytic cycle.
Head
Capsid
(proteins heath)
Neck
Whiskers
Tail
Base
plate
Tail fiber
(a) Bacteriophage
• At other times the virus integrates their nucleic
acid into the host genome but does not replicate.
• This is called the lysogenic cycle.
A T4 BACTERIOPHAGE
T4 bacteriophage
DNA
Bacterial
chromosome
Bacterial
cell
(b)
(a)
HOW ANIMAL VIRUSES
ENTER CELLS
• Animal viruses
typically enter their
host cells by
membrane fusion.
• A diverse array of
viruses occur
among animals.
• One example of an
animal virus is human
immunodeficiency
virus (HIV).
Envelope
protein
Envelope
Capsid
Enzyme
RNA
(c) Human immunodeficiency virus (HIV)
HOW ANIMAL VIRUSES
ENTER CELLS
• HIV infects only certain cells.
• Macrophages are attacked by HIV.
• The normal role of macrophages is to pick up
cellular debris.
DISEASE VIRUSES
• Emerging viruses are viruses that originate in
one organism and pass to another
organism.
• For example, influenza is fundamentally a bird
virus and smallpox is thought to have passed from
cows to humans.
• Air travel and world trade in animals make
emerging viruses a greater threat today than in
the past.
DISEASE VIRUSES
• The influenza virus is one of the most lethal
viruses in human history.
• New strains of influenza usually originate in
the Far East, where influenza hosts are
common.
• The most common hosts are ducks, chickens, and
pigs.
• These hosts live in close proximity to each other
and to humans.