1. Prokaryotes: The First Single-Celled Creatures
Chapter 16

2. 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.

3. 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.

4. 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.

5. A chemical process involving bubbles may have preceded the origin of life
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When the bubbles
persisted long enough
to rise to the surface,
they popped, releasing
their contents to the air.
Bombarded by the sun's ultraviolet
radiation, lightning, and other energy
sources, the simple organic molecules
released from the bubbles reacted to
form more complex organic molecules. 2 2
The gases, concentrated
inside the bubbles, reacted
to produce simple organic
molecules. 5 5
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. 1 1
Volcanoes erupted
under the sea, releasing
gases enclosed in bubbles.

6. 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.

7. 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.

8. 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.

9. 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
Afternoon
Morning
Oldest
definite
fossils of
eukaryotes
3 BILLION
YEARS
Noon
2 BILLION YEARS
Oldest definite fossils
of prokaryotes
Appearance of
oxygen in atmosphere

10. 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.

11. 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.

12. 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.

13. Prokaryotes come in many shapes
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Rod-shaped
Spherical
Spirally-coiled
E.coli
Streptococcus
Spirillum
Flagellated
Filamentous
Stalked
Helicobacter pylori
Cyanobacteria
Gliding bacteria
(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.

14. 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 gram-negative.
Bacteria who lack this layer are gram-positive.

15. 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.

16. 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.

17. 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.

18. Bacterial conjugation1
Donor
cell
Recipient
cell
Conjugation
bridge 2 3
Plasmid
Bacterial
chromosome 4 5

19. 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.

20. Comparing Prokaryotes to Eukaryotes
Internal compartmentalization.
Unlike eukaryotic cells, prokaryotic
cells contain no internal
compartments, no internal
membrane system, and no cell
nucleus.
Cell size. Most prokaryotic cells
are only about 1 micrometer in
diameter, whereas most eukaryotic
cells are well over 10 times that
size
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.
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.
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.
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.
Eukaryotic cell
Prokaryotic cell
Unicellular bacteria
Eukaryotic
chromosomes
Prokaryotic
chromosome
Mitosis in
eukaryotes
Binary fission
in prokaryotes
Simple bacterial
flagellum
Chemoautotrophs
cell
Example
Feature
TABLE 16.1
PROKARYOTES COMPARED
TO EUKARYOTES
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.

21. 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.

22. 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.

23. 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.

24. 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.

25. 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.

26. 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 pest-control 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.

27. Using bacteria to clean up oil spills

28. 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.

29. Thermoacidophiles live in hot springs

30. 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.

31. 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.

32. 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.
(b) Tobacco mosaic virus (TMV)
Proteins
RNA

33. The structure of bacterial, plant, and animal viruses
DNA
Head
Capsid
(proteins heath)
Neck
Whiskers
Tail fiber
Tail
(a) Bacteriophage
Base
plate
(b) Tobacco mosaic virus (TMV)
Proteins
RNA
(c) Human immunodeficiency virus (HIV)
Envelope
protein
Capsid
Enzyme
RNA

34. How Bacteriophages Enter Prokaryotic Cells
DNA
Head
Capsid
(proteins heath)
Neck
Whiskers
Tail fiber
Tail
(a) Bacteriophage
Base
plate
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.
At other times the virus integrates their nucleic acid into the host genome but does not replicate.
This is called the lysogenic cycle.

35. A T4 bacteriophage T4 bacteriophage DNA Bacterial chromosome Bacterial
cell
(b)
(a)

36. 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).
(c) Human immunodeficiency virus (HIV)
Envelope
protein
Capsid
Enzyme
RNA

37. 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.

38. 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.

39. 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.