Chapter 27 Prokaryotes
Overview: They’re (Almost) Everywhere!
• Prokaryotes were the earliest organisms on Earth.
• Today, they still dominate the biosphere.
• Their collective biomass outweighs all eukaryotes combined at
• More prokaryotes inhabit a handful of fertile soil or the mouth or
skin of a human than the total number of people who have ever
• Prokaryotes are wherever there is life.
• They thrive in habitats that are too cold, too hot, too salty, too acidic,
or too alkaline for any eukaryote.
• Prokaryotes have even been discovered in rocks two miles
below the surface of the Earth.
• Why have these organisms dominated the biosphere since the origin
of life on Earth?
• Prokaryotes display diverse adaptations that allow them to
inhabit many environments.
• They have great genetic diversity.
• Prokaryotes are classified into two domains, Bacteria and Archaea,
which differ in structure, physiology and biochemistry.
Concept 27.1 Structural, functional, and genetic adaptations
contribute to prokaryotic success
Prokaryotes are small.
• Most prokaryotes are unicellular.
• Some species may aggregate transiently or form true colonies,
showing division of labor between specialized cell types. • Most prokaryotes have diameters in the range of 1–5 ?m, compared
to 10–100 ?m for most eukaryotic cells.
• The largest prokaryote discovered so far has a diameter of 750
• The most common shapes among prokaryotes are spheres (cocci),
rods (bacilli), and helices.
Nearly all prokaryotes have a cell wall external to the plasma
• In nearly all prokaryotes, a cell wall maintains the shape of the cell,
affords physical protection, and prevents the cell from bursting in a
• In a hypertonic environment, most prokaryotes lose water and
plasmolyze, like other walled cells.
• Severe water loss inhibits the reproduction of prokaryotes,
which explains why salt can be used to preserve foods.
• Most bacterial cell walls contain peptidoglycan, a polymer of modified
sugars cross-linked by short polypeptides.
• The walls of archaea lack peptidoglycan.
• The Gram stain is a valuable tool for identifying specific bacteria
based on differences in their cell walls.
• Gram-positive bacteria have simple cell walls with large
amounts of peptidoglycans.
• Gram-negative bacteria have more complex cell walls with less
• An outer membrane on the cell wall of gram-negative
cells contains lipopolysaccharides, carbohydrates bonded
• Among pathogenic bacteria, gram-negative species are generally
more deadly than gram-positive species. • The lipopolysaccharides on the walls of gram-negative bacteria
are often toxic, and the outer membrane protects the pathogens
from the defenses of their hosts.
• Gram-negative bacteria are commonly more resistant than
gram-positive species to antibiotics because the outer
membrane impedes entry of the drugs.
• Many antibiotics, including penicillin, inhibit the synthesis of cross-
links in peptidoglycans, preventing the formation of a functional wall,
especially in gram-positive species.
• These drugs cripple many species of bacteria, without affecting
human and other eukaryote cells that do not synthesize
• Many prokaryotes secrete another sticky protective layer of
polysaccharide or protein, the capsule, outside the cell wall.
• Capsules allow cells to adhere to their substratum.
• They may increase resistance to host defenses.
• They glue together the cells of those prokaryotes that live as
• Another way for prokaryotes to adhere to one another or to the
substratum is by surface appendages called fimbriae and pili.
• Fimbriae are usually more numerous and shorter than pili.
• These structures can fasten pathogenic bacteria to the mucous
membranes of the host.
• Sex pili are specialized for holding two prokaryote cells together
long enough to transfer DNA during conjugation.
Many prokaryotes are motile.
• About half of all prokaryotes are capable of directional movement.
• Some species can move at speeds exceeding 50 ?m/sec,
about 100 times their body length per second. • The beating of flagella scattered over the entire surface or
concentrated at one or both ends is the most common method of
• The flagella of prokaryotes differ in structure and function from
those of eukaryotes.
• In a heterogeneous environment, many prokaryotes are capable of
taxis, movement toward or away from a stimulus.
• Prokaryotes that exhibit chemotaxis respond to chemicals by
changing their movement patterns.
• Solitary E. coli may exhibit positive chemotaxis toward other
members of their species, enabling the formation of colonies.
The cellular and genomic organization of prokaryotes is
fundamentally different from that of eukaryotes.
• The cells of prokaryotes are simpler than those of eukaryotes in both
internal structure and genomic organization.
• Prokaryotic cells lack the complex compartmentalization found in
• Instead, prokaryotes use specialized infolded regions of the
plasma membrane to perform many metabolic functions,
including cellular respiration and photosynthesis.
• Prokaryotes have smaller, simpler genomes than eukaryotes.
• On average, a prokaryote has only about one-thousandth as
much DNA as a eukaryote.
• In the majority of prokaryotes, the genome consists of a ring of DNA
with few associated proteins.
• The prokaryotic chromosome is located in the nucleoid region.
• Prokaryotes may also have smaller rings of DNA called plasmids,
which consist of only a few genes. • Prokaryotes can survive in most environments without their
plasmids because their chromosomes program all essential
• Plasmid genes provide resistance to antibiotics, direct
metabolism of unusual nutrients, and other special contingency
• Plasmids replicate independently of the chromosome and can
be transferred between partners during conjugation.
• Although the general processes for DNA replication and translation of
mRNA into proteins are fundamentally alike in eukaryotes and
prokaryotes, some of the details differ.
• For example, prokaryotic ribosomes are slightly smaller than
the eukaryotic version and differ in protein and RNA content.
• These differences are great enough that selective antibiotics,
including tetracycline and erythromycin, bind to prokaryotic
ribosomes to block protein synthesis in prokaryotes but not in
Populations of prokaryotes grow and adapt rapidly.
• Prokaryotes have the potential to reproduce quickly in a favorable
• Prokaryotes reproduce asexually via binary fission, synthesizing DNA
• While most prokaryotes have generation times of 1–3 hours,
some species can produce a new generation in 20 minutes
under optimal conditions.
• A single cell in favorable conditions will produce a large colony
of offspring very quickly.
• Of course, prokaryotic reproduction is limited because cells
eventually exhaust their nutrient supply, accumulate metabolic
wastes, or are consumed by other organisms. • Some bacteria form resistant cells called endospores when an
essential nutrient is lacking in the environment.
• A cell replicates its chromosome and surrounds one
chromosome with a durable wall to form the endospore.
• The original cell then disintegrates to leave the endospore
• An endospore is resistant to all sorts of trauma.
• Endospores can survive lack of nutrients and water, extreme
heat or cold, and most poisons.
• Most endospores can survive in boiling water.
• Endospores may be dormant for centuries or more.
• When the environment becomes more hospitable, the
endospore absorbs water and resumes growth.
• Sterilization in an autoclave kills endospores by heating them to
120°C under high pressure.
• Lacking meiotic sex, mutation is the major source of genetic variation
• With generation times of minutes or hours, prokaryotic populations
can adapt very rapidly to environmental changes as natural selection
favors gene mutations that confer greater fitness.
• As a consequence, prokaryotes are important model organisms for
scientists who study evolution in the laboratory.
• Richard Lenski and his colleagues have maintained colonies of E. coli
through more than 20,000 generations since 1988.
• The researchers regularly freeze samples of the colonies and
later thaw them to compare their characteristics to those of their
• Such comparisons have revealed that the colonies in Lenski’s
laboratory can grow 60% faster than those that were frozen in
1988. • Lenski’s team is studying the genetic changes underlying the
adaptation of the bacteria to their environment.
• By measuring RNA production, the researchers found that two
separate colonies showed changes in expression of the same
59 genes, compared to the original colonies.
• The direction of change—increased or decreased
expression—was the same for every gene.
• This is an apparent case of parallel adaptive evolution.
• Horizontal gene transfer also facilitates rapid evolution of
• Conjugation can permit exchange of a plasmid containing a few
genes or large groups of genes.
• Once the transferred genes are incorporated into the
prokaryote’s genome, they are subject to natural selection.
• Horizontal gene transfer is a major force in the long-term
evolution of pathogenic bacteria.
Concept 27.2 A great diversity of nutritional and metabolic
adaptations have evolved in prokaryotes
• Organisms can be categorized by their nutrition, based on how they
obtain energy and carbon to build the organic molecules that make
up their cells.
• Nutritional diversity is greater among prokaryotes than among all
• Every type of nutrition observed in eukaryotes is found in
prokaryotes, along with some nutritional modes unique to
• Organisms that obtain energy from light are phototrophs.
• Organisms that obtain energy from chemicals in their environment
• Organisms that need only CO2 as a carbon source are autotrophs. • Organisms that require at least one organic nutrient—such as
glucose—as a carbon source are heterotrophs.
• These categories of energy source and carbon source can be
combined to group prokaryotes according to four major modes of
1. Photoautotrophs are photosynthetic organisms that harness
light energy to drive the synthesis of organic compounds from
• Among the photoautotrophic prokaryotes are the
• Among the photosynthetic eukaryotes are plants and
2. Chemoautotrophs need only CO2 as a carbon source but
obtain energy by oxidizing inorganic substances.
• These substances include hydrogen sulfide (H2S),
ammonia (NH3), and ferrous ions (Fe2+) among others.
• This nutritional mode is unique to prokaryotes.
3. Photoheterotrophs use light to generate ATP but obtain their
carbon in organic form.
• This mode is restricted to a few marine prokaryotes.
4. Chemoheterotrophs must consume organic molecules for both
energy and carbon.
• This nutritional mode is found widely in prokaryotes, protists,
fungi, animals, and even some parasitic plants.
• Prokaryotic metabolism also varies with respect to oxygen.
• Obligate aerobes require O2 for cellular respiration.
• Facultative anaerobes will use O2 if present but can also grow
by fermentation in an anaerobic environment.
• Obligate anaerobes are poisoned by O2 and use either
fermentation or anaerobic respiration. • In anaerobic respiration, inorganic molecules other than
O2 accept electrons from electron transport chains.
• Nitrogen is an essential component of proteins and nucleic acids in all
• Eukaryotes are limited in the forms of nitrogen they can use.
• In contrast, diverse prokaryotes can metabolize a wide variety
of nitrogenous compounds.
• Nitrogen-fixing prokaryotes convert N2 to NH3, making atmospheric
nitrogen available to themselves (and eventually to other organisms)
for incorporation into organic molecules.
• Nitrogen-fixing cyanobacteria are the most self-sufficient of all
• They require only light energy, CO2, N2, water, and some
minerals to grow.
• Prokaryotes were once thought of as single-celled individualists.
• Microbiologists now recognize that cooperation between prokaryotes
allows them to use environmental resources they cannot exploit as
• Cooperation may involve specialization in cells of a prokaryotic
• For example, the cyanobacterium Anabaena forms filamentous
colonies with specialized cells to carry out nitrogen fixation.
• Photosynthesis produces O2, which inactivates the enzymes
involved in nitrogen fixation.
• Most cells in the filament are photosynthetic, while a few
specialized cells called heterocysts carry out only nitrogen
• A heterocyst is surrounded by a thickened cell wall that
restricts the entry of oxygen produced by neighboring
photosynthetic cells. • Heterocysts transport fixed nitrogen to neighboring cells
in exchange for carbohydrates.
• In some prokaryotic species, metabolic cooperation occurs in
surface-coating colonies known as biofilms.
• Cells in a colony secrete signaling molecules to recruit nearby
cells, causing the colony to grow.
• Once the colony is sufficiently large, the cells begin producing
proteins that adhere the cells to the substrate and to one
• Channels in the biofilms allow nutrients to reach cells in the
interior and allow wastes to be expelled.
• In some cases, different species of prokaryotes may cooperate.
• For example, sulfate-consuming bacteria and methane-
consuming archaea coexist in ball-shaped aggregates in the
mud of the ocean floor.
• The bacteria use the archaea’s waste products.
• In turn, the bacteria produce compounds that facilitate methane
consumption by the archaea.
• Each year, these archaea consume an estimated 300 billion kg
of methane, a major greenhouse gas.
Concept 27.3 Molecular systematics is illuminating prokaryotic