Chapter 27 Prokaryotes and the Origins of Metabolic Diversity
They’re (almost) everywhere! An overview of prokaryotic life
• Prokaryotes were the earliest organisms on Earth and evolved alone for 1.5 billion years.
• Live in diverse environments.
• Most are benign or beneficial but others cause disease.
• Diverse in structure and in metabolism.
• About 5,000 species are known, but actual diversity may range from about 400,000 to 4
million species.
Bacteria and archaea are the two main branches of prokaryote evolution
The Structure, Function, and Reproduction of Prokaryotes
Most prokaryotes are unicellular.
The most common shapes among prokaryotes are spheres (cocci), rods (bacilli), and
helices.
Nearly all prokaryotes have a cell wall external to the plasma membrane
• Most bacterial cell walls contain peptidoglycan, a polymer of modified sugars crosslinked by short polypeptides.
Gram stain- used identify bacteria based on differences in their cell walls.
• Gram-positive bacteria have simpler cell walls, with large amounts of peptidoglycans.
• Gram-negative bacteria have more complex cell walls and less peptidoglycan.
• Generally more threatening. The outer membrane protects against host
defenses. More resistant to antibiotics.
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Many prokaryotes secrete a sticky protective layer, the capsule, outside the cell wall.
Pili- an appendage used to adhere to surfaces or other prokaryotes. Also used for
conjugation.
Many prokaryotes are motile
• Flagella
• Taxis- movement toward or away from a stimulus (chemicals, light, magnetic fields).
The cellular and genomic organization of prokaryotes is fundamentally different from that of
eukaryotes
• Prokaryotic cells lack a nucleus enclosed by membrane. They have smaller, simpler
genomes than eukaryotes and have small rings of DNA, plasmids, that consist of only a
few genes.
Populations of prokaryotes grow and adapt rapidly
• Prokaryotes reproduce only asexually via binary fission, synthesizing DNA almost
continuously.
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Transformation- a cell can absorb and integrate fragments of DNA from their
environment.
• Conjugation- one cell directly transfers genes to another cell.
• Transduction- viruses transfer genes between prokaryotes.
Endospore- forms when a cell replicates its chromosome and surrounds one chromosome with
a durable wall.
Can survive lack of nutrients and water, extreme heat or cold, and most poisons.
May be dormant for centuries or more.
When the environment becomes more hospitable, the endospore absorbs water and
resumes growth.
Nutrition and Metabolic Diversity
Prokaryotes can be grouped into four categories according to how they obtain energy and
carbon
• Species that use light energy are phototrophs.
• Species that obtain energy from chemicals in their environment are chemotrophs.
• Organisms that need only CO2 as a carbon source are autotrophs.
• Organisms that require at least one organic nutrient as a carbon source are
heterotrophs.
• Photoautotrophs- photosynthetic organisms that harness light energy to drive the
synthesis of organic compounds from carbon dioxide.
• Ex: cyanobacteria.
• Chemoautotrophs need only CO2 as a carbon source, but they obtain energy by
oxidizing inorganic substances, rather than light.
• These substances include hydrogen sulfide (H2S), ammonia (NH3), and ferrous
ions (Fe2+) among others.
• Photoheterotrophs use light to generate ATP but obtain their carbon in organic form.
• Chemoheterotrophs must consume organic molecules for both energy and carbon.
Prokaryotes are responsible for the key steps in the cycling of nitrogen through ecosystems.
• Some convert ammonium (NH4+) to nitrite (NO2-).
• Others convert nitrite or nitrate (NO3-) to N2 (gas).
• Nitrogen fixation- convert N2 to NH4+, making atmospheric nitrogen available to other
organisms for incorporation into organic molecules.
Obligate aerobes- require O2 for cellular respiration.
Facultative anerobes- 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.
Photosynthesis evolved early in prokaryotic life
• The very first prokaryotes were heterotrophs, but photosynthesis capabilities soon
followed.
A Survey of Prokaryotic Diversity
Molecular systematics is leading to phylogenetic classification of prokaryotes
• Carl Woese clustered prokarotes into taxonomic groups based on comparisons of
nucleic acid sequences- small-subunit ribosomal RNA (SSU-rRNA)
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Researchers are identifying a great diversity of archaea in extreme environments and in the
oceans
• Archaea are extremophiles, “lovers” of extreme environments.
Methanogens obtain energy by using CO2 to oxidize H2 replacing methane as a waste.
• They live in swamps and marshes where other microbes have consumed all the oxygen.
• Methanogens are important decomposers in sewage treatment.
• Other methanogens live in the anaerobic guts of herbivorous animals, playing an
important role in their nutrition.
Extreme halophiles live in such saline places as the Great Salt Lake and the Dead Sea.
Extreme thermophiles thrive in hot environments, 60oC-80oC, hot sulfur springs or at 105oC
water near deep-sea hydrothermal vents
Most known prokarotes are bacteria
Proteobacteria, Chlamydias, Spirochetes, Gram-Positive Bacteria, Cyanobacteria
The Ecological Impact of Prokaryotes
Prokaryotes are indispensable links in the recycling of chemical elements in ecosystems
• Decomposers
• Metabolize inorganic molecules containing elements such as iron, sulfur, nitrogen, and
hydrogen
• Restore oxygen to the atmosphere
• Fix nitrogen
Many prokaryotes are symbiotic
• Commensalism, one symbiont receives benefits while the other is not harmed or helped
by the relationship.
• Parasitism, one symbiont, the parasite, benefits at the expense of the host.
• Mutualism, both symbionts benefit.
Pathogenic prokaryotes cause many human diseases
• Some pathogens are opportunistic. These are normal residents of the host, but only
cause illness when the host’s defenses are weakened.
Exotoxins are proteins secreted by prokaryotes.
C. botulinum, V. cholerae, E. coli
Endotoxins are components of the outer membranes of some gram-negative bacteria.
Salmonella
Humans use prokaryotes in research and technology
• The application of organisms to remove pollutants from air, water, and soil is
bioremediation.
• Decomposers treat human sewage.
• Soil bacteria have been developed to decompose petroleum products at the site of oil
spills or to decompose pesticides.
• The chemical industry produces acetone, butanol, and other products from bacteria.
• The pharmaceutical industry cultures bacteria to produce vitamins and antibiotics.
• The food industry used bacteria to convert milk to yogurt and various kinds of cheese.
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