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CH 27
Bacteria & Archaea Contributing factors to Prokaryotic success Earth’s first organisms were likely prokaryotes. Most prokaryotes are unicellular, although some species form colonies. Most prokaryotic cells are 0.5–5 μm, much smaller than the 10–100 μm of many eukaryotic cells. Prokaryotic cells have a variety of shapes. The three most common shapes are spheres (cocci), rods (bacilli), and spirals.
Cell-Surface Structures An important feature of nearly all prokaryotic cells is their cell wall, which maintains cell shape, protects the cell, and prevents it from bursting in a hypotonic environment. Eukaryote cell walls are made of cellulose or chitin. Bacterial cell walls contain peptidoglycan, a network of sugar polymers cross-linked by polypeptides.
Archaea contain polysaccharides and proteins but lack peptidoglycan. Scientists use the Gram stain to classify bacteria by cell wall composition. Gram-positive bacteria have simpler walls with a large amount of peptidoglycan. Gram-negative bacteria have less peptidoglycan and an outer membrane that can be toxic.
Motility In a heterogeneous environment, many bacteria exhibit taxis, the ability to move toward or away from a stimulus. Chemotaxis is the movement toward or away from a chemical stimulus. Most motile bacteria propel themselves by flagella scattered about the surface or concentrated at one or both ends. Flagella of bacteria, archaea, and eukaryotes are composed of different proteins and likely evolved independently. Oscillatoria VIDEO Flagella VIDEO
Origins of Bacteria Flagella Bacterial flagella are composed of a motor, hook, and filament. Many of the flagella’s proteins are modified versions of proteins that perform other tasks in bacteria. Flagella likely evolved as existing proteins were added to an ancestral secretory system. This is an example of exaptation, where existing structures take on new functions through descent with modification.
Internal Organization
Prokaryotic cells usually lack complex compartmentalization. Some prokaryotes do have specialized membranes that perform metabolic functions. These are usually infoldings of the plasma membrane.
The chromosome is not surrounded by a membrane; it is located in the nucleoid region. Some species of bacteria also have smaller rings of DNA called plasmids.
There are some differences between prokaryotes and eukaryotes in DNA replication, transcription, and translation. These allow people to use some antibiotics to inhibit bacterial growth without harming themselves.
Reproduction & Adaptation Prokaryotes reproduce quickly by binary fission and can divide every 1–3 hours. Key features of prokaryotic reproduction: They are small They reproduce by binary fission They have short generation times Many prokaryotes form metabolically inactive endospores, which can remain viable in harsh conditions for centuries.
Their short generation time allows prokaryotes to evolve quickly. For example, adaptive evolution in a bacterial colony was documented in a lab over 8 years Prokaryotes are not “primitive” but are highly evolved.
Rapid reproduction, mutation, and genetic recombination promote genetic diversity in prokaryotes Prokaryotes have considerable genetic variation. Three factors contribute to this genetic diversity: Rapid reproduction Mutation Genetic recombination
Rapid Reproduction & Mutation Prokaryotes reproduce by binary fission, and offspring cells are generally identical. Mutation rates during binary fission are low, but because of rapid reproduction, mutations can accumulate rapidly in a population. High diversity from mutations allows for rapid evolution.
Genetic Recombination Genetic recombination, the combining of DNA from two sources, contributes to diversity.
Methanogens live in swamps and marshes and produce methane as a waste product. Methanogens are strict anaerobes and are poisoned by O2. In recent years, genetic prospecting has revealed many new groups of archaea. Some of these may offer clues to the early evolution of life on Earth.
Prokaryotes play crucial roles in the biosphere Prokaryotes are so important that if they were to disappear the prospects for any other life surviving would be dim. Prokaryotes play a major role in the recycling of chemical elements between the living and nonliving components of ecosystems. Chemoheterotrophic prokaryotes function as decomposers, breaking down dead organisms and waste products. Prokaryotes can sometimes increase the availability of nitrogen, phosphorus, and potassium for plant growth.
Ecological Interactions In mutualism, both symbiotic organisms benefit. In commensalism, one organism benefits while neither harming nor helping the other in any significant way. In parasitism, an organism called a parasite harms but does not kill its host. Parasites that cause disease are called pathogens.
Beneficial & Harmful Impacts Mutualistic Bacteria Human intestines are home to about 500–1,000 species of bacteria Many of these are mutalists and break down food that is undigested by our intestines Pathogenic Bacteria Prokaryotes cause about half of all human diseases For example, Lyme disease is caused by a bacterium and carried by ticks
