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Ch. 27: Bacteria and Archea (Metabolic Adaptations (Organisms that obtain…
Ch. 27: Bacteria and Archea
Structure
Internal Organization and DNA
A typical prokaryotic cell may also have much smaller rings of independently replicating DNA molecules called plasmids, most carrying only a few genes.
Prokaryotes lack a nucleus. Their chromosome is located in the nucleoid, a region of cytoplasm that is not enclosed by a membrane.
Prokaryotes generally have less DNA, circular chromosomes, and fewer proteins.
Some prokaryotic cells do have specialized membranes that perform metabolic functions. These membranes are usually infoldings of the plasma membrane.
Prokaryotic cells lack the complex compartmentalization associated with the membrane-enclosed organelles found in eukaryotic cells.
Prokaryotic cells have a variety of shapes.
Cocci: spherical prokaryotes that can appear in pairs, chains, or clusters.
Bacilli: rod-shaped prokaryotes that are usually solitary or can be arranged in chains.
Spirilla: spiral prokaryotes range from comma-like shapes to loose coils.
Motility
About half of all prokaryotes are capable of taxis, a directed movement toward or away from a stimulus.
The most common structure that allow prokaryotes to move are flagella. Flagella may be scattered over the entire surface of the cell or concentrated at one or both ends.
Prokaryotic flagella differ from eukaryotic flagella in size, molecular composition, and their mechanism of propulsion.
Prokaryotic cells typically have diameters of 0.5-5 micrometers.
Prokaryotes are well organized, achieving all of an organism's life functions with a single cell.
Most prokaryotes are unicellular, although the cells of some species remain attached to each other after cell division.
Cell Surface Structure
The cell wall of many prokaryotes is surrounded by a sticky layer of polysaccharide or protein. This layer is called a capsule if it is dense and well defined or a slime layer if it is not as well organized.
To withstand harsh conditions, certain bacteria develop resistant cells called endospores when they lack water or essential nutrients.
Archaeal cell walls contain a variety of polysaccharides and proteins but lack peptidoglycan.
Some prokaryotes stick to their substrate or to one another by means of hairlike appendages called fimbriae
Most bacterial cell walls contain peptidoglycan, a polymer composed of modified sugars cross-linked by short polypeptides.
Gram Stain
Gram staining is a technique developed by Hans Christian Gram that allows scientists to categorize many bacterial species according to differences in cell wall composition.
Samples are stained with crystal violet dye and iodine, then rinsed in alcohol, and finally stained with a red dye such as safranin that enters the cells and binds to DNA.
The structure of a bacterium's cell wall determines the staining response.
Gram-positive bacteria have relatively simple walls composed of a thick layer of peptidoglycan.
The cell walls of gram-negative bacteria have less peptidoglycan and are structurally more complex, with an outer membrane that contains lipopolysaccharides.
Gram-positive stains the cell purple and gram-negative stains the cell pink or red.
Cell wall maintains cell shape, protects the cell, and prevents it from bursting in a hypotonic environment.
Fimbriae are usually shorted and more numerous than pili, appendages that pull together prior to DNA transfer from one cell to the other; pili are sometimes referred to as sex pili.
Reproduction
Many prokaryotic species can reproduce quickly by binary fission, leading to the formation of extremely large populations.
By binary fission, a single prokaryotic cell divides into 2 cells, which then divide into 4, 8, 16, and so on.
Many prokaryotes can reproduce quickly in favorable environments.
Three factors gave rise to high levels of genetic diversity in prokaryotes.
Because prokaryotes can often proliferate rapidly, mutations can quickly increase a population's genetic variation. As a result, prokaryotic populations often can evolve in short periods of time in response to changing conditions.
Genetic Recombustion
Genetic diversity in prokaryotes also can arise by recombination of the DNA from two different cells (via transformation, transduction, or conjugation). By transferring advantageous alleles, such as ones for antibiotic resistance, recombination can promote adaptive evolution in prokaryotic evolutions.
In transformation, the genotype and possibly phenotype of a prokaryotic cell are altered by the uptake of foreign DNA from its surroundings.
In transduction, phages carry prokaryotic genes from one host cell to another.
In a process called conjugation, DNA is transferred between two prokaryotic cells (usually the same species) that are temporarily joined.
The ability to form pili and donate DNA during conjugation results from the presence of a particular piece of DNA called the F factor (F for fertility).
The F factor in its plasmid form is called the F plasmid. Cells containing the F plasmid function as DNA donors during conjugation.
R plasmids (R for resistance) are resistant genes carried by plasmids.
Prokaryotes do not reproduce sexually which allows for the variation to result to rapid reproduction.
Metabolic Adaptations
Organisms that obtain energy from light are called phototrophs.
Organisms that obtain energy from chemicals are called chemotrophs.
Like all organisms prokaryotes can be categorized by how they obtain energy and the carbon used in building the organic molecules that make up cells.
Organisms that need only CO2 or related compounds as a carbon source are called autotrophs.
Heterotrophs require at least one organic nutrient, such as glucose, to make other organic compounds.
The four modes of nutrition include photoautotrophy, chemoautotrophy, photoheterotrophy, and chemoheterotrophy.
Obligate anaerobes require oxygen.
Obligate anaerobes are poisoned by oxygen.
Facultative anaerobes can survive with or without oxygen.