Prokaryotes lack internal, membrane-bound organelles but exhibit diverse internal structures and some differentiation (Fig. 5.5). In bacteria, the cytoplasm is enclosed by a cytoplasmic membrane consisting of ester-linked, straight-chained fatty acids. The lipids comprising the membrane form a bilayer, with nonpolar hydrophobic ends associating with each other and polar, hydrophilic ends externalized. Cytoplasmic membranes in Archaea contain only a single lipid layer consisting of ether-linked, branched aliphatic acids. For all prokaryotes, the cyto-plasmic membrane provides an important link between the cell and its environment and contains many proteins required for import of nutrients, export of waste products, and production of extracellular enzymes for breakdown of high-molecular-weight compounds. It is also involved in generating energy in respiring cells through oxidative phosphorylation and can provide protection through toxin or antibiotic degrading enzymes.
The bacterial chromosome is present as a single, double-stranded, covalently closed, circular DNA molecule forming a nucleoid. Additional genetic material may be present as one or many small DNA molecules, termed plasmids, having a similar structure. These can be transmitted vertically (from generation to generation) and horizontally or laterally, between different bacterial strains. Other mechanisms of genetic exchange in prokaryotes are transformation, which involves direct uptake of DNA and incorporation of genes into the host chromosome, and transduction, in which gene transfer is mediated by a bacteriophage.
Proteins are synthesized within the cytoplasm by thousands of ribosomes, often forming structures termed polysomes and attached to mRNA. Storage products can also accumulate in cells. Examples include poly-|3-hydroxybutyrate (a glucose polymer), polyphosphate, and glycogen, while the sulfur oxidizer Beggiatoa stores elemental sulfur, which can be seen as yellow granules within the cell.
Some intracellular structures are associated with specific metabolic processes. Autotrophic bacteria possess carboxysomes, which are particulate bodies involved in fixing carbon dioxide. Photosynthetic bacteria possess complex intracellular membrane structures, which are the site of the energy-trapping, photosynthetic processes. Photosynthetic bacteria show relatively close evolutionary relationships to other functional groups with similar complex membrane structures involved in other reactions, including ammonia, methane, and iron oxidation. Membranes can occur in distinctive patterns, for example, forming a layer within the cytoplasmic membrane or in an equatorial plane, and are diagnostic for some groups.
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