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Structure and Function of Bacterial Cells (page 7)

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The Plasma Membrane

The plasma membrane, also called the cytoplasmic membrane, is the most dynamic structure of a procaryotic cell. Its main function is a s a selective permeability barrier that regulates the passage of substances into and out of the cell. The plasma membrane is the definitive structure of a cell since it sequesters the molecules of life in a unit, separating it from the environment. The bacterial membrane allows passage of water and uncharged molecules up to mw of about 100 daltons, but does not allow passage of larger molecules or any charged substances except by means special membrane transport processes and transport systems.

Bacterial membranes are composed of 40 percent phospholipid and 60 percent protein. The phospholipids are amphiphilic molecules with a polar hydrophilic glycerol "head" attached via an ester bond to two nonpolar hydrophobic fatty acid tails, which naturally form a bilayer in aqueous environments. Dispersed within the bilayer are various structural and enzymatic proteins which carry out most membrane functions. At one time, it was thought that the proteins were neatly organized along the inner and outer faces of the membrane and that this accounted for the double track appearance of the membrane in electron micrographs. However, it is now known that while some membrane proteins are located and function on one side or another of the membrane, most proteins are partly inserted into the membrane, or possibly even traverse the membrane as channels from the outside to the inside. It is possible that proteins can move laterally along a surface of the membrane, but it is thermodynamically unlikely that proteins can be rotated within a membrane, which discounts early theories of how transport systems might work. The arrangement of proteins and lipids to form a membrane is called the fluid mosaic model, and is illustrated in Figure 20.


Figure 20. Fluid mosaic model of a biological membrane. In aqueous environments membrane phospholipids arrange themselves in such a way that they spontaneously form a fluid bilayer. Membrane proteins, which may be structural or functional, may be permanently or transiently associated with one side or the other of the membrane, or even be permanently built into the bilayer, while other proteins span the bilayer and may form transport channels through the membrane.

The membranes of Bacteria are structurally similar to the cell membranes of eucaryotes, except that bacterial membranes consist of saturated or monounsaturated fatty acids (rarely, polyunsaturated fatty acids) and do not normally contain sterols. The membranes of Archaea form bilayers functionally equivalent to bacterial membranes, but archaeal lipids are saturated, branched, repeating isoprenoid subunits that attach to glycerol via an ether linkage as opposed to the ester linkage found in glycerides of eukaryotic and bacterial membrane lipids (Figure 21). The structure of archaeal membranes is thought to be an adaptation to their existence and survival in extreme environments.


Figure 21. Generalized structure of a membrane lipids. (top). A phospholipid in the membrane of the bacterium Escherichia coli. The R1 and R2 positions on glycerol are substituted with saturated or monounsaturated fatty acids, with ester linkages to the glyceride. The R3 position is substituted with phosphatidylethanolamine, the most common substituent in this position in Bacteria. (bottom). An Archaeal membrane lipid. In contrast to bacterial phospholipids, which are glycerol esters of fatty acids, the lipids in membranes of Archaea are diethers of glycerol and long-chain, branched, saturated hydrocarbons called isoprenoids or which are made up of repeating C5 subunits. One of the major isoprenoids is the C20 molecule phytanol. The R3 position of glycerol may or may not be substituted. The structure of archaeal membrane lipids is thought to be an adaptation to extreme environments such as hot and acidic conditions where Archaea prevail in nature.

Functions of the Cytoplasmic Membrane

Since procaryotes lack any intracellular organelles for processes such as respiration or photosynthesis or secretion, the plasma membrane subsumes these processes for the cell and consequently has a variety of functions in energy generation, and biosynthesis. For example, the electron transport system that couples aerobic respiration and ATP synthesis is found in the procaryotic membrane. The photosynthetic chromophores that harvest light energy for conversion into chemical energy are located in the membrane. Hence, the plasma membrane is the site of oxidative phosphorylation and photophosphorylation in procaryotes, analogous to the functions of mitochondria and chloroplasts in eukaryotic cells. Besides transport proteins that selectively mediate the passage of substances into and out of the cell, procaryotic membranes may contain sensing proteins that measure concentrations of molecules in the environment or binding proteins that translocate signals to genetic and metabolic machinery in the cytoplasm. Membranes also contain enzymes involved in many metabolic processes such as cell wall synthesis, septum formation, membrane synthesis, DNA replication, CO2 fixation and ammonia oxidation. The predominant functions of procaryotic membranes are listed in Table 7 and discussed below.


Table 7. Functions of the procaryotic plasma membrane
    1. Osmotic or permeability barrier

    2. Location of transport systems for specific solutes (nutrients and ions)

    3. Energy generating functions, involving respiratory and photosynthetic electron transport systems, establishment of proton motive force, and transmembranous, ATP-synthesizing ATPase

    4. Synthesis of membrane lipids (including lipopolysaccharide in Gram-negative cells)

    5. Synthesis of murein (cell wall peptidoglycan)

    6. Assembly and secretion of extracytoplasmic proteins

    7. Coordination of DNA replication and segregation with septum formation and cell division

    8. Chemotaxis (both motility per se and sensing functions)

    9. Location of specialized enzyme system





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