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Biological membrane
A biological membrane or biomembrane is an enclosing or separating membrane that acts as a selectively permeable barrier within living things. Biological membranes, in the form of cell membranes, often consist of a phospholipid bilayer with embedded, integral and peripheral proteins used in communication and transportation of chemicals and ions. Bulk lipid in membrane provides a fluid matrix for proteins to rotate and laterally diffuse for physiological functioning. Proteins are adapted to high membrane fluidity environment of lipid bilayer with the presence of an annular lipid shell, consisting of lipid molecules bound tightly to surface of integral membrane proteins. The cellular membranes should not be confused with isolating tissues formed by layers of cells, such as mucous membranes and basement membranes.
==Function==
Biological molecules are amphiphilic or amphipathic, or are simultaneously hydrophobic and hydrophilic. 〔 The phospholipid bilayer contains charged hydrophilic headgroups, which interact with polar water. The lipids also contain hydrophobic tails, which meet with the hydrophobic tails of the complementary layer. The hydrophobic tails are usually fatty acids that differ in lengths. The interactions of lipids, especially the hydrophobic tails, determine the lipid bilayer physical properties such as fluidity.
The phospholipid bilayer is formed due to formation of aggregate in aqueous solutions. Aggregating is caused by the hydrophobic effect, where hydrophobic ends are kept away from water and hydrophilic ends are in contact with it.〔 This creates a favorable molecular arrangement by reducing unfavorable contact between hydrophobic tails and water and increasing hydrogen bonding between the hydrophilic heads and water.〔 This aggregation of nonpolar tails increases the system entropy by reducing the surface area of the nonpolar tails and, thereby decreasing the interactions between the non polar tails and water. Less water is allowed to interact with the hydrophobic ends and, therefore, hydrogen bonding between hydrophilic heads and water is increased. The increase in hydrogen bonding increases the entropy of the system, creating a spontaneous process. Aggregation of non polar substances in water is, therefore, entropically driven and spontaneously occurring. 〔 The aggregation formed due to the hydrophobic effect is partially responsible for the shape of biological membranes. 〔
Membranes in cells typically define enclosed spaces or compartments in which cells may maintain a chemical or biochemical environment that differs from the outside. For example, the membrane around peroxisomes shields the rest of the cell from peroxides, chemicals that can be toxic to the cell, and the cell membrane separates a cell from its surrounding medium. Peroxisomes are one form of vacuole found in the cell that contain by-products of chemical reactions within the cell. Most organelles are defined by such membranes, and are called "membrane-bound" organelles.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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