A. What Membranes Are
A membrane is a thermodynamically stable, non-covalent assembly of lipid and protein; often these major components also have carbohydrate residues attached to them. It is arranged as a bilayered sheet, with each layer being termed a “leaflet.” Because interactions among components are non-covalent, each leaflet is fluid in that individual molecules are free to move within the plane of that leaflet. A membrane also is asymmetric in that one leaflet is different from the other, and many directional functions are carried out by the membrane.
B. What Membranes Do
Permeability barriers. A membrane forms the boundary between a cell and its environment, and also forms compartments within a cell. But a membrane is highly selective in what it allows to pass through it. Some molecules cross a membrane easily; other hardly at all, or only with the aid of a transport mechanism. The presence of transport mechanisms enables a membrane to selectively regulate what ions and molecules may pass through it.
Fuse. Membranes may fuse with each other on close approach. Fusion is an integral part of cell functions such as endocytosis, exocytosis, budding of vesicles from the Golgi, and fusion of sperm and egg. A reversal of fusion occurs on the separation of membranes during cell division.
Create and maintain gradients. Membranes not only regulate what may diffuse through, but also contain energy-dependent devices for creation of specific gradients, and for maintenance of different concentrations of a substance on each side.
Regulate flow of information. A membrane can have receptors on one side which are specific for a particular informational entity such as a hormone. Binding of the hormone leads to transmission of a signal across the membrane, with diverse physiological effects.
Convert energy. A membrane often has structural elements which convert one form of chemical energy to another. Examples: use of ATP energy to produce a gradient (as in active transport); use of a gradient to generate ATP (as in oxidative phosphorylation).
KeywordsLipase Luminal Cytosol Epinephrine Choline
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