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            The fluid mosaic model is the only accepted proper description of a biological membrane. Its publication in 1972 by Singer and Nicolson (Science 1972 Feb. 18 175:23 720-31) put an end to other existing and competing models of biological membranes. These competing models assumed triple layers, with a lipid core layer and two protein layers, one on each side, but the proteins would not penetrate the lipid core, i.e., did not span the membrane. The fluid mosaic model, based on thermodynamic and functional considerations, postulated instead that proteins as the active components of signal transduction and transport, must and can span the entire thickness of a lipid bilayer. Protein-lipid interaction was thought to be stabilized through hydrophobic (fat soluble) contact points on the surface of the proteins. .
             The fluid mosaic model of a biological membrane defines a phospholipid bilayer mixed with (integral) membrane proteins. The lipid bilayer, therefore, is not a continuous layer, but contains non-lipid molecules, in particular proteins. .
             Today, the fluid-mosaic model is the only accepted model of the structure of biological membranes, including those of animals, plants, and microorganisms (see figure above for membranes of the cell wall of the bacteria Escherichia coli). the use of the same structure among all living organisms is consistent with the idea of a common ancestor organism (or organisms) from which all modern life forms have evolved. Since 1972 no modifications of the model have been necessary, also individual membranes differ in molecular composition, lipid types, or lipid to protein ratios. .
             b) The components of membrane Lipid -- cholesterol, phospholipid and sphingolipid, Proteins, Carbohydrate -- as glycoprotein. Differences in composition among membranes (e.g. myelin vs. inner mitochondrial membrane) Illustrate the variability of membrane structure. This is due to the differences in function.

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