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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 c