A widely accepted model, proposed by S. J. Singer and G. L. Nicholson in the 1970s, for the structure of cell membranes. It is based on a lipid bilayer forming the framework of the membrane, with proteins embedded in it; these proteins may be restricted to either lipid layer (extrinsic proteins) or they may span both (intrinsic proteins), producing an asymmetric structure (see illustration). This structure allows the lateral movement of the constituent phospholipid and protein molecules of the membrane. The proteins may associate in distinct patches to carry out particular functions within the membrane. Moreover, areas of membrane rich in cholesterol and sphingolipids form lipid rafts, or lipid microdomains, along with associated proteins. These can diffuse in the plane of the membrane, fold to form flask-shaped structures (see caveola), and rapidly assemble and disassemble. Lipid rafts distribute signalling proteins over the cell surface, and their caveolae can also perform endocytosis.

Fluid mosaic model of cell membranes