Biological membranes consist of a continuous double layer of lipid molecules in which membrane proteins are embedded.Although the lipid bilayer provides the basic structure of biological membranes, the membrane proteins perform most of the membrane’s specific tasks and therefore give each type of cell membrane its characteristic functional properties.The amounts and types of proteins in a membrane are highly variable.In the myelin membrane, which serves mainly as electrical insulation for nerve-cell axons, less than 25% of the membrane mass is protein. By contrast, in the membranes involved in ATP production (such as the internal membranes of mitochondria and chloroplasts), approximately 75% is protein.A typical plasma membrane is somewhere in between, with protein accounting for about half of its mass.Because lipid molecules are small compared with protein molecules, however, there are always many more lipid molecules than protein molecules in cell membranes—about 50 lipid molecules for each protein molecule in cell membranes that are 50% protein by mass.Membrane proteins vary widely in structure and in the way they associate with the lipid bilayer, which reflects their diverse functions.