The membranes of cells are largely composed on of a phospholipid bilayer and proteins. Most of our current information concerning biological membranes is summarized by the fluid mosaic model proposed by S. J. Singer and G. L. Nicholson in 1972. This is the model depicted in Figure 10.10.

The fluid, asymmetric lipid bilayer carries within it a host of proteins. Some of them, called peripheral membrane proteins, are only partially buried in the lipid matrix and are exposed at only one membrane face or the other. Other proteins, the integral membrane proteins, are largely buried within the membrane but are exposed on both faces. Integral proteins are frequently involved in transmitting either specific substances or chemical signals through the membrane. The whole membrane is a fluid mosaic of lipids and proteins.