Singer and Nicolson (1972) proposed fluid mosaic model of plasma membrane. According to this model plasma membrane contains a lipid bilayer, which are interrupted by protein molecules. Some proteins attached at the polar surface of lipid (called, extrinsic proteins) while other partialy penetrate the lipid bilayer (called, intrinsic proteins). Intrinsic proteins are tightly binded whereas the extrinsic proteins are loosely binded to the lipid bilayer.