Depending on the ease of extraction, proteins of the plasma membrane are of two types.
(a) External or extrinsic proteins: It is a peripheral protein (30% of total protein). They lie on the surface of the membrane. It can be easily removed e.g. Spectrin in RBC.
(b) Integral or intrinsic proteins: It is about 70% of total protein. They are buried partially or totally in the plasma membrane. These cannot be separated easily. e.g. Cytochrome oxidase, Porin proteins. They may function as carriers, permeases, enzymes, receptors. Some large globular intrinsic proteins pass as a helix into the lipid bilayer from outside to inside to form tunnel proteins or transmembrane proteins. The transmembrane proteins act as channels for the passage of water-soluble materials and water.
