Plasma membrane is called quasi-fluid membrane because of the lateral movement of protein molecules through the membrane bilayer.
Structure of a plasma membrane:
(a) The cell membrane is composed of lipid bilayer of phospholipids molecules into which variety of globular proteins are embedded.
(b) The lipid is a phospholipid and each molecule has a polar “head” region and a nonpolar “tail” region. When such molecules are surrounded by water, they align in a characteristic 2-layered sheet with the heads pointed outward and the tails pointed inward and water excluded from the middle - lipid bilayer. This layer is semipermeable i.e.; it allows only certain molecules to pass through.
(c) Protein molecules are embedded in the lipid bilayer that forms passage ways for materials.
(d) Some are embedded in the outer surface, inner surface, and some extend all the way from one side to another.
(e) These proteins often recognize specific materials and allow them to pass through the membrane or they transport them across. This is why plasma membrane is called selectively permeable membrane.
