Keeping in view the 'fluid mosaic model' for the structure of cell...

Singer and Nicolson (1972) suggested the widely accepted fluid mosaic model of biological membranes. According to this model the plasma membrane contains a bimolecular lipid layer, both surfaces of which are interrupted by protein molecules. Some proteins are attached at the polar surface of the lipid (ie, called extrinsic protein) while, other (called intrinsic proteins) either partially penetrate the bilayer or span the membrane entirely to stick out on both sides (called transmembrane proteins). Mobility of membrane proteins due to fluid property of lipid bilayer was demonstrated by classical experiment of D Frye and M Edidin (1970). The movement of lipid molecules from one lipid monolayer to another monolayer is called flip-flop-movement. Flip-flop movement is rarely found in lipid molecules whereas it remain absent in protein molecules.