A - Integral protein, B - Phospholipid bilayer, C - D- Peripheral protein, D- Glycolipids (oligosaccharide with lipids).
Singer and Nicholson (1972) put forward the "fluid mosaic model" of biomembrane. It is the latest and most widely accepted model. According to this model, the cell membrane consists of a highly viscous fluid matrix of two phospholipid molecules layers. These serve as a relatively impermeable barrier to the passage of most water-soluble molecules. Protein molecules or their complexes occur in the membrane, but not in a continuous layer. Instead, these appear as separate particles asymmetrically arranged in a mosaic pattern. Some of these (peripheral or extrinsic proteins) are loosely bound at the opposite surfaces of lipid layers. According to the mosaic model, the correct sequence of plasmalemma is P-L-L-P. Others (called integral or intrinsic proteins) penetrate deeply into the lipid layer. Some of the essential proteins penetrate through the phospholipid layers and project on both the surfaces. These are called transmembrane or tunnel proteins. They are in the form of a single helix, e.g., glycophorins.
