Q.List the major molecular components of the fluid mosaic model of the plasma membrane, and explain what is meant by describing the membrane as 'quasi-fluid'.

The cell membrane, also called the plasma membrane or biomembrane, is a thin, quasi-fluid structure found both around the whole cell (extracellularly) and around most organelles inside a eukaryotic cell (intracellularly), where it separates each organelle from the cytosol. It is about 75 Angstrom thick and appears trilamellate (three-layered) under the electron microscope, built mainly from a bilayer of phospholipids -- each molecule having one hydrophilic head and two hydrophobic tails (amphipathic), arranged so the tails face inward, away from the surrounding water -- along with proteins and carbohydrates in ratios that vary between cell types. The most accepted description of its structure is the fluid mosaic model, proposed by Singer and Nicolson in 1972: proteins are embedded in the phospholipid bilayer like icebergs in a sea of lipid and are free to move within it, a mobility that is measured as the membrane's fluidity and comes mainly from the lipid component. Integral (intrinsic) proteins sit at different depths in the bilayer, some spanning it completely as transmembrane proteins that form channels, for instance for water; peripheral (extrinsic) proteins sit only on one surface or the other. This model replaced an earlier, rigid 'sandwich' model (Danielli and Davson), which pictured static protein layers coating the lipid core and so could not explain the mobility membrane proteins actually show. The membrane's main …