Cell Membrane

The cell membrane - also called the cell surface membrane or plasma membrane - is the thin boundary that holds in a cell's cytoplasmic content (its cytosol); it is remarkably thin, well under 10 nanometres across, yet it manages to control almost everything that enters or leaves the cell. The accepted model of its structure is the Fluid Mosaic Model, proposed in 1972 by Jonathan Singer and Garth Nicolson. In this model the membrane's basic framework is a double layer (bilayer) of phospholipid molecules, each phospholipid having a water-avoiding (hydrophobic) fatty-acid tail and a water-attracting (hydrophilic) phosphate-containing head; the two layers arrange themselves tail-to-tail, so that both of the bilayer's outer faces present their hydrophilic heads to the watery environment on either side. Embedded within and across this lipid bilayer sit globular proteins: 'integral proteins', which are deeply embedded and mostly project out beyond the lipid layer on one or both sides, and 'peripheral proteins', which sit only superficially attached to one surface of the bilayer. These membrane proteins are far more than structural filler - they act as channels and carriers that move specific molecules across the membrane, and they also function as enzymes, receptors, or antigens. A smaller amount of carbohydrate is also present, as short-chain polysaccharides bound either to membrane proteins (forming glycoproteins) or to membrane lipids (forming glycolipids); together these form a surface coat called the glycocalyx. Because phospholipids have relatively small polar head regions, they are able (though only slowly) to flip from one layer of the bilayer to the other, a movement called flip-flop or flip-flopping; membrane proteins, whose polar regions are far more extensive, essentially cannot flip-flop at all. The membrane's functions flow directly from this structure: it acts as a selectively permeable transport channel (moving molecules both by energy-requiring and energy-independent processes, using its embedded channel and carrier proteins), it manages bulk transport of solids and liquids via endocytosis (bringing material into the cell, either by phagocytosis of particles or pinocytosis of fluid droplets) and exocytosis (ejecting material suc …