A lamellar organization (lamellar phase) of phospholipids packed as a bilayer ~7 nm thick with the hydrophobic acyl tails inwardly directed and polar head groups on the outside surfaces. It is this fluid bilayer that forms the basis of membranes in cells, though in most cellular membranes a very substantial proportion of the area may be occupied by integral membrane proteins that are free to move in the plane of the membrane. Some proteins are anchored, some may be surrounded by atypical lipid domains, lipid domains (rafts) may also exist, and the inner and outler leaflets may have differences in phospholipid composition. Other proteins (peripheral membrane proteins) are anchored to the membrane but the protein portion is not embedded within the hydrophobic region. The ubiquitous triple-layered appearance of membranes (the so-called unit membrane) seen in electron microscopy is thought to arise because osmium tetroxide binds to the polar regions leaving a central, unstained, hydrophobic region.
Subjects: Medicine and Health.