ABSTRACT

Cell membranes are crucial to the life of the cell. The plasma membrane encloses the cell, defines its boundaries, and maintains the essential differences between the cytosol and the extracellular environment. Despite their differing functions, all biological membranes have a common general structure: each is a very thin film of lipid and protein molecules, held together mainly by noncovalent interactions. The lipid bilayer provides the basic structure for all cell membranes. Lipid molecules constitute about 50" of the mass of most animal cell membranes, nearly all of the remainder being protein. In addition to phospholipids, the lipid bilayers in many cell membranes contain glycolipids and cholesterol. The shape and amphiphilic nature of the phospholipid molecules cause them to form bilayers spontaneously in aqueous environments. The same forces that drive phospholipids to form bilayers also provide a self-sealing property. Various techniques have been used to measure the motion of individual lipid molecules and their components.