What are the characteristics of the plasma membrane?

Answer 1

The membrane that surrounds the cytoplasm of a cell; it is also called the plasma membrane or, in a more general sense, a unit membrane. This is a very thin, semifluid, sheetlike structure made of four continuous monolayers of molecules. The plasma membrane and the membranes making up all the intracellular membranous organelles display a common molecular architectural pattern of organization, the unit membrane pattern, even though the particular molecular species making up the membranes differ considerably. All unit membranes consist of a bilayer of lipid molecules, the polar surfaces of which are directed outward and covered by at least one monolayer of nonlipid molecules on each side, most of which are protein, packed on the lipid bilayer surfaces and held there by various intermolecular forces. Some of these proteins, called intrinsic proteins, traverse the bilayer and are represented on both sides. The segments of the polypeptide chains of these transverse proteins within the core of the lipid bilayer may form channels that provide low-resistance pathways for ions and small molecules to get across the membrane in a controlled fashion. Sugar moieties are found in both the proteins and lipids of the outer half of the unit membrane, but not on the inside next to the cytoplasm. The molecular composition of each lipid monolayer making up the lipid bilayer is different. The unit membrane is thus chemically asymmetric.