(1) Semi-permiable barrier around the cytoplasm, that only allows certian materials to pass in or out of the cell.
(2) The cell membrane is the outer covering of the cell. Although some cells form another layer above the cell membrane (called cell wall), other cells have the cell membrane as the only protective barrier between the cytoplasm and the outside of the cell.
The cell membrane is a phospholipid bilayer interspersed with proteins as depicted in the Fluid Mosaic Model. Its structure and composition makes it selectively permeable (or semipermeable), which means not every substance is allowed to enter or leave the cell. The cell membrane controls which substances can go in and out of the cell. It can allow a particular substance to pass through at a certain time, and then reject the same substance at a later time.
Its other main functions include cell adhesion, ion channel conductance, cell signaling, and attachment point for cytoskeleton (which is important in keeping the shape of the cell).
Another interesting feature of the cell membrane is the presence of surface molecules (e.g. glycoproteins, glycolipids, etc.) that act like a ‘signature’ for a cell. Every cell has a different ‘signature’ or ‘marker’ that is thought to function in cell recognition, or in a sort of cellular identification system.
Synonym: plasma membrane, plasmalemma, cytomembrane