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.
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.