The outer surface of a cell is surrounded by the plasma membrane, which marks the boundary between the cellular content and the external medium. The plasma membrane is a continuous film made up of molecules of lipids, proteins, and carbohydrates. Lipids form a double layer and proteins are arranged in an irregular and asymmetric way between them. These components have mobility, which gives the membrane a high degree of fluidity. It acts as a semipermeable and selective barrier regulating the chemical composition of the cell.