cell membrane

Thin layer of protein and fat surrounding cells that keeps the cells together and controls substances passing between the cytoplasm and the intercellular space. The cell membrane is semipermeable, allowing some substances to pass through and some not. Generally, small molecules such as water, glucose, and amino acids can penetrate the membrane, while large molecules, such as starch, cannot. Substances often cross the membrane by diffusion, a spontaneous passage of molecules. Water movement across the membrane is a special case of diffusion known as osmosis.

Membranes also play a part in active transport, hormonal response (see hormones), and cell metabolism.