The plasma membrane plays a vital function in controlling what enters or leaves any cell. It manages this by several ways. Water is one of the substances that freely moves in and out of the cell. It does this by osmosis through the membrane until equilibrium of concentration of water is reached. Simple Diffusuion and Facilitated Diffusion are also processes with which the plasma membrane controls what comes in and out. Active transport is another way in which by the use of energy gained from glucose molecules, substances are introduced into the cell even if the concentration is not favourable.
