The cell membrane is a membrane on the surface of the cell, which is liquid and fluid. The cell membrane has the role of separating, maintaining cellular morphology, and controlling the entry and exit of substances. 1. Separation: the separation of the cell membrane, for the cell’s life activities to provide a relatively stable internal environment, the membrane area greatly increased to improve the biological functions occurring in the membrane. 2. Maintaining cellular morphology: the cell membrane can maintain cellular morphology, play a protective role for the cell, so that the living cells continue to carry out metabolism, not only from the surrounding environment, nutrients and oxygen intake, but also the discharge of metabolic products. 3. Control the entry and exit of substances: the cell membrane is permeable, small molecules, such as carbon dioxide, oxygen, water, etc. can enter and exit the cell membrane without energy and carriers, known as free diffusion, from high to low diffusion.