The cell membrane is a thin film on the surface of the cell, which is liquid and can flow. The function of the cell membrane is to maintain the cell in a certain shape and to protect the cell. Living cells are constantly metabolizing, taking in essential nutrients and oxygen from the surrounding environment and excreting metabolic products. The cell membrane is very clear under the microscope and is a lipid bilayer structure, which selectively allows substances to pass through, allowing some small molecules to pass through, while restricting the passage of some harmful substances or large molecules, keeping the stability of intracellular substances.