Cell membrane, also called _ _ _ _ _ _ _ membrane, thin membrane that surrounds every living cell, delimiting the cell from the environment around it. Enclosed by this cell membrane (also known as the plasma membrane) are the cell’s constituents, often large, water-soluble, highly charged molecules such as _ _ _ _ _ _ _ , nucleic acids, carbohydrates, and substances involved in _ _ _ _ _ _ _ metabolism. Outside the cell, in the surrounding water-based environment, are ions, acids, and _ _ _ _ _ _ _ that are toxic to the cell, as well as nutrients that the cell must absorb in order to live and _ _ _ _ _ _ _ . The cell membrane, therefore, has two functions: first, to be a _ _ _ _ _ _ _ keeping the constituents of the cell in and _ _ _ _ _ _ _ substances out and, second, to be a _ _ _ _ _ _ _ allowing transport into the cell of essential nutrients and movement from the cell of waste products.