plasma membrane, the outer covering of a cell, often having projecting microvilli and containing the cellular cytoplasm. The plasma membrane is so thin and delicate that it is barely visible with a light microscope and can be studied in detail only with an electron microscope. The membrane controls the exchange of materials between the cell and its environment by various processes, such as osmosis, phagocytosis, pinocytosis, and secretion. Also called cell membrane.
