Color enhanced transmission electron micrograph showing the cell membrane of Meniodium. Magnification 250,000x. Cell membrane, the thin membrane that forms the outer surface of the protoplasm of a cell and regulates the passage of materials in and out of the cell. It is made up of proteins and lipids and often contains molecular receptors. The membranes of organelles within the cell are made of the same basic material as the cell membrane. Also called plasmalemma, plasma membrane.