The cell membrane forms a cell's outer boundary and controls what moves into and out of it.
Also called the plasma membrane, it separates the cell's interior from the surrounding environment. Its structure is mainly a flexible phospholipid bilayer containing proteins. Some proteins form channels or carriers, while others act as receptors or help attach the membrane to the cell's internal cytoskeleton.
The membrane is selectively permeable rather than completely sealed. Small molecules may cross by diffusion, while water moves by osmosis. Cells can also use active transport, which requires energy, to move substances against a concentration gradient. Larger material can enter or leave through endocytosis and exocytosis.
A frequent mix-up is the cell wall. Plants, fungi, bacteria, and many archaea have cell walls outside their membranes, but animal cells do not. The wall provides extra structural support; the membrane is the living, selectively regulating boundary found in all cells.