A component of every biological cell, the selectively permeable cell membrane (or plasma membrane or plasmalemma) is a thin and structured bilayer of phospholipid and protein molecules that envelopes the cell. It separates a cell's interior from its surroundings and controls what moves in and out. Cell surface membranes often contain receptor proteins and cell adhesion proteins. There are also other proteins with a variety of functions. These membrane proteins are important for the regulation of cell behavior and the organization of cells in tissues.
In animal cells, the cell membrane establishes this separation alone, whereas in yeast, bacteria and plants an additional cell wall forms the outermost boundary, providing primarily mechanical support. The plasma membrane is only about 10 nm thick and may be discerned only faintly with a transmission electron microscope. One of the key roles of the membrane is to maintain the cell potential.
As a lipid bilayer, the cell membrane is semi-permeable. This means that only some molecules can pass unhindered in or out of the cell. These molecules are either small or lipophilic. Other molecules can pass in or out of the cell, if there are specific transport molecules.
Depending on the molecule, transport occurs by different mechanisms, which can be separated into those that do not consume energy in the form of ATP (passive transport) and those that do (active transport).
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