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Cell Biology (cytology)

What is the function of the human cell membrane?


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October 26, 2008 4:47AM

In all cells, the cell membrane acts as a combination of brick wall, window, and security guard.

The cell membrane is to a considerable extent made up of a double sheet of fatty molecules (the phospholipid bilayer) which is impermeable to most substances. Only small, uncharged molecules such as those of oxygen, carbon dioxide, and water can cross this bilayer.

There are apertures in the cell membrane, each of which is formed by one or more protein molecules. These allow across the membrane substances that cannot cross the phospholipid bilayer.

The proteins embedded in the phospholipid bilayer are selective. Through such features as channel (pore) size, charge, and three-dimensional shape, these transport proteins (channel and carrier proteins) regulate which substances enter and leave the cell. An important function of the cell membrane is therefore to regulate the passage of atoms, molecules, and ions in and out of the cell. This in turn maintains a stable chemical environment within the cell, which is important, for example, for keeping conditions optimal for intracellular enzymes.

Carrier proteins are capable of pumping substances up (against) their concentration gradient, in a process called active transport.

The cell membrane has further functions. Protein receptor molecules recognize specific hormones and growth factors, and thus determine whether a cell will respond to a particular signal molecule. And chemical compounds attached to the outside of the cell membrane play a role in adhesion (keeping cells together in a tissue). Furthermore, movements of the cytoskeleton just interior to the cell membrane cause changes to the shape of the membrane, for example in endocytosis and exocytosis.