Drawing of a cell membrane

A component of every biological cell, the cell membrane (or plasma membrane) is a thin and structured bilayer of phospholipid and protein molecules that encapsulate 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. 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.

A Fluid Mosaic

The basic composition and structure of the plasma membrane is the same as that of the membranes that surround organelles and other subcellular compartments. The foundation is a phospholipid bilayer, and the membrane as a whole is often described as a 'fluid mosaic' - a two-dimensional fluid of freely diffusing lipids, dotted or embedded with proteins which may function as channels or transporters across the membrane, or as receptors. Some of these proteins simply adhere to the membrane (extrinsic or peripheral proteins), while others might be said to reside within it or to span it (intrinsic proteins -- more at integral membrane protein). Glycoproteins have carbohydrates attached to their extracellular domains. Cells may vary the variety and the relative amounts of different lipids to maintain the fluidity of their membranes despite changes in temperature. Cholesterol molecules (in case of eukaryotes) or hopanoids (in case of prokaryotes) in the bilayer assist in regulating fluidity.

Detailed Structure

In fact, not all lipid molecules in the cell membrane are "fluid," in the sense of free to diffuse. Lipid rafts and caveolae are examples of more cohesive membrane regions. Across the membrane globally, also many proteins are not entirely free to diffuse. The cytoskeleton undergirds the cell membrane and provides anchoring points for integral membrane proteins. Anchoring restricts them to a particular cell face or surface--for example, the "apical" surface of epithelial cells that line the vertebrate gut--and limits how far they may diffuse within the bilayer. Finally, rather than presenting always a formless and fluid contour, the plasma membrane surface of cells may show structure. Returning to the example of epithelial cells in the gut, the apical surfaces of many such cells are dense with involutions, all similar in size. The finger-like projections, called "microvilli", increase cell surface area and facilitate the absorption of molecules from the outside. Synapses are another example of highly structured membrane.

Transport across membranes

Depending on the molecule, transport occurs by different mechanisms, which can be separated into those that do not consume ATP energy (passive transport) and those that do (active transport):

Passive transport

Passive transport is a means of moving biochemicals, and other atomic/molecular substances, across membranes through diffusion of hydrophobic (non polar) and small polar molecules, or facilitated diffusion of polar and ionic molecules, which relies on a transport protein to provide a channel or bind to specific molecules. This spontaneous process decreases free energy, and increases entropy in a system. Unlike active transport, this process does not involve chemical energy (ATP).

Active transport

Typically moves molecules from low concentration to high, or against their concentration gradient, an process that would be entropically unfavorable were it not stoichiometrically coupled with the hydrolysis of ATP.

Examples include:

1. endocytosis
2. exocytosis, in which molecules packaged in membrane vesicles are either imported or exported, respectively. Molecular exchangers, transporters and pumps represent other examples.

References

• R.R. Dogonadze, Z.D. Urushadze, "Investigation of the Electrochemical Processes on the Border of Division Electrolite-Biological Membrane", Dep. VINITI (No 3633-71), Moscow, 1971, 20 pp. (In Russian)
• Z.D. Urushadze, "The Charge Transfer Across the Model Biological Membrane".- Proceedings of the Georgian Academy of Sciences, Series of Biology, 12, No 5, 1986, pp. 347-352 (In Russian, English summary)