Cell membrane

A component of every biological cell, the cell membrane (or plasma membrane) is a thin and structured layer of lipid and protein molecules, which surrounds the cell. It separates a cell's interior from its surroundings and attemps to control what moves in and out.

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. Some of these proteins simply adhere to the membrane (extrinsic 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 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 membrane 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 contor, 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 mechanisms include diffusion, which is the entropic flow of molecules across the membrane from a region where they are in high concentration to where they are in low concentration. This is accomplished primarily only by large, hydrophobic molecules, because the oily core of the bilayer poses a barrier to others. An exception is water, in which case the diffusion process is typically referred to as osmosis. In "facilitated diffusion" specialized carrier molecules, such as ion channels or chelatorss catalyze the passive flow of their substrates across the membrane. Facilitated diffusion of water, for example in the kidneys, occurs via water channels.
• 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 endocytosis and exocytosis, in which molecules packaged in membrane vesicles are either imported or exported, respectively. Molecular exchangers, transporters and pumps represent other examples.