The cell membrane (or plasma membrane) is a thin, structured layer of lipid and protein molecules that completely envelopes the cell, separates its interior from the surroundings and strictly controls what moves in and out. In animal cells, the 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 lipid 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), while others might be said to reside within it or to span it (intrinsic -- see integral membrane protein). Glycoproteins are proteins with carbohydrates that are (in the plasma membrane) attached to extracellular domains. Cells may vary the lipid composition of their membranes (i.e. vary the lipid species and the particular mix of lipid species) to maintain the membranes' fluidity under changes in temperature. Cholesterol molecules in the bilayer assist in the maintenance of fluidity.
Transportation across membranes
Depending on the molecule, transportation 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 chelators 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.