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 attached to extracellular domains. Cells may vary the variety and particular mix of lipid species in their cell membrane to maintain its fluidity under changes in temperature. Cholesterol molecules in the bilayer assist in the maintenance of fluidity.
Detailed Structure
In fact, not all lipid molecules in the cell membrane are "liquid" or free to diffuse. Lipid rafts[?] and caveolae are examples of more rigid membrane regions. The membrane is undergirded, furthermore, by the membrane cytoskeleton[?], to which many integral membrane proteins are anchored. This anchoring serves to restrict membrane proteins to a particular cell face or surface (for example, the "apical" surface of epithelial cells[?] that line the vertebrate gut) and constrains protein's motion within the bilayer. Finally, rather than presenting always a gentle and haphazard contor, the plasma membrane surface of many cells can be dense with orderly involutions. The resulting finger-like projections ("microvilli") increase cell surface area and facilitate the absorption of molecules from the outside.
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.
Common misspelling and questions (FAQ)
ell-membrane cll-membrane cel-membrane cel-membrane cellmembrane cell-embrane cell-mmbrane cell-mebrane cell-memrane cell-membane cell-membrne cell-membrae cell-membran ecll-membrane clel-membrane cell-membrane cel-lmembrane cellm-embrane cell-emmbrane cell-mmebrane cell-mebmrane cell-memrbane cell-membarne cell-membrnae cell-membraen cell-membran ccell-membrane ceell-membrane celll-membrane celll-membrane cell--membrane cell-mmembrane cell-meembrane cell-memmbrane cell-membbrane cell-membrrane cell-membraane cell-membranne cell-membranee dell-membrane xell-membrane fell-membrane fell-membrane vell-membrane c3ll-membrane cwll-membrane csll-membrane c4ll-membrane cdll-membrane c4ll-membrane crll-membrane cdll-membrane ceol-membrane cekl-membrane ce,l-membrane cepl-membrane ce.l-membrane cepl-membrane ce;l-membrane ce.l-membrane celo-membrane celk-membrane cel,-membrane celp-membrane cel.-membrane celp-membrane cel;-membrane cel.-membrane cell0membrane cellpmembrane cell[membrane cell-jembrane cell-nembrane cell-kembrane cell-kembrane cell-,embrane cell-m3mbrane cell-mwmbrane cell-msmbrane cell-m4mbrane cell-mdmbrane cell-m4mbrane cell-mrmbrane cell-mdmbrane cell-mejbrane cell-menbrane cell-mekbrane cell-mekbrane cell-me,brane cell-memgrane cell-memvrane cell-memhrane cell-memhrane cell-memnrane cell-memb4ane cell-membeane cell-membdane cell-memb5ane cell-membfane cell-memb5ane cell-membtane cell-membfane cell-membrqne cell-membrwne cell-membrzne cell-membrwne cell-membrsne cell-membrzne cell-membrahe cell-membrabe cell-membraje cell-membraje cell-membrame cell-membran3 cell-membranw cell-membrans cell-membran4 cell-membrand cell-membran4 cell-membranr cell-membrand cell-membranesIt was vanity, I suppose. If the crop came on well I could claim that if the crop failed Mary would have the plough and harrow against me, Anyway, I'd want a plough and harrow later on, and I might as well get it now; and the first thing I saw was old Corny George ploughing the flat. with the trace-chains and the spare horses, and had made him clear off Old Corny looked pretty grumpy on it -- he'd broken all his ploughshares Mary had an old felt hat and a new pair of 'lastic-side boots of mine on, to get a rotten old stump out of the way by the time Corny came round were a bone of contention between us; but she generally got them off but I said that would be all right -- we'd want a plough anyway. `I thought you wanted old Corny to plough the ground,' she said. `I never said so.' `But when I sent Jim after you about the hoe to put the spuds in, When Corny was done, James and I cross-ploughed the land, and got and added nearly an acre, and ploughed that. James was all right he liked ploughing or fencing, or any graft he could make a show at. We sliced the potatoes of an evening -- and there was trouble There was no time for the hoe -- and besides it wasn't a novelty to James -- .