Copyright © Philip M. Parker, INSEAD. Terms of Use.
Definition: Cell Membrane |
Cell MembraneNoun1. A thin membrane around the cytoplasm of a cell; controls passage of substances in and out of the cell. Source: WordNet 1.7.1 Copyright © 2001 by Princeton University. All rights reserved. |
| Domain | Definition |
Health | Cell membrane = plasma membrane. The structure enveloping a cell, enclosing the cytoplasm, and forming a selective permeability barrier; it consists of lipids, proteins, and some carbohydrates, the lipids thought to form a bilayer in which integral proteins are embedded to varying degrees. (references) |
Source: compiled by the editor from various references; see credits. | |
(From Wikipedia, the free Encyclopedia)
A component of every biological cell, 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 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 archoring 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.
Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Cell membrane."
Synonyms: Cell MembraneSynonyms: cell wall (n), plasma membrane (n). (additional references) |
| Domain | Title |
Books | |
Source: compiled by the editor from various references; see credits. | |
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| Thumbnail | Description & Credit | Thumbnail | Description & Credit |
 | A. Peripheral blood leukocyte from an AIDS patient producing HIV. There are several particles budding from the periphery of the cell. Other virus particles cluster outside the cell membrane. (x30) b. Enlargement of far right center area of top photo showing mature HIV particles. (x90) c. HIV particle budding from cell membrane. (x200) d. Mature hiv particle with characteristic core. (x200). See artwork: GR-07. Credit: Matt Gonda (photographer). |  | The earliest visible stage of HIV replication occurs when viral proteins accumulate under the cell membrane in a process called budding (a). In the next stage a crescent shaped early bud has constricted, forming a membrane-encapsulated sphere, with the dense center called a viral nucleoid (b). As the constricting process continues, the virus pinches off and becomes free extracellular infectious virus (c). At this stage, the dark circular mucleoid condenses into a bar; this morphologic feature is used to discriminate HIV-I from HTLV-II and HTLV-III. See artwork: GR-31. Credit: Dr. Matthew Gonda (photographer). |
 | The human immunodeficiency virus (HIV-I) enters the T-lymphocyte where the virus loses its outer envelop, releasing its RNA and its reverse transcriptase. The reverse transcriptase builds a complimentary DNA strand from the viral RNA template. The DNA helix is inserted into the host genome. When this is transcribed by the infected cell, the new viral RNA and proteins are produced to form new viruses that then bud from the cell membrane, thus completing the life cycle of the virus. See artwork: GR-32. Credit: Trudy Nicholson (artist). |  | This is a histological slide of a normal cell. The cell membrane, nucleus and the nucleolus are all distinguishable. Credit: Unknown photographer/artist. |
Source: pictures compiled by the editor from various references; see picture credits. | |||
| Subject | Topic | Quote |
Health | In addition to CFTR, other chloride channels exist in the cell membrane. (references) | |
Each neuron has a cell body, an axon, and many dendrites, all surrounded by a cell membrane. (references) | ||
Conversely, virus in the salivary glands buds primarily from the cell membrane into the acinar lumen. (references) | ||
Source: compiled by the editor from ICON Group International, Inc.; see credits. | ||
Expressions using "cell membrane": Cell Membrane Permeability ♦ Cell Membrane Structures. Additional references. | |
| Source: compiled by the editor from various references; see credits. |
| The following statistics estimate the number of searches per day across the major English-language search engines as identified by various trade publications. Hyperlinks lead to commercial use of the expression at Amazon.com. |
| Expression | Frequency per Day |
cell membrane | 120 |
picture of cell membrane | 7 |
cell membrane structure | 7 |
alcohol cell membrane | 2 |
| Source: compiled by the editor from various references; see credits. | |
| Language | Translations for "cell membrane"; alternative meanings/domain in parentheses. | |
Danish | cellemembran. (various references) | |
Dutch | celmembraan. (various references) | |
Finnish | soluketto. (various references) | |
French | membrane cellulaire (f). (various references) | |
German | Zellmembran. (various references) | |
Greek | ατέλεια της κυτταρικής μεμβράνης (cell membrane defect). (various references) | |
Italian | membrana cellulare (cell wall). (various references) | |
Pig Latin | ellcay embranemay.(various references) | |
Portuguese | membrana celular (cell wall). (various references) | |
Spanish | unir las proteinas con la parte interna de la membrana celular (to attach proteins to the inside of the cell membrane), repolarizar la membrana celular (repolarizing the cell membrane), porción de la molécula que sobresale de la membrana (portion lying outside the cell membrane), permeabilidad de la membrana celular (permeability of cell membrane), la proteina se ancla en la membrana celular (the protein becomes anchored in the cell membrane), la membrana celular se invagina (the cell membrane buckles inward to form a pocket), defecto de la membrana celular (cell membrane defect). (various references) | |
Swedish | repolarisering av cellmembran (repolarizing the cell membrane). (various references) | |
| Source: compiled by the editor from various translation references. | ||
Misspellings | |
"Cell Membrane" is suggested in spellcheckers for the following: cell mambrane, cell membrabe, cell membrain. (additional references) | |
| Source: compiled by the editor, based on several corpora (additional references). | |
Scrabble® Enable2K-Verified Anagrams | |
| Words within the letters "a-b-c-e-e-e-l-l-m-m-n-r" | |
-3 letters: cerebella. | |
-4 letters: cameleer, crenelle, embalmer, enameler, membrane, reelable. | |
-5 letters: bellman, bellmen, clamber, clammer, cleaner, embrace, enabler, labeler, menacer, nacelle, reclame, reclean, relabel. | |
| Source: compiled by the editor from various references; see credits. SCRABBLE® is a registered trademark. All intellectual property rights in and to the game are owned in the U.S.A and Canada by Hasbro Inc., and throughout the rest of the world by J.W. Spear & Sons Limited of Maidenhead, Berkshire, England, a subsidiary of Mattel Inc. Mattel and Spear are not affiliated with Hasbro. | |
Copyright © Philip M. Parker, INSEAD. Terms of Use.
