Skip to main content

Membrane

Membrane


All living creatures are made of cells. One cellular component, the membrane, plays a crucial role in almost all cellular activities. The primary function of all cell membranes is to act as barriers between the intracellular and extracellular environments, and as sites for diverse biochemical activities. The cell itself is encapsulated by its own membrane, the plasma membrane. Although the composition of membranes varies, in general, lipid molecules make up approximately 40 percent of their dry weight; proteins, approximately 60 percent. The lipids and proteins are held together by noncovalent interactions.

Among several possible stable arrangements of protein and lipid molecules in membranes, the bilayer model, first described over seventy years ago, characterizes most biological membranes. An important feature of this model is that the hydrophilic groups of the lipid molecules are oriented toward the surfaces of the bilayer, and the hydrophobic groups toward the interior. In 1972 Jonathan Singer and Garth Nicolson postulated a unified theory of membrane structure called the fluid-mosaic model. They proposed that the matrix, or continuous part, of membrane structure is a fluid bilayer, and that globular amphiphilic proteins are embedded in a single monolayer, with some proteins spanning the thickness of both monolayers. Both proteins and lipids are mobile and, thus, the membrane can be viewed as a twodimensional solution of proteins in lipids.

The major class of lipids in plasma membranes is phospholipids. Phospholipids consist of a glycerol backbone and two fatty acids joined by ester linkage to the first two carbons of glycerol, and a phosphate group joined to the third. Different groups can be esterified to the phosphate, and these groups define the different classes of phospholipids. In addition, the fatty acids have varying chain lengths and degrees of unsaturation. The presence of the nonpolar acyl chain regions and the polar head groups gives the phospholipid molecules their amphipathic character, which allows them to assume the bilayer arrangements of membranes. In addition to phospholipids, two other kinds of lipids are found in the membranes of animal cells: glycolipids and cholesterol. Glycolipids usually make up only a small fraction of the lipids in the membrane but have been shown to possess many biological functions, one of which is their capacity to function as recognition sites. Cholesterol is an important component of plasma membranes and has been shown to play a key role in the control of membrane fluidity.

Several membrane functions are believed to be largely mediated by proteins. Membrane proteins have been put into two general categories: peripheral and integral. Peripheral proteins (or extrinsic proteins) are those that do not penetrate the bilayer to any significant degree and are associated with it by virtue of noncovalent interactions (ionic interactions and hydrogen bonds ) between membrane surfaces and protein surfaces. Integral proteins (or intrinsic proteins), in contrast, possess hydrophobic surfaces that readily penetrate the lipid bilayer, as well as other surfaces that prefer contact with aqueous medium. These proteins can either insert into the membrane or extend all the way across it and expose themselves to the aqueous solutions on both sides.

One of the main functions of the plasma membrane is to separate cytoplasm from extracellular surroundings. In fact, membranes are highly selective permeability barriers, as they contain specific channels and pumps that enable the transport of substances across membranes. These transport systems to a large degree regulate the molecular and ionic composition of intracellular media. Membranes also control the flow of information between cells, and between cells and their extracellular environments, and they contain specific receptors that make membranes sensible to external stimuli. In addition, some membranes conduct and pass on signals that can be chemical or electrical, as in the transmission of nerve impulses. Thus, membranes play a central role in signal transduction processes and in biological communication.

RECOGNITION SITES

Glycolipids and glycoproteins can act as recognition sites in a variety of processes involving recognition between cell types or recognition of cellular structures by other molecules. Recognition events are important in normal cell growth, fertilization, transformation of cells, and other processes.

see also Cholesterol; Lipids; Phospholipids; Transmembrane Protein.

Michèle Auger

Bibliography

Garrett, Reginald H., and Grisham, Charles M. (2002). Principles of Biochemistry with a Human Focus. Fort Worth, TX: Harcourt College Publishers.

Harrison, Roger, and Lunt, George G. (1980). Biological Membranes: Their Structure and Function. New York: Wiley.

Singer, S. Jonathan, and Nicolson, Garth L. (1972). "The Fluid Mosaic Model of the Structure of Cell Membranes." Nature 175:720731.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Membrane." Chemistry: Foundations and Applications. . Encyclopedia.com. 25 Apr. 2017 <http://www.encyclopedia.com>.

"Membrane." Chemistry: Foundations and Applications. . Encyclopedia.com. (April 25, 2017). http://www.encyclopedia.com/science/news-wires-white-papers-and-books/membrane

"Membrane." Chemistry: Foundations and Applications. . Retrieved April 25, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/news-wires-white-papers-and-books/membrane

membrane

membrane, structure composed mostly of lipid and protein that forms the external boundary of cells and of major structures within cells. Membrane organization is based on a sheet two molecules thick—a double layer of lipids aligned with their long hydrocarbon tails tucked inside—studded with protein molecules, some of which extend completely through the lipid bilayer. The basic function of the membrane is to provide for the integrity of the cell—e.g., to separate the outside from the inside. While water and a few substances, such as carbon dioxide and oxygen, can diffuse across the membrane, most molecules necessary for cellular functions traverse the membrane by means of transport mechanisms. There are several such mechanisms and they rely upon interactions between a transportable molecule and specific protein molecules in the membrane. Among these is the Na+-K+ pump, by which sodium ions within the cell are exchanged with potassium ions from without. Such transport functions permit selective entry of particular materials into the cell and into structures within the cell. Information can also be transmitted across the membrane. In this case, specific membrane proteins called receptors bind hormones or other such informational molecules and subsequently transmit a signal to the interior of the cell. Endocytosis also allows the bulk transport of materials across the membrane.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"membrane." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. 25 Apr. 2017 <http://www.encyclopedia.com>.

"membrane." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (April 25, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/membrane

"membrane." The Columbia Encyclopedia, 6th ed.. . Retrieved April 25, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/membrane

membrane

mem·brane / ˈmemˌbrān/ • n. Anat. & Zool. a pliable sheetlike structure acting as a boundary, lining, or partition in an organism. ∎  a thin pliable sheet or skin of various kinds: the concrete should include a membrane to prevent water seepage. ∎ Biol. a microscopic double layer of lipids and proteins that bounds cells and organelles and forms structures within cells. DERIVATIVES: mem·bra·na·ceous / ˌmembrəˈnāshəs/ adj. mem·bra·ne·ous / memˈbrānēəs/ adj. mem·bra·nous / ˈmembrənəs; memˈbrānəs/ adj.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"membrane." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. 25 Apr. 2017 <http://www.encyclopedia.com>.

"membrane." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. (April 25, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/membrane-0

"membrane." The Oxford Pocket Dictionary of Current English. . Retrieved April 25, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/membrane-0

membrane

membrane In biology, boundary layer or layers inside or around a living cell or tissue. Cell membranes include the plasma membrane surrounding the cell, the network of membranes inside the cell (endoplasmic reticulum), and the double membrane surrounding the nucleus. The multicellular membranes of the body comprise mucous membranes of the respiratory, digestive and urinogenital passages, synovial membranes of the joints, and the membranes that coat the inner walls of the abdomen, thorax and the surfaces of organs. See also epithelium; synovial fluid

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"membrane." World Encyclopedia. . Encyclopedia.com. 25 Apr. 2017 <http://www.encyclopedia.com>.

"membrane." World Encyclopedia. . Encyclopedia.com. (April 25, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/membrane

"membrane." World Encyclopedia. . Retrieved April 25, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/membrane

membrane

membrane A sheet-like structure, 7–10 nm wide, that forms the boundary between a cell and its environment and also between various compartments within the cell. It is composed mainly of lipids, proteins, and some carbohydrates, the structural arrangement of which is still subject to speculation. Membranes function as selective barriers and also as a structural base for enzymes, which may form an integral part of the membrane itself.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"membrane." A Dictionary of Plant Sciences. . Encyclopedia.com. 25 Apr. 2017 <http://www.encyclopedia.com>.

"membrane." A Dictionary of Plant Sciences. . Encyclopedia.com. (April 25, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/membrane

"membrane." A Dictionary of Plant Sciences. . Retrieved April 25, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/membrane

membrane

membrane A sheet-like structure, 7–10 nm wide, that forms the boundary between a cell and its environment and also between various compartments within the cell. It is composed mainly of lipids, proteins, and some carbohydrates, the structural arrangement of which is still subject to speculation. Membranes function as selective barriers and also as a structural base for enzymes, which may form an integral part of the membrane itself.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"membrane." A Dictionary of Zoology. . Encyclopedia.com. 25 Apr. 2017 <http://www.encyclopedia.com>.

"membrane." A Dictionary of Zoology. . Encyclopedia.com. (April 25, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/membrane-0

"membrane." A Dictionary of Zoology. . Retrieved April 25, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/membrane-0

membrane

membrane
1. A thin sheet of tissue or other material that lines a body cavity, forms a partition, or connects various structures.

2. Any of the various flexible sheetlike structures, composed predominantly of lipids and proteins, that occur in living cells, such as the plasma membrane forming the cell boundary. See cell membrane.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"membrane." A Dictionary of Biology. . Encyclopedia.com. 25 Apr. 2017 <http://www.encyclopedia.com>.

"membrane." A Dictionary of Biology. . Encyclopedia.com. (April 25, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/membrane-1

"membrane." A Dictionary of Biology. . Retrieved April 25, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/membrane-1

membrane

membrane (mem-brayn) n.
1. a thin layer of tissue surrounding an organ or tissue, lining a cavity, or separating adjacent structures or cavities. See also basement membrane, mucous membrane, serous membrane.

2. the lipoprotein envelope surrounding a cell (cell m.).
membranous (mem-brăn-ŭs) adj.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"membrane." A Dictionary of Nursing. . Encyclopedia.com. 25 Apr. 2017 <http://www.encyclopedia.com>.

"membrane." A Dictionary of Nursing. . Encyclopedia.com. (April 25, 2017). http://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/membrane

"membrane." A Dictionary of Nursing. . Retrieved April 25, 2017 from Encyclopedia.com: http://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/membrane

membrane

membrane XVI (first in sense ‘parchment’). — L. membrāna (partly through F.) ‘skin covering a part of the body’ (prop. sb. use of fem. of adj. in -ānus -AN), f. membrum MEMBER.
So membranaceous XVII. f. late L. membranous XVI. — F.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"membrane." The Concise Oxford Dictionary of English Etymology. . Encyclopedia.com. 25 Apr. 2017 <http://www.encyclopedia.com>.

"membrane." The Concise Oxford Dictionary of English Etymology. . Encyclopedia.com. (April 25, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/membrane-1

"membrane." The Concise Oxford Dictionary of English Etymology. . Retrieved April 25, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/membrane-1

membrane

membraneairplane, terreplein •sailplane, tailplane •mainplane •seaplane, ski-plane •chilblain •biplane, triplane •warplane • towplane • Tamerlane •monoplane • aeroplane • gyroplane •hydroplane • châtelaine • aquaplane •balletomane • Moulmein •Charlemagne • frangipane • propane •windowpane • counterpane •membrane • checkrein • lamebrain •migraine • ingrain • quatrain •grosgrain • wholegrain • scatterbrain •suzerain • birdbrain • sixain •elastane • Beltane • Aquitaine •octane •Fonteyn, montane •Holstein • Bloemfontein • butane •bloodstain • chevrotain •ethane, methane •polyurethane • Alfvén • paravane •weathervane • vervain • Gawain

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"membrane." Oxford Dictionary of Rhymes. . Encyclopedia.com. 25 Apr. 2017 <http://www.encyclopedia.com>.

"membrane." Oxford Dictionary of Rhymes. . Encyclopedia.com. (April 25, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/membrane

"membrane." Oxford Dictionary of Rhymes. . Retrieved April 25, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/membrane