Difference between revisions of "Cell membrane" - New World Encyclopedia

Latest revision as of 00:16, 4 January 2020

The structure of a cell membrane, showing its lipid bilayer and embedded proteins

The cell membrane (or plasma membrane) is the thin outer layer of the cell that differentiates the cell from its environment. In animals, 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.

As a semi-permeable barrier, the cell membrane maintains an essential balance between individual distinctness and communal interaction: it functions to retain key components of the cell and to keep out toxic or unwanted substances, while selectively controlling the flow of nutrients and biochemical signals into the cell.

The cell membrane is composed mainly of phospholipid and protein molecules arranged in organized but flexible sheets. The phospholipid components form a bilayer that contributes structural stability and creates the semi-permeable environment, while the proteins are responsible for most of the dynamic processes carried out by cell membranes, such as the transport of molecules into and out of the cell.

Transport across the cell membrane underlies a variety of physiological processes, from the beating of an animal’s heart to the opening of tiny pores in leaves that enables gas exchange with the environment. A major cellular manifestation of motor neuron disease is the inability of nerve cells to stimulate the opening of channels through the membranes of muscle cells, which would result in normal muscle function.

The regulation of transport, though a crucial function of the cell membrane, is not its only role. Cell membranes assist in the organization of individual cells to form tissues. They are also involved in biological communication: the binding of a specific substance to the exterior of the membrane can initiate, modify, or turn off a cell function.

The structure of cell membranes

Components

Lipids

The three major types of lipids found in the cell membrane are phospholipids, glycolipids, and cholesterol molecules.

A phospholipid is composed of a polar head (a negatively charged phosphate group) and two non-polar tails (its two fatty acid chains). Phospholipids are said to be amphipathic molecules because they contain a hydrophilic (water-loving) head and two hydrophobic (water-fearing) tails. This amphipathic property causes multitudes of phospholipids suspended in water to organize naturally into a spherical, three-dimensional bilayer, which, becomes the cell membrane. Two phospholipid molecules in water will tend to join together via their water-fearing tails, and billions of them will cluster together side-by-side in the same fashion until the growing sheet curves back to make a closed sphere.

The length and properties of the fatty-acid components of phospholipids determine the fluidity of the cell membrane. At reduced temperatures, some organisms may vary the type and relative amounts of lipids to maintain the fluidity of their membranes. These changes in membrane lipid components contribute to the survival of plants, bacteria, and hibernating animals during winter.

The regulation of membrane fluidity is assisted by another lipid, cholesterol, which is primarily found in eukaryotes. (In prokaryotes, hopanoids perform a similar function.)

Carbohydrate components are linked to lipids (to form glycolipids) or to proteins (glycoproteins) on the outside of the cell membrane. They are crucial in recognizing specific molecules or other cells. For example, the carbohydrate unit of some glycolipids changes when a cell becomes cancerous, which may allow white blood cells to target cancer cells for destruction.

Proteins

Cell membranes contain two types of proteins:

Extrinsic or peripheral proteins simply adhere to the membrane and are bound by polar interactions.
Intrinsic proteins or integral membrane proteins may be said to reside within the membrane or to span it. They interact extensively with the fatty acid chains of membrane lipids and can be released only by agents that compete for these non-polar interactions. In addition, the cytoskeleton, which undergirds the cell membrane, provides anchoring points for integral membrane proteins.

The protein components of cell membranes may function as channels or transporters across the membrane or as receptors of biochemical information.

The relative number of proteins and lipids depends on the specialized function of the cell. For example, myelin, a membrane that encloses some nerve cells, uses properties of lipids to act as an insulator, and so contains only one protein per 70 lipids. In general, most cell membranes are about 50 percent protein by weight.

The fluid-mosaic model

The cell membrane is often described as a fluid mosaic—a two-dimensional fluid of freely diffusing lipids, dotted or embedded with proteins. The model was first proposed by S. Jonathan Singer (1971) as a lipid-protein model and extended to include the fluid character in a publication with Garth L. Nicolson in Science (1972). Proteins are free to diffuse laterally in the lipid matrix unless restricted by specific interactions, but not to rotate from one side of the membrane to another.

Rather than presenting always a formless and fluid contour, however, the surface of the cell membrane may show structure. Synapses, the junctions between nerve cells, are one example of a highly structured membrane.

Transport across the cell membrane

As the cell membrane is semi-permeable, only some molecules can pass unhindered into or out of the cell. These molecules are typically either small or non-polar. The cell membrane has a low permeability to ions and most polar molecules, with water being a notable exception.

There are two major mechanisms for moving chemical substances across membranes: passive transport (which does not require the input of outside energy) and active transport (which is driven by the direct or indirect input of chemical energy in the form of ATP).

Passive transport

Passive transport processes rely on a concentration gradient (a difference in concentration between the two sides of the membrane). This spontaneous process acts to decrease free energy and increase entropy in a system. There are two types of passive transport:

Simple diffusion of hydrophobic (non-polar) and small polar molecules through the phospholipid bilayer.
Facilitated diffusion of polar and ionic molecules, which relies on a transport protein to provide a channel or bind to specific molecules. Channels form continuous polar pathways across membranes that allow ions to flow rapidly down their electrochemical gradients (i.e., in a thermodynamically favorable direction).

Active transport

Charged or polar molecules (such as amino acids, sugars, and ions) do not pass readily through the lipid bilayer. Protein pumps use a source of free energy, like ATP or light, to drive the uphill transport. That is, active transport typically moves molecules against their electrochemical gradient, a process that would be entropically unfavorable were it not coupled with the hydrolysis of ATP. This coupling can be either primary or secondary:

Primary active transport involves the direct participation of ATP.
In secondary active transport, energy derived from the transport of a molecule (such as sodium) in the direction of its electrochemical gradient is used to move another molecule against its gradient.

In general, active transport is much slower than pasive transport through channels.

The processes of endocytosis, which bring macromolecules, large particles, and even small cells into a eukaryotic cell, can be thought of as examples of active transport. In endocytosis, the plasma membrane folds inward around materials from the environment, forming a small pocket. The pocket deepens, forming a vesicle that separates from the membrane and migrates into the cell interior. In exocytosis, materials packaged in vesicles are exported from a cell when the vesicle membrane fuses with the cell membrane.

Other functions of cell membranes

Organization. Some receptors on the external surface of the cell membrane participate in the grouping of cells to form tissues (cellular adhesion).
Information processing. Membrane proteins may act as receptors for the various chemical messages that pass between cells. The movement of bacteria toward food and the response of target cells to hormones such as insulin are two examples of processes that hinge on the detection of a signal by a specific receptor in a cell membrane.
Enzyme assembly. Cell membranes can serve as an assembly that organizes the specific enzymes involved in a given metabolic pathway. Binding the enzymes to the membrane in sequential order enables the series of chemical reactions in the pathway to be carried out efficiently.
Biological communication. Some membranes generate chemical or electrical signals. Cell membranes of nerve cells, muscle cells, and some eggs are excitable electrically. In nerve cells, for example, the plasma membrane conducts the nerve impulse from one end of the cell to the other.

References
ISBN links support NWE through referral fees

Purves, W., D. Sadava, G. Orians, and C. Heller. 2004. Life: The Science of Biology, 7th edition. Sunderland, MA: Sinauer.
Stryer, L. 1995. Biochemistry, 4th edition. New York: W.H. Freeman.
Tsuneshige, A., K. Imai, and I. Tyuma. 1987. “The binding of hemoglobin to red cell membrane lowers its oxygen affinity.” Biochemistry 101(3): 695-704.

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History of "Cell membrane"

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@@ Line 1: / Line 1: @@
-{{unreferenced}}
+{{Ebapproved}}{{Copyedited}}{{Paid}}{{Approved}}{{Images OK}}{{Submitted}}
-[[Image:CellMembraneDrawing.jpg|thumb|400px|Drawing of a cell membrane]]
-A '''cell membrane''', '''plasma membrane''' or '''plasmalemma''' is a selectively  [[permeability|permeable]] [[lipid bilayer|bilayer]] which comprises the outer layer of a cell. It is comprised of, among other components, [[phospholipid]] and [[protein]] [[molecule]]s which separate the cell interior from its surroundings within animal cells, and control the input and output of cell through the use of [[Transmembrane receptor|receptor]] and [[Cell adhesion|cell adhesion proteins]], which also play a role in cell behavior and the organization of cells within tissues.
-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 [[Nanometre|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]].
+[[Image:CellMembraneDrawing.jpg|thumb|400px|The structure of a cell membrane, showing its lipid bilayer and embedded proteins]]
+The '''cell membrane''' (or '''plasma membrane''') is the thin outer layer of the [[cell (biology)|cell]] that differentiates the cell from its environment. In [[animal]]s, the cell membrane establishes this separation alone, whereas in [[yeast]], [[bacteria]] and [[plant]]s, an additional [[cell wall]] forms the outermost boundary, providing primarily mechanical support.
-==Structure - fluid mosaic==
+As a [[#Transport across the cell membrane|semi-permeable]] barrier, the cell membrane maintains an essential balance between individual distinctness and communal interaction: it functions to retain key components of the cell and to keep out toxic or unwanted substances, while selectively controlling the flow of nutrients and biochemical signals into the cell.
-The basic composition and structure of the plasma membrane is the same as that of the membranes that surround [[organelle]]s and other subcellular compartments. The foundation is a [[lipid bilayer|phospholipid bilayer]], and the membrane as a whole is often described as a ''fluid mosaic'' &ndash; a two-dimensional fluid of freely diffusing lipids, dotted or embedded with proteins, which may function as channels or transporters across the membrane, or as receptors. The model was first proposed by S.J. Singer (1971) as a lipid protein model and extended to include the fluid character in a publication with G.L. Nicolson in "Science" (1972).
+The cell membrane is composed mainly of [[phospholipid]] and [[protein]] [[molecule]]s arranged in organized but flexible sheets. The phospholipid components form a bilayer that contributes structural stability and creates the semi-permeable environment, while the proteins are responsible for most of the dynamic processes carried out by cell membranes, such as the transport of molecules into and out of the cell.
-Some of these proteins simply adhere to the membrane (''extrinsic'' or ''peripheral'' proteins), whereas others may be said to reside within it or to span it (''intrinsic'' proteins &ndash; 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. The regulation is assisted the bilayer by [[cholesterol]] molecules in [[eukaryote]]s and [[hopanoids]] in prokaryotes.
+Transport across the cell membrane underlies a variety of physiological processes, from the beating of an animal’s [[heart]] to the opening of tiny pores in [[leaf|leaves]] that enables gas exchange with the environment. A major cellular manifestation of motor neuron disease is the inability of nerve cells to stimulate the opening of channels through the membranes of [[muscle]] cells, which would result in normal muscle function.
-===Detailed Structure===
+The regulation of transport, though a crucial function of the cell membrane, is not its only role. Cell membranes assist in the organization of individual cells to form tissues. They are also involved in biological communication: the binding of a specific substance to the exterior of the membrane can initiate, modify, or turn off a cell function.
+==The structure of cell membranes==
+===Components===
+====Lipids====
-Phospholipid molecules in the cell membrane are "fluid," in the sense that they are free to diffuse and exhibit rapid lateral diffusion. [[Lipid rafts]] and [[caveolae]] are examples of cholesterol-enriched microdomains in the cell membrane.
+The three major types of [[lipid]]s found in the cell membrane are phospholipids, glycolipids, and [[cholesterol]] molecules.
-Many proteins are not free to diffuse. The [[cytoskeleton]] undergirds the cell membrane and provides anchoring points for integral membrane proteins. Anchoring restricts them to a particular cell face or surface &ndash; for example, the "apical" surface of [[epithelial cell]]s that line the [[vertebrate]] [[gastrointestinal tract|gut]] &ndash; and limits how far they may diffuse within the bilayer. Rather than presenting always a formless and fluid contour, 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. [[Synapse]]s are another example of highly-structured membrane.
+A phospholipid is composed of a polar head (a negatively charged phosphate group) and two non-polar tails (its two [[fatty acid]] chains). Phospholipids are said to be '''amphipathic''' molecules because they contain a [[hydrophilic]] (water-loving) head and two [[hydrophobic]] (water-fearing) tails. This amphipathic property causes multitudes of phospholipids suspended in water to organize naturally into a spherical, three-dimensional bilayer, which, becomes the cell membrane. Two phospholipid molecules in water will tend to join together via their water-fearing tails, and billions of them will cluster together side-by-side in the same fashion until the growing sheet curves back to make a closed sphere.
-New material is incorporated into the membrane, or deleted from it, by a variety of mechanisms.
+The length and properties of the fatty-acid components of phospholipids determine the fluidity of the cell membrane. At reduced temperatures, some organisms may vary the type and relative amounts of lipids to maintain the fluidity of their membranes. These changes in membrane lipid components contribute to the survival of plants, bacteria, and hibernating animals during winter.
-* Fusion of intracellular [[vesicles]] with the membrane not only excretes the contents of the vesicle, but also incorporates the vesicle membrane's components into the cell membrane. The membrane may form blebs that pinch off to become vesicles.
-* If a membrane is continuous with a tubular structure made of membrane material, then material from the tube can be drawn into the membrane continuously.
-* Although the concentration of membrane components in the aqueous phase is low (stable membrane components have low solubility in water), exchange of molecules with this small reservoir is possible.
-In all cases, the mechanical tension in the membrane has an effect on the rate of exchange. In some cells, usually having a smooth shape, the membrane tension and area are interrelated by elastic and dynamical mechanical properties, and the time-dependent interrelation is sometimes called ''homeostasis'', ''area regulation'' or ''tension regulation''.
-Structure:
+The regulation of membrane fluidity is assisted by another lipid, [[cholesterol]], which is primarily found in [[eukaryote]]s. (In [[prokaryote]]s, hopanoids perform a similar function.)
-The outer cell membrane and the membranes surrounding inner cell organelles are bilipid layers. To perform the function of the organelle, the membrane is specialized in that it contains specific proteins and lipid components that enable it to perform its unique roles for that cell or organelle. In a the cell membrane phospholipid molecules create a spherical three dimensional lipid bilayer shell around the cell. A phospholipid molecule is composed of a head and two tails. The circle, or head, is the negatively charged phosphate group and the two tails are the two highly hydrophobic fatty acid chains of the phospholipid.
+[[Carbohydrate]] components are linked to lipids (to form ''glycolipids'') or to proteins (''glycoproteins'') on the outside of the cell membrane. They are crucial in recognizing specific molecules or other cells. For example, the carbohydrate unit of some glycolipids changes when a cell becomes [[cancer]]ous, which may allow white blood cells to target cancer cells for destruction.
-Functions:
+====Proteins====
+Cell membranes contain two types of [[protein]]s:
+*''Extrinsic'' or ''peripheral'' proteins simply adhere to the membrane and are bound by polar interactions.
+*''Intrinsic proteins'' or ''integral membrane proteins'' may be said to reside within the membrane or to span it. They interact extensively with the [[fatty acid]] chains of membrane lipids and can be released only by agents that compete for these non-polar interactions. In addition, the cytoskeleton, which undergirds the cell membrane, provides anchoring points for integral membrane proteins.
-. It attaches parts of the cytoskeleton to the cell membrane in order to provide shape.
+The protein components of cell membranes may function as channels or transporters across the membrane or as receptors of biochemical information.
-. It attaches cells to an extra-cellular matrix in grouping cells together to form tissues.
+The relative number of proteins and lipids depends on the specialized function of the cell. For example, [[myelin]], a membrane that encloses some nerve cells, uses properties of lipids to act as an insulator, and so contains only one protein per 70 lipids. In general, most cell membranes are about 50 percent protein by weight.
-. It transports molecules into and out of cells by such methods as ion pumps, channel proteins and carrier proteins.
+===The fluid-mosaic model===
+The cell membrane is often described as a ''fluid mosaic''&mdash;a two-dimensional fluid of freely diffusing lipids, dotted or embedded with proteins. The model was first proposed by S. Jonathan Singer (1971) as a lipid-protein model and extended to include the fluid character in a publication with Garth L. Nicolson in ''Science'' (1972). Proteins are free to diffuse laterally in the lipid matrix unless restricted by specific interactions, but not to rotate from one side of the membrane to another.
-. It acts as receptor for the various chemical messages which pass between cells such as nerve impulses and hormone activity.
+Rather than presenting always a formless and fluid contour, however, the surface of the cell membrane may show structure. [[Synapses]], the junctions between nerve cells, are one example of a highly structured membrane.
-. It takes part in enzyme activity which can be important in the metabolism or as part of the body's defense mechanism.
+==Transport across the cell membrane==
+As the cell membrane is semi-permeable, only some molecules can pass unhindered into or out of the cell. These molecules are typically either small or non-polar. The cell membrane has a low permeability to [[ion]]s and most polar molecules, with [[water]] being a notable exception.
-==Transport across membranes==
+There are two major mechanisms for moving chemical substances across membranes: ''passive transport'' (which does not require the input of outside energy) and ''active transport'' (which is driven by the direct or indirect input of chemical energy in the form of [[ATP]]).
-As a lipid bilayer, the cell membrane is [[Semipermeable membrane|semi-permeable]]. This means that only some molecules can pass unhindered in or out of the cell. These molecules are either small or [[lipophilic]]. Other molecules can pass if there are specific transport molecules.
-Depending on the molecule, transport occurs by different mechanisms, which can be separated into those that do not consume energy in the form of [[Adenosine triphosphate|ATP]] (passive transport) and those that do (active transport).
 ===Passive transport===
-{{main|Passive transport}}
+Passive transport processes rely on a concentration gradient (a difference in concentration between the two sides of the membrane). This spontaneous process acts to decrease free energy and increase [[entropy]] in a system. There are two types of passive transport:
-Passive transport is a means of moving different chemical substances across membranes through [[diffusion]] of [[hydrophobic]] (non-polar) and small polar molecules, or facilitated diffusion of polar and ionic molecules, which relies on a [[transport protein]] to provide a channel or bind to specific molecules. This spontaneous process decreases free energy, and increases [[entropy]] in a system. Unlike active transport, this process does not involve any chemical energy (from [[adenosine triphosphate|ATP]]).
-The outer cell membrane and the membranes surrounding inner cell organelles are bilipid layers. To perform the function of the organelle, the membrane is specialized in that it contains specific proteins and lipid components that enable it to perform its unique roles for that cell or organelle. In the cell membrane phospholipid molecules create a spherical three dimensional lipid bilayer shell around the cell. A phospholipid molecule is composed of a head and two tails. The circle, or head, is the negatively charged phosphate group and the two tails are the two highly hydrophobic fatty acid chains of the phospholipid.
+*Simple diffusion of hydrophobic (non-polar) and small polar molecules through the phospholipid bilayer.
+*Facilitated diffusion of polar and ionic molecules, which relies on a [[transport protein]] to provide a channel or bind to specific molecules. Channels form continuous polar pathways across membranes that allow ions to flow rapidly down their electrochemical gradients (i.e., in a thermodynamically favorable direction).
 ===Active transport===
-{{main|Active transport}}
+Charged or polar molecules (such as [[amino acid]]s, [[sugar]]s, and ions) do not pass readily through the lipid bilayer. Protein pumps use a source of free energy, like [[ATP]] or [[light]], to drive the uphill transport. That is, active transport typically moves molecules against their electrochemical gradient, a process that would be [[entropy|entropically]] unfavorable were it not coupled with the [[hydrolysis]] of ATP. This coupling can be either primary or secondary:
+*''Primary active transport'' involves the direct participation of ATP.
+*In ''secondary active transport'', energy derived from the transport of a molecule (such as [[sodium]]) in the direction of its electrochemical gradient is used to move another molecule against its gradient.
-Typically moves molecules against their [[electrochemical gradient]], a process that would be [[entropy|entropically]] unfavorable were it not [[stoichiometry|stoichiometrically]] coupled with the hydrolysis of ATP. This coupling can be either primary or secondary. In the primary active transport, transporters that move molecules against their electrical/chemical gradient, hydrolyze ATP. In the secondary active transport, transporters use energy derived from transport of another molecule in the direction of their gradient, to move other molecules in the direction against their gradient. This can be either [[symport]] (in the same direction) or [[antiport]] (in the opposite direction).
+In general, active transport is much slower than pasive transport through channels.
-Examples include:
+The processes of ''endocytosis'', which bring macromolecules, large particles, and even small cells into a eukaryotic cell, can be thought of as examples of active transport. In endocytosis, the plasma membrane folds inward around materials from the environment, forming a small pocket. The pocket deepens, forming a vesicle that separates from the membrane and migrates into the cell interior. In ''exocytosis'', materials packaged in vesicles are exported from a cell when the vesicle membrane fuses with the cell membrane.
-#The usual cases of [[molecular exchanger]]s, [[Molecular transporter|transporters]] and [[transmembrane ATPase|pumps]]
-#[[endocytosis]] and [[exocytosis]], where molecules packaged in membrane [[vesicle (biology)|vesicle]]s are either imported or exported respectively, can be thought of as active.
-==See also==
+==Other functions of cell membranes==
-*[[Bacterial cell structure]]
+#''Organization''. Some receptors on the external surface of the cell membrane participate in the grouping of cells to form tissues (cellular adhesion).
+#''Information processing''. Membrane proteins may act as receptors for the various chemical messages that pass between cells. The movement of [[bacteria]] toward food and the response of target cells to [[hormone]]s such as insulin are two examples of processes that hinge on the detection of a signal by a specific receptor in a cell membrane.
+#''Enzyme assembly''. Cell membranes can serve as an assembly that organizes the specific [[enzyme]]s involved in a given [[metabolism|metabolic]] pathway. Binding the enzymes to the membrane in sequential order enables the series of chemical reactions in the pathway to be carried out efficiently.
+#''Biological communication''. Some membranes generate chemical or electrical signals. Cell membranes of nerve cells, muscle cells, and some eggs are excitable electrically. In nerve cells, for example, the plasma membrane conducts the nerve impulse from one end of the cell to the other.
-==External links==
+==References==
-*[http://www.biochemweb.org/lipids_membranes.shtml Lipids, Membranes and Vesicle Trafficking - The Virtual Library of Biochemistry and Cell Biology]
+* Purves, W., D. Sadava, G. Orians, and C. Heller. 2004. ''Life: The Science of Biology,'' 7th edition. Sunderland, MA: Sinauer.
-*[http://www.westernblotting.org/protocol%20membrane%20extraction.htm Cell membrane protein extraction protocol]
+* Stryer, L. 1995. ''Biochemistry,'' 4th edition. New York: W.H. Freeman.
-*[http://www.phys.unsw.edu.au/~jw/tension.html Membrane homeostasis, tension regulation, mechanosensitive membrane exchange and membrane traffic]
+* Tsuneshige, A., K. Imai, and I. Tyuma. 1987. “The binding of hemoglobin to red cell membrane lowers its oxygen affinity.” ''Biochemistry'' 101(3): 695-704.
-{{Organelles}}
 {{credit|71761057}}
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 [[Category:Life sciences]]
+[[Category:Cell biology]]