Cell membrane
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Reverted edits by [[Special:Contributions/190.25.48.7|190.25.48.7]] to last version by TimVickers (using [[WP:HG|Huggle]])
[[Image:Cell membrane detailed diagram 4.svg|thumb|400px|Illustration of a Eukaryotic cell membrane]]
The '''cell membrane''' (also called the '''plasma membrane''', '''plasmalemma''', or "phospholipid bilayer") is a [[selectively permeable]] [[lipid bilayer]] found in all [[cell (biology)|cells]].<ref name = MBOC>{{cite book | author = Alberts B, Johnson A, Lewis J, ''et al'' | title = Molecular Biology of the Cell | edition = 4th ed. | isbn = 0-8153-3218-1 |url = http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.1864}}</ref> It contains a wide variety of biological [[molecule]]s, primarily [[protein]]s and [[lipids]], which are involved in a vast array of cellular processes such as cell adhesion, ion channel conductance and cell signaling. The plasma membrane also serves as the attachment point for both the intracellular [[cytoskeleton]] and, if present, the [[cell wall]].
==Function==
The cell membrane surrounds the [[cytoplasm]] of a cell and, in animal cells, physically separates the intracellular components from the extracellular environment, thereby serving a function similar to that of [[skin]]. In [[fungi]], some [[bacteria]], and [[plants]], an additional cell wall forms the outermost boundary; however, the cell wall plays mostly a mechanical support role rather than a role as a selective boundary. The cell membrane also plays a role in anchoring the [[cytoskeleton]] to provide shape to the cell, and in attaching to the [[extracellular matrix]] to help group cells together in the formation of [[Tissue (biology)|tissues]].
The barrier is [[Semipermeable membrane|selectively permeable]] and able to regulate what enters and exits the cell, thus facilitating the [[membrane transport protein|transport]] of materials needed for survival. The movement of substances across the membrane can be either ''passive'', occurring without the input of cellular energy, or ''active'', requiring the cell to expend energy in moving it. The membrane also maintains the [[cell potential]].
Specific proteins embedded in the cell membrane can act as molecular signals that allow cells to communicate with each other. [[Protein receptor]]s are found ubiquitously and function to receive signals from both the environment and other cells. These signals are ''[[signal transduction|transduced]]'' into a form that the cell can use to directly effect a response. Other proteins on the surface of the cell membrane serve as "markers" that identify a cell to other cells. The interaction of these markers with their respective receptors forms the basis of cell-cell interaction in the [[immune system]].
==Structure==
=== Lipid bilayer ===
[[Image:Fluid Mosaic.svg|thumb|260px|right|Diagram of the arrangement of amphipathic lipid molecules to form a [[lipid bilayer]]. The yellow [[Chemical polarity|polar]] head groups separate the grey hydrophobic tails from the aqueous cytosolic and extracellular environments.]]
The cell membrane consists primarily of a thin layer of [[amphipathic]] [[phospholipids]] which spontaneously arrange so that the hydrophobic "tail" regions are shielded from the surrounding polar fluid, causing the more hydrophilic "head" regions to associate with the cytosolic and extracellular faces of the resulting bilayer. This forms a continuous, spherical [[lipid bilayer]] approximately 7 [[Nanometre|nm]] thick, barely discernible with a [[transmission electron microscope]].<ref name = MBOC />
The arrangement of hydrophilic and hydrophobic heads of the lipid bilayer prevent polar solutes (e.g. amino acids, nucleic acids, carbohydrates, proteins, and ions) from diffusing across the membrane, but generally allows for the passive diffusion of hydrophobic molecules. This affords the cell the ability to control the movement of these substances via [[transmembrane protein]] complexes such as pores and gates.
[[Flippase]]s and [[Scramblase]]s concentrate [[phosphatidyl serine]], which carries a negative charge, on the inner membrane. Along with [[sialic acid|NANA]], this creates an extra barrier to charged [[Moiety|moities]] moving through the membrane.
Membranes serve diverse functions in eukaryotic and prokaryotic cells. One important role is to regulate the movement of materials into and out of cells. The phospholipid bilayer structure (fluid mosaic model) with specific membrane proteins accounts for the selective permeability of the membrane and passive and active transport mechanisms. In addition, membranes in prokaryotes and in the mitochondria and chloroplasts of eukaryotes facilitate the synthesis of ATP through chemiosmosis.
===Integral membrane proteins===
The cell membrane contains many [[integral membrane protein]]s, which pepper the entire surface. These structures, which can be visualized by [[electron microscopy]] or [[fluorescence microscopy]], can be found on the inside of the membrane, the outside, or [[Transmembrane protein|membrane spanning]]. These may include [[integrins]], [[cadherins]], [[desmosome]]s,
[[clathrin|clathrin-coated pits]], [[caveolae]]s, and different structures involved in [[cell adhesion]].
=== Membrane skeleton ===
The [[cytoskeleton]] is found underlying the cell membrane in the cytoplasm and provides a scaffolding for membrane proteins to anchor to, as well as forming organelles that extend from the cell. Anchoring proteins restricts them to a particular cell surface — for example, the ''apical surface'' of [[epithelial cell]]s that line the [[vertebrate]] [[gastrointestinal tract|gut]] — and limits how far they may diffuse within the bilayer. The cytoskeleton is able to form appendage-like organelles, such as [[cilia]], which are [[microtubule]]-based extensions covered by the cell membrane, and [[filopodia]], which are [[actin]]-based extensions. These extensions are ensheathed in membrane and project from the surface of the cell in order to sense the external environment and/or make contact with the substrate or other cells. The apical surfaces of epithelial cells are dense with actin-based finger-like projections known as [[microvilli]], which increase cell surface area and thereby increase the absorption rate of nutrients. Localized decoupling of the cytoskeleton and cell membrane results in formation of a [[bleb (cell biology)|bleb]].
=== Structure and the ''Fluid mosaic model'' ===
{{Expand|date=June 2008}}
According to the fluid mosaic model of [[S. J. Singer]] and [[Garth Nicolson]], the biological membranes can be considered as a two-dimensional liquid where all lipid and protein molecules diffuse more or less freely<ref>[http://www.ncbi.nlm.nih.gov/sites/entrez?Db=pubmed&Cmd=ShowDetailView&TermToSearch=4333397&ordinalpos=6&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum The fluid mosaic model of the structure of cell membranes] by S. J. Singer and G. L. Nicolson in [[Science (journal)|Science]] (1972) Volume 175, pages 720-731.</ref>. This picture may be valid in the space scale of 10 nm. However, the plasma membranes contain different structures or domains that can be classified as (a) protein-protein complexes; (b) [[lipid raft]]s, (c) pickets and fences formed by the actin-based [[cytoskeleton]]; and (d) large stable structures, such as synapses or desmosomes.
The fluid mosaic model can be seen when the membrane proteins of two cells (e.g., a human cell and a mouse cell) are tagged with different-coloured fluorescent labels. When the two cells are fused, the two colours intermix, indicating that the proteins are free to move in the 2D plane.
==Composition==
Cell membranes contain a variety of biological molecules, notable lipids and proteins. Material is incorporated into the membrane, or deleted from it, by a variety of mechanisms:
* Fusion of intracellular [[vesicles]] with the membrane ([[exocytosis]]) 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 around extracellular material that pinch off to become vesicles ([[endocytosis]]).
* 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''.
===Lipids===
[[Image:Membrane lipids.png|thumb|right|310px|Examples of the major membrane phospholipids and glycolipids: [[phosphatidylcholine]] (PtdCho), [[phosphatidylethanolamine]] (PtdEtn), [[phosphatidylinositol]] (PtdIns), [[phosphatidylserine]] (PtdSer).]]
The cell membrane consists of three classes of [[amphipathic]] lipids: [[phospholipid]]s, [[glycolipid]]s, and [[steroid]]s. The amount of each depends upon the type of cell, but in the majority of cases phospholipids are the most abundant.<ref name=Lodish>{{cite book | title=Molecular Cell Biology| author = Lodish H, Berk A, Zipursky LS, ''et al'' | edition = 4th ed. | year = 2004 | isbn = 0-7167-3136-31986}}</ref> In [[red blood cell|RBC]] studies, 30% of the plasma membrane is lipid.
The fatty chains in phospholipids and glycolipids usually contain an even number of carbon atoms, typically between 14 and 24. The 16- and 18-carbon fatty acids are the most common. Fatty acids may be saturated or unsaturated, with the configuration of the double bonds nearly always ''cis''. The length and the degree of unsaturation of fatty acids chains have a profound effect on membranes fluidity<ref name="flashbio">{{cite web|url=http://www.bio.davidson.edu/people/macampbell/111/memb-swf/membranes.swf|title=Membrane Structure|accessdate = 2007-01-11|publisher=Davidson College|year=2002|author=Jesse Gray, Shana Groeschler, Tony Le, Zara Gonzalez|format=SWF}}</ref> as unsaturated lipids create a kink, preventing the fatty acids from packing together as tightly, thus decreasing the melting point (increasing the fluidity) of the membrane. The ability of some organisms to regulate the fluidity of their cell membranes by altering lipid composition is called [[homeoviscous adaptation]].
The entire membrane is held together via [[non-covalent]] interaction of hydrophobic tails, however the structure is quite fluid and not fixed rigidly in place. Phospholipid molecules in the cell membrane are "fluid" in the sense that they are free to diffuse and exhibit rapid lateral diffusion along the layer in which they are present. However, movement of phospholipid molecules between layers is not energetically favourable and does not occur to an appreciable extent. [[Lipid rafts]] and [[caveolae]] are examples of [[cholesterol]]-enriched microdomains in the cell membrane.
In animal cells cholesterol is normally found dispersed in varying degrees throughout cell membranes, in the irregular spaces between the hydrophobic tails of the membrane lipids, where it confers a stiffening and strengthening effect on the membrane.<ref name = MBOC />
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===Carbohydrates===
About 5% of the plasma membrane weight is [[carbohydrate]], predominantly [[glycoprotein]], but with some [[lipoprotein]] ([[cerebroside]]s and [[ganglioside]]s). For the most part, no [[glycosylation]] occurs on other unit membranes, and only ever occurs on the extracellular surface of cell membranes.
The [[glycocalyx]] is an important feature in all cells, especially [[epithelium|epithelia]] with microvilli. Recent data suggest the glycocalyx participates in cell adhesion, [[lymphocyte homing]], and many others.
The [[penultimate]] sugar is [[galactose]] and the terminal sugar is [[sialic acid]], as the sugar backbone is modified in the [[golgi apparatus]]. Sialic acid carries a negative charge, providing an external barrier to charged particles.
=== Proteins ===
{| class = "prettytable" style = "float:right; font-size:85%; margin-left:15px; width:60%"
| '''Type''' || '''Description''' || '''Examples'''
|-
| [[Integral protein]]s<br />or ''transmembrane proteins'' || Span the membrane and have a hydrophilic [[cytosol]]ic [[protein domains|domain]], which interacts with internal molecules, a hydrophobic membrane-spanning domain that anchors it within the cell membrane, and a hydrophilic extracellular domain that interacts with external molecules. The hydrophobic domain consists of one, multiple, or a combination of [[Alpha helix|α-helices]] and [[Beta sheet|β sheet]] protein [[Structural motif|motif]]s. || [[Ion channel]]s, [[proton pump]]s, [[G protein-coupled receptor]]
|-
|[[Lipid anchored protein]]s || Covalently-bound to single or multiple lipid molecules; hydrophobically insert into the cell membrane and anchor the protein. The protein itself is not in contact with the membrane. || [[G protein]]s
|-
|[[Peripheral protein]]s || Attached to integral membrane proteins, or associated with peripheral regions of the [[lipid bilayer]]. These proteins tend to have only temporary interactions with biological membranes, and, once reacted the molecule, dissociates to carry on its work in the cytoplasm. || [[Peripheral protein#Enzymes|Some enzymes]], [[Peripheral protein#Polypeptide ligands .28hormones.2C inhibitors.2C toxins.2C antimicrobial peptides.29|some hormones]]
|}
The cell membrane plays host to a large amount of protein that is responsible for its various activities. The amount of protein differs between species and according to function, however the typical amount in a cell membrane is 50%.<ref name="flashbio">{{Cite web|url=http://www.bio.davidson.edu/people/macampbell/111/memb-swf/membranes.swf|title=Membrane Structure|accessdate = 2007-01-11|publisher=Davidson College|year=2002|author=Jesse Gray, Shana Groeschler, Tony Le, Zara Gonzalez|format=SWF}}</ref> These proteins are undoubtedly important to a cell: Approximately a third of the [[gene]]s in [[yeast]] code specifically for them, and this number is even higher in multicellular organisms.<ref name=Lodish />
The cell membrane, being exposed to the outside environment, is an important site of cell-cell communication. As such, a large variety of [[protein receptor]]s and identification proteins, such as [[antigen]]s, are present on the surface of the membrane. Functions of membrane proteins can also include cell-cell contact, surface recognition, cytoskeleton contact, signalling, enzymic activity, or transporting substances across the membrane.
Most membrane proteins must be inserted in some way into the membrane. For this to occur, an N-terminus "signal sequence" of amino acids directs proteins to the [[endoplasmic reticulum]], which inserts the proteins into a lipid bilayer. Once inserted, the proteins is then transported to its final destination in [[vesicles]], where the vesicle fuses with the target membrane/
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==Variation==
The cell membrane has slightly different composition in different [[cell types]] and has therefore different denominations in different cell types:
*[[Sarcolemma]] in [[myocytes]]
*Oolemma in [[oocytes]].
==Permeability== <!--Membrane permeability redirects here-->
The permeability of membranes is the ease of molecules to pass it. This depends mainly on [[electric charge]] and, to a slightly lesser extent, on the [[molar mass]] of the molecule. Electrically-neutral and small molecules pass the membrane easier than charged, large ones.
The electric charge phenomenon results in [[pH parturition]] of substances throughout the [[fluid compartment]]s of the body.
==See also==
* [[Ammonium transporter]]
* [[AP2 adaptors]]
* [[Bacterial cell structure]]
* [[Cell adhesion]]
* [[Efflux (microbiology)]]
* [[Elasticity of cell membranes]]
* [[Gram-negative bacteria]]
* [[Gram-positive bacteria]]
==References==
{{reflist}}
==External links==
{{Commonscat|Cell membrane}}
* [http://www.biochemweb.org/lipids_membranes.shtml Lipids, Membranes and Vesicle Trafficking - The Virtual Library of Biochemistry and Cell Biology]
* [http://www.westernblotting.org/protocol%20membrane%20extraction.htm Cell membrane protein extraction protocol]
* [http://www.phys.unsw.edu.au/~jw/tension.html Membrane homeostasis, tension regulation, mechanosensitive membrane exchange and membrane traffic]
* [http://opm.phar.umich.edu/localization.php?localization=Eukaryotic%20plasma%20membrane 3D structures of proteins associated with plasma membrane of eukaryotic cells]
{{Organelles}}
{{Structures of the cell membrane}}
[[Category:Cell anatomy]]
[[Category:Membrane biology]]
[[Category:Organelles]]
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[[bg:Клетъчна мембрана]]
[[ca:Membrana plasmàtica]]
[[cs:Cytoplazmatická membrána]]
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[[es:Membrana plasmática]]
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[[it:Membrana cellulare]]
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[[simple:Cell membrane]]
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