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| Cell membranes form the boundary between a cell's interior environment and its exterior environment. They consist of a 5 nm thick bilayer of [[Lipid|lipids]] which interact in a [[Hydrophobic|hydrophobic]] manner to [[Transmembrane protein|transmembrane proteins]] embedded within the membrane. Cell membranes contain 500-1000 [[Lipid|lipid]] types which give rise to the permeability barrier to [[Water|water]]-soluble [[Molecules|molecules]]. The embedded [[Proteins|proteins]] function as either substrate transporters, [[Receptor|receptors]], [[Enzyme|enzymes]] or provide links to the [[Cytoskeleton|cytoskeleton]]. Cell membranes also contain [[Sterols|sterols]], [[Glycolipids|glycolipids]], and [[Glycoproteins|glycoproteins]].
| | See [[Cell_membrane|cell membrane]] |
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| The [[Lipid|lipids]] that constitute a cell membrane are synthesised in the cytosolic monolayer of the [[Endoplasmic reticulum|endoplasmic reticulum]]. They are [[Amphiphilic|amphiphilic]] in nature since they have a [[Hydrophilic|hydrophilic]] head group and two [[Hydrophobic|hydrophobic]] tails. It is this [[Amphiphilic|amphiphilic]] property that leads to a natural tendency of lipids to form bilayers. In a bilayer, the tails face inwards and interact through [[Van der waals forces|van der Waals forces]] whereas the head groups interact with water molecules either side of the membrane. [[Phospholipids|Phospholipids]] are the most abundant membrane [[Lipid|lipid]] and feature a group called [[Phosphoglycerides|phosphoglycerides]] of which there are three types: [[Phosphatidylserine|phosphatidylserine]], [[Phosphatidylethanolamine|phosphatidylethanolamine]], and [[Phosphatidylcholine|phosphatidylcholine]]. The tails of [[Phosphoglycerides|phosphoglycerides]] are fatty acids containing between 14-24 carbon atoms. Two [[Fatty acid|fatty acids]] form [[Ester bond|ester bonds]] with [[Glycerol|glycerol]], which also binds to a polar head group consisting of a [[Phosphate|phosphate]] group bound to either [[Choline|choline]], [[Ethanolamine|ethanolamine]], or [[Serine|serine]]. The chain length and saturation of [[Fatty acid|fatty acids]] affects the fluidity of the cell membrane. Microorganisms which have temperatures dependent on the environment, like [[Bacteria|bacteria]] and [[Yeast|yeast]], are able to change the properties of their membrane lipids. For example, when temperatures fall, lipids with short chain length and highly unsaturated fatty acids are produced which lowers the temperature at which the membrane lipids change phase from a liquid to a crystalline state. This maintains the cell membrane at a relatively constant fluidity. The [[Lipid|lipids]] within a membrane are able to rotate and move laterally within a monolayer but movement between monolayers, known as 'flip-flop', rarely occurs. However, there is great asymmetry in the [[Lipid|lipid]] compositions of the two monolayers. [[Phospholipid|Phospholipid]] translocators catalyse the 'flip-flop' of specific [[Lipid|lipids]]. In [[Red blood cells|red blood cells]], [[Choline|choline]]-containing [[Lipid|lipids]] are concentrated in the outer monolayer whereas [[Amino acid|amino acid]]-containing [[Lipid|lipids]] (ethanolamine- or serine-containing lipids) are in the [[Cytosol|cytosolic]] monolayer. Another membrane [[Lipid|lipid]] is [[Sphingomyelin|sphingomyelin]]. It is derived form [[Shingosine|shingosine]], an acyl chain which features an [[Amine group|amine group]] and two [[Hydroxyl group|hydroxyl groups]] at one end of the [[Molecule|molecule]]. [[Sphingomyelin|Sphingomyelin]] is made through the addition of a [[Fatty acid|fatty acid]] and phosphocholine group to an amine group and hydroxyl group in [[Sphingosine|sphingosine]], respectively. [[Sphingomyelin|Sphingomyelin]] is involved in [[Lipid rafts|lipid rafts]] which are membrane domains that are thicker than that normal of a cell membrane and can better accommodate some [[Transmembrane proteins|transmembrane proteins]]. [[Lipid rafts|Lipid rafts]] are involved in localising [[Protein|proteins]] for [[Vesicular transport|vesicular transport]] or in the formation of protein assemblies.<br>
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| [[Cholesterol|Cholesterol]] and [[Glycolipids|glycolipids]] are also cell membrane constituents. [[Cholesterol|Cholesterol]] is a sterol and its presence in membranes reduces the permeability of the membrane to [[Water|water]]-soluble substances and prevents crystallization occuring between the highly concentrated hydrocarbon tails of lipids. It is also important in stabilising [[Lipid rafts|lipid rafts]]. Bacterial cell membranes have no [[Cholesterol|cholesterol]]. Glycolipids are exposed to the extracellular environment due to [[Glycosylation|glycosylation]] within the lumen of the [[Golgi apparatus|Golgi apparatus]]. They are involved in cell-cell recognition. Complex [[Glycolipids|glycolipids]] called [[Gangliosides|gangliosides]] can provide entry points for bacterial toxins like the [[Cholera toxin|cholera toxin]] <ref>Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2008) Molecular Biology of the Cell, 5th Edition, Pages 617-629, New York: Garland Science</ref>. <br>
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| === References ===
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| <references /><br>
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Latest revision as of 18:11, 22 November 2011