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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; they have a [[Hydrophilic|hydrophilic]] head group and two [[Hydrophobic|hydrophobic]] tails. It is this [[Amphiphilic|amphiphilic]] property that leads to a natural tendency 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 [[Lipid|lipid]] type and mainly feature [[Phosphoglycerides|phosphoglycerides]] of which there are three: [[Phosphatidylserine|phosphatidylserine]], [[Phosphatidylethanolamine|phosphatidylethanolamine]], and [[Phosphatidylcholine|phosphatidylcholine]]. The tails of [[Phosphoglycerides|phosphoglycerides]] are fatty acids containing 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 featuring a [[Phosphate|phosphate]] group bound to either [[Choline|choline]], [[Ethanolamine|ethanolamine]], or [[Serine|serine]]. The length and saturation of the [[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 a phase transition between a liquid to crystalline state occurs. The [[Lipid|lipids]] within a membrane are able to rotate, 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]] 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 addition of a [[Fatty acid|fatty acid]] to the amine group and a [[Phosphocholine|phosphocholine]] group to a [[Hydroxyl group|hydroxyl group]] in [[Sphingosine|sphingosine]]. [[Sphingomyelin|Sphingomyelin]] is involved in [[Lipid rafts|lipid rafts]], membrane domains which are thicker and 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 that is present in membranes. They reduce the permeability of the membrane to [[Water|water]]-soluble substances and prevent crystallization 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]] 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