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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|fatty acids ]]containing between 14-24 carbon atoms. Two [[Fatty acid|fatty acids]] form [[Ester bond|ester bonds]] with [[Glycerol|glycerol]]. [[Glycerol|Glycerol]] further 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|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