Lipid bi-layer

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The lipid bi-layer provides a basic impermeable barrier around cells, and is approximately 5nm thick. They help to maintain different environments between cytosol and the extracellular environment/organelles, and also control the transport of solutes between compartments. The lipid bilayer consists of four major phospholipids which are; phopshatidylethanolamine, phosphatidylserine, phosphatidycholine and sphingomyelin. These are derived from glycerol apart from sphingomyelin, which is derived form serine. Phosphatidylserine is the only one negatively charged [1].

The fluidity of the membrane is dependant on its composition and the temperature. If the lipid bi layer has a greater fluidity then the temperature at which the membrane freezes will be lower. However, the fluidity must be maintained or certain transport processes and enzyme activities could cease if the viscosity is increased beyond its threshold level.The fluidity of the membrane can be altered depending on the amount of cholesterol in the plasma membrane.  It slots in tightly between the hydrocarbon chains of the phospholipids and prevents them from packing together, so at high temperatures, the membrane is less fluid [2].
The lipid molecules in a bilayer are fluid and not fixed in place. They are able to diffuse laterally, rotate, flex their fatty acid chains and can flip vertically exchanging positions with the opposing lipid. However, the vertical exchange rarely happens. Lipids present in the bilayer can also be unsaturated due to double bonds being present in the fatty acid chains. This prevents the lipids from being to pack as close together. This causes the lipid bilayer to be more fluid as it is harder to freeze and due to the lipids being further apart because of unsaturated hydrocarbon chains, the bilayer is thinner than one formed of saturated lipids. A shorter chain length also increases the fluidity. This is because a shorter chain length reduces the interaction between the lipids in the same and opposite monolayer.

References

  1. Molecular Biology of the Cell, Alberts et al., 5th Edition (2007) Garland Science, New York Chapter 10 p617-65
  2. The World of the Cell, Becker et al., 7th Edition (2009), Pearson, San Fransisco, p171
 


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