When lipids are together they spontaneously form a lipid bilayer due to their amphiphatic nature. Lipids consist of a 'head' and a 'tail' section. The 'tail' is hydrophobic and will group together in the centre of the bilayer and the hydrophilic heads will form the edges of the bilayer protecting the 'tails' from contact with water. There are 3 main classes of lipids used in the forming of a lipid bilayer for function as a membrane: phospholipids; glycolipids and cholesterol. The receptors needed on the membrane, sometimes along with the cholesterol, are structures which protrude from the surface of the bilayer; hence not buried within the bilayer structure. The lipid mostly used for membranes is the phospholipid. In all lipids, the 'tail' is formed from hydrocarbon chains, varying in length and saturation. In phospholipids, the 'head' is formed from a phosphate attached to an alcohol and a 'platform' either glycerol or serine, giving the lipid its hydrophillic properties.
The lipid bilayer is seen as the universal basis for membrane structure. It can be seen using electron microscopy. However, to reveal the details of its organisation, specialised techniques such as freeze-fracture electron microscopy and x-ray diffraction are required.
There are four major phospholipids in the mammalian plasma membrane; phosphatidylethanolamine. phosphatidylserine, phosphatidylcholine and sphingomyelin. Phosphatidylserine is the only charged phospholipid, with a charge of -1. Sphingomyelin is the only phospholipid based on serine as opposed to glycerol. These four phospholipids are distributed asymmetrically between the two leaflets of the bilayer - both sphingomyelin and phosphatidylcholine tend to be in the non-cytosolic monolayer - this results in an asymmetry in the bilayer.
Lipid asymmetry is functionally important, especially in converting extracellular signals into intracellular ones. Many cytosolic proteins bind to specific lipid head groups found in the cytosolic monolayer of the lipid bilayer. The enzyme protein kinase C (PKC), for example, which is activated in response to various extracellular signals, binds to the cytosolic face of the plasma membrane, where phosphatidylserine is concentrated and requires this negatively charged phospholipid for its activity.
All lipids are not soluble in water. Both triglycerides and phospholipids are lipids but the major difference between the two molecules is the number of fatty acids which varies between them with triglycerides possessing three fatty acids, unlike phospholipids in which one of the three fatty acids is replaced by a phosphate group. The phosphate group is electrically charged and attracted to water thereby, forming the hydrophilic region(attracts water) on phospholipids while the two fatty acids from the hydrophobic region(repels water). These properties make the phospholipid molecules very essential in the cell membrane. The cell membrane is made up of a double layer of phospholipid molecules and this structure is as a result of the fact that a group of phospholipid molecules arranges itself into a bilayer when present in water, with the hydrophilic region (i.e head) facing outward, interacting with the watery environment while the hydrophobic region(i.e tail) faces inward thereby, avoiding the watery environment. The cell membrane is also made up of proteins, cholesterol molecules. glycolipids(phospholipids + short chains of carbohydrates) and glycoproteins( proteins + short chains of carbohydrates). The structure of the cell membrane is called "The Fluid Mosaic Model".
'Fluid' because the exchange of phospholipids within monolayers is frequent, along with individual rotations. 'Mosaic' because of the dotted nature of the protein molecules within the membrane and 'model' because, nobody has ever seen the membrane the way it is on diagrams due to the small nature of its components but the structure has been worked out because it explains the behaviour of the membrane.
The fluidity of the lipid bilayer depends on how many unsaturated fatty acid tails are present. Unsaturated tails have kinks due to a cis-double bond, which means the tails are less tightly packed together and therefore more fluid.
- ↑ Berg, JM. (2006) "Biochemistry" 6th Ed. p239, New York, W.H. Freeman
- ↑ Alberts et al. (2002). Molecular Biology of the Cell. 4th Edition. US Garland Science. Page 584.
- ↑ Alberts et al (2007). Molecular Biology of the Cell. 5th ed. New York: Garland Science. 617-650
- ↑ Alberts et al (2015). Molecular Biology of the Cell. 6th ed. New York: Garland Science. 574
- ↑ Mary Jones (2010) Biology Revision Guide, 1st edition, London NW1 3BH: Hodder Education
- ↑ Alberts et al. (2002) Molecular Biology of the Cell. 4th Edition. US Garland Science. 584