Phospholipid bilayer

Phospholipid bilayers form the plasma membrane that surrounds all cells. The molecules that make up the cell membrane are called phospholipids.

Phosholipid Structure

Phospholipids are amphipathic molecules. This means that they have a hydrophilic, polar phosphate head and two hydrophobic fatty acid tails. These components of the phospholipids cause them to orientate themselves, so the phosphate head can interact with water and the fatty acid tails can't, hence forming a bilayer. This arrangement can also be called a bio-molecular sheet, as the hydrophobic tails from each individual lipid sheet interact with one another forming a hydrophobic interior that acts as a permeability barrier^[1]. The hydrophilic head is made up of Gycerol and a phosphate group - it is the phosphate group which makes the head hydrophilic. The hydrophobic tail is made up of 2 fatty acid chains, one of which usually contains a Cis double bond (C=C). This double bind causes the tail to 'kink' which affects the packing structure and fluidity of the bilayer. In mammalian plasma membranes 4 main structures of phospholipids can be found^[2]:

• phosphatidylethanolamine
• phosphatidylserine (negatively charged)
• phosphatidylcholine
• sphingomyelin

As well as these, the membrane also contains various other types of lipid such as cholesterol and proteins. These molecules contribute significantly to the mass of the membrane. Some of the fatty acids in the phospholipid molecules are unsaturated, with one or more carbon-carbon double bonds in its hydrocarbon chain. These double bonds create a kink in the hydrophobic tails. These kinks prevent adjacent phospholipid molecules from packing too close together, which causes an increase in the fluidity of the bilayer. The length of the fatty acid tails also has an effect on the fluidity of the bilayer. If the bilayer has shorter fatty acid chains they are less likely to 'stick' together and they'll be less tightly packed together increasing the fluidity of the membrane. The bilayer is arranged so that the phospholipid heads face outwards and the fatty acid chains face inwards, with cholesterol and proteins scattered throughout the membrane. This structure is described as fluid because the phospholipids can diffuse along the membrane^[3][4]. The bilayer can form spontaneously when in an aqueous environment which means it is also self-sealing. This is due to how the hydrophobic tail and hydrophilic head react when they come in contact with water. The hydrophilic head is soluble in water due to it being charged or polar. This allows it to form electrostatic forces or hydrogen bonds with the water molecules. However, the hydrophobic tail is insoluble in water due to it being uncharged and non-polar meaning it cannot form any interactions with water molecules. Therefore as the bilayer forms, the phospholipids are arranged so that the tails are in the middle of the bilayer and the heads are on the outside^[5][6].

References

1. ↑ Berg M, Stryer L and Tymoczko J. (2007) Biochemistry, 7th edition, New York: WH Freeman
2. ↑ Molecular Biology of the Cell, Alberts et all, 5th Edition, 2009, Garland Science, New York
3. ↑ Bradley P and Calvert J. (2008) Compendium for medical sciences, 1st edition, Banbury: Scion Publishing Limited. (pages 33-34)
4. ↑ Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P. (2012). Molecular Biology of the Cell - Fifth Edition, New York: Garland Science
5. ↑ Alberts A, Johnson A, Lewis J, Raff M, Roberts K, Walter P, (2008) Molecular Biology of the Cell, 5th edition, New York: Garland Science
6. ↑ Alberts, B (2008). Molecular biology of the cell. 5th ed. New York: Garland Science. 622 - 624.