Phospholipid

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The structure of a phospholipid
Phospholipids are lipid molecules which have a phosphate group attached.

They are highly abundant in cell membranes, where they form a lipid bilayer, due to the amphiphatic nature of their hydrophilic heads and hydrophobic tails[1]. The majority of atoms in the hydrophobic tails are nonpolar and have no charge, this is what makes them insoluble when dissolved in water. However, the hydrophilic heads are soluble in water due to the presence of either charged groups which are electrostatically attracted to the water molecules, or polar groups which enable hydrogen bonds to be form between the heads and the water molecules[2].

The most common constituent of any lipid bilayer making up a cell membrane is the phospholipid. Phospholipids are amphiphilic. They have a polar head and two hydrocarbon tails, which are nonpolar. The phospholipids that make up the cell membranes of plants, bacterial or animal cells often have fatty acids tails. Of these two fatty acid tails one is unsaturated (contains double bonds) and the other is saturated. This difference causes variation in the length of the tails and thus alters the fluidity of the membrane[3]. For instance, in an unsaturated hydrocarbon tail, the double bonds create kinks so that the molecules do not pack as tightly together, giving rise to a more fluid consistency[4]. Whereas, in a saturated hydrocarbon tail, there are no kinks and thus, the molecules pack together tightly resulting in a less fluid membrane[5].The chemical makes up of the tails can differ. This means that there are many different phospholipids that can make up a cell membrane. The main type found in mammalian cells are phosphoglycerides such as phosphatidylcholine, phosphatidylserine and phosphatidylethanolamine. These differ in the fact that they have choline, serine and ethanolamine (respectively) attached to their phosphate groups. phosphatidylserine is the only one that carries a negative charge and all the other phosphoglycerides have no charge at physiological pH[6]. The phospholipid bilayer acts as a barrier allowing the smaller molecules such as O2 and glycerol in, but keeping larger molecules such as amino acids and ions out. The bilayer can be broken down by detergents.

Aside from phosphoglycerides, another class of lipids commonly found within the body are sphingolipids[7].

References

  1. Alberts et al. (2002) Molecular Biology Of The Cell, 4th edition, New York: Garland Science. p62
  2. Alberts et al. (2008) Molecular Biology Of The Cell, 5th edition, New York: Garland Science. p620
  3. Bruce Alberts,Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, Peter Walter, Molecular Biology of the Cell, Fifth Edition, 2008, Garland Science, New York. p618
  4. Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K and Walter, P. (2008) Molecular Biology of the cell. New York, Garland Science. pp 59.
  5. Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K and Walter, P. (2008) Molecular Biology of the cell. New York, Garland Science. pp 59.
  6. Alberts, Johnson, Lewis, Raff, Roberts, Walter, (2008), Molecular biology of the cell, 5th edition, New York, Garland science, pp620.
  7. Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K and Walter, P. (2008) Molecular Biology of the cell. New York, Garland Science. pp 59.
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