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Chemistry of life

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Lipids: phospholipids, waxes and steroids

Every cell contains high levels of inorganic phosphate ions (-PO43-). Sometimes one of the fatty acids in a triglyceride is replaced by a phosphate group to form a phospholipid. Phospholipid molecules have a hydrophilic region around the phosphate group which is soluble in water and hydrophobic regions around the fatty acids which are not soluble and repel water. This is a key element in the structure of cell membranes and so in the nature of life itself.

Structure of a phospholipid – the hydrophilic and hydrophobic regions of the molecule have a major effect on the structure of cell membranes

The waterproof waxy layer of a leaf stops water getting into or out of the plant cells underneath.

 

Waxes

Waxes are lipids made up of very long chain fatty acids joined to alcohols by ester bonds that are insoluble in water but soluble in organic polar solvents. The big difference between waxes and triglycerides is that in waxes there is only one fatty acid joined to a single alcohol, because alcohols only have one available hydroxyl group, unlike glycerol which had three. Waxes on the surface of leaves and insect cuticles, along with oils on feathers and fur, form a water-proof layer which enables the organisms to survive in their environments.

 

Steroids

Steroids are insoluble in water but otherwise are not typical lipids – they are made up of large numbers of carbon atoms arranged in complex ring structures. They are very important in biological systems as hormones.

Other lipids or lipid-derived molecules which are important in biological systems include:

  • Carotenoids - are involved in photosynthesis
  • Lipid-rich myelin – found wrapped around neurones providing electrical insulation which makes rapid transmission of impulses possible
  • Fat in the bodies of animals acts to protect and cushion internal organs and gives thermal insulation (e.g. blubber in seals and polar bears).

Blubber beneath the skin of a polar bear helps it keep warm and provides buoyancy for swimming. (Photo credit: Understanding Animal Research)

Activity

For years the scientific evidence suggested saturated fats in the diet increased the risk of coronary heart disease. A recent massive study throws doubt on this.

Investigate this using a variety of sources (for example the internet, scientific journals or interviews with experts if you can). Then write two articles on the topic, one for a red-top newspaper and one to be published in New Scientist magazine. Consider the different audiences for each type of publication.