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Chemistry of life

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Lipids: triglycerides and fatty acids

Most macromolecules are polymers. Lipids are macromolecules – they can be very large indeed – but they are not polymers. Lipids are very varied and they play a number of key roles in living things that include playing an important part in the structure of cell membranes, as a high-energy food and as hormones, chemical messages which control the physiology of many living organisms.

All lipids contain carbon, hydrogen and oxygen, but they contain relatively few oxygen atoms. They all dissolve in organic solvents but do not dissolve in water. They are all macromolecules, but they contain quite a variety of different structures.

 

The triglycerides - fats and oils

Chemically fats and oils have the same basic structure, but fats are solids at room temperature and oils are liquids. Fats and oils are very compact molecules which are used as energy stores in both the plant and animal kingdoms. Lipid-rich plant and animal tissue make energy-rich food sources. The triglycerides are made up of glycerol with 3 fatty acids attached.

 

Glycerol (propane-1,2,3-triol)

All of the triglycerides are based around glycerol. The IUPAC name for this molecule is propane-1,2,3-triol but it is still always referred to in biochemistry as glycerol. It is a colourless, odourless viscous sweet liquid which is the core of all the triglycerides.

 

Lipids are very compact molecules which provide a lot of energy as they are digested, such as the suet this sparrow is eating.

(Photo credit: stevevoght, licensed under the Creative Commons Attribution-Share Alike 2.0 Generic license.)

 

Fatty acids

Fatty acids are carboxylic acids and over 70 different types have been extracted from living tissues alone. They have a very long pleated backbone of carbon atoms with hydrogen atoms attached, and a carboxyl group (-COOH) at one end. The long carbon chain is often represented by the letter R in diagrams. Fatty acids vary in two main ways:

  • The length of the carbon chain (in living organisms the length of the chain is often 15-17 carbon atoms long)
  • The type of bonds between the carbon atoms. If the bonds are all single bonds, it is a saturated fatty acid. If the fatty acid contains one or more double bonds in the carbon chain it is an unsaturated fatty acid. If the carbon chain has just one double bond it is a monounsaturated fatty acid. If it has two or more it is a polyunsaturated fatty acid.

 

Saturated and unsaturated fatty acids

 

Triglycerides are formed by condensation reactions between glycerol and three fatty acids to form an ester bond. Each condensation reaction between glycerol and a fatty acid is an example of esterification. This bond in an ester can be broken down by a hydrolysis reaction.

 

Reversible esterification and hydrolysis reactions between a fatty acid and glycerol