Lipids (AQA A Level Biology)

Lipids

Lipids

• Macromolecules which contain carbon, hydrogen and oxygen atoms. However, unlike carbohydrates lipids contain a lower proportion of oxygen

• Non-polar and hydrophobic (insoluble in water)

• There are two groups of lipid that you need to know:

  • Triglycerides (the main component of fats and oils)

  • Phospholipids

Triglycerides

• Are non-polar, hydrophobic molecules

• The monomers are glycerol and fatty acids

• Glycerol is an alcohol (an organic molecule that contains a hydroxyl group bonded to a carbon atom)

• Fatty acids contain a methyl group at one end of a hydrocarbon chain known as the R group (chains of hydrogens bonded to carbon atoms, typically 4 to 24 carbons long) and at the other is a carboxyl group

  • The shorthand chemical formula for a fatty acid is RCOOH

• Fatty acids can vary in two ways:

  • Length of the hydrocarbon chain (R group)

  • The fatty acid chain (R group) may be saturated (mainly in animal fat) or unsaturated (mainly vegetable oils, although there are exceptions e.g. coconut and palm oil)

• Unsaturated fatty acids can be mono or  poly-unsaturated:

  • If H atoms are on the same side of the double bond they are cis-fatty acids and are metabolised by enzymes

  • If H atoms are on opposite sides of the double bond they are trans-fatty acids and cannot form enzyme-substrate complexes, therefore, are not metabolised. They are linked with coronary heart disease

Examples of different types of fatty acids with the functional groups and presence of double bonds highlighted

• Triglycerides are formed by esterification

• An ester bond forms when a hydroxyl (-OH) group on glycerol bonds with the carboxyl (-COOH) group of the fatty acid:

  • An H from glycerol combines with an OH from the fatty acid to make water

  • The formation of an ester bond is a condensation reaction

  • For each ester bond formed a water molecule is released

  • Three fatty acids join to one glycerol molecule to form a triglyceride

  • Therefore for one triglyceride to form, three water molecules are released

    

Formation of a triglyceride from a glycerol molecule and three fatty acid molecules by the process of esterification.