Lipids (AQA A Level Biology)

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.