AQA AS Biology

Revision Notes

1.2.1 Lipids

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

Triglycerides_Basics - types of fatty acids (1), downloadable IGCSE & GCSE Biology revision notes

Triglycerides_Basics - types of fatty acids (2), downloadable IGCSE & GCSE Biology revision notes
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 the hydroxyl (-OH) group of the glycerol bonds with the carboxyl group (-COOH) of the fatty acid
    • 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

Triglycerides_Basics - Formation of a triglyceride (1), downloadable IGCSE & GCSE Biology revision notes
Formation of a triglyceride from a glycerol molecule and three fatty acid molecules by the process of esterification

Exam Tip

Ensure that you are familiar with the structure of a triglyceride and that you can recognise whether the fatty acids are saturated or unsaturated.

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