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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While studying Biochemistry at Oxford University, Amelia started her own tutoring service, helping to connect Science tutors with students in her local area. Amelia has experience teaching the sciences and Maths at all levels to UK and international students and, as well as being our Biology Lead, designs revision resources for Chemistry.
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