Edexcel A (SNAB) A Level Biology:复习笔记1.2.9 Lipids & Ester Bonds



  • Lipids are macromolecules that contain carbon, hydrogen and oxygen atoms. Unlike carbohydrates, lipids contain a lower proportion of oxygen
  • Lipids are non-polar and hydrophobic (insoluble in water)
  • Triglycerides are a kind of lipid that forms the main component of fats and oils
  • Lipids play an important role in energy yield, energy storage, insulation and hormonal communication


  • 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)
  • Saturated fatty acids contain no carbon-carbon double bonds
    • The hydrocarbon chain is saturated with hydrogen atoms due to the absence of double bonds
    • They form unbranched, linear chains
  • 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

Function of lipids

  • Triglycerides are fats and oils
  • Fatty acid and glycerol molecules are the components that make up triglycerides
  • Fats and oils have a number of important functions in organisms: energy storage, insulation, buoyancy, and protection

The Ester Bond

  • Triglycerides are formed by esterification
  • An ester bond forms when a hydroxyl (-OH) group from glycerol bonds with the carboxyl (-COOH) group 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


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

Exam Tip

Ensure you are confident with the structure of a triglyceride so you can recognise whether they are saturated or unsaturated.