• is a variable group
  • contains C, H ,O atoms
  • high ratio of H:O than carbohydrates
  • insoluble in water
  • eg triglycerides, phospholipids, cholesterol etc



  • they are insoluble in water, but soluble in ethanol
  • they are less dense than water (they float)
  • act as good insulators – both electrical and thermal.
  • contains the molecule glycerol and three fatty acids.
  • Three fatty acids and one glycerol molecule join together in a condensation reaction forming ester linkage
  • Three molecules of water are released
  • The reaction can be reversed using hydrolysis to turn the triglyceride back into its individual molecules


Fatty acids

  • Have long chains of C and H atoms.
  • Saturated fatty acids has only single C-O bonds.
  • An unsaturated fatty acid will have carbon-to-carbon (C=C) double bonds
  • It makes the molecules in the lipid push apart and so makes them more fluid
  • Unsaturated lipids tend to have lower melting points than saturated lipids.


Explain how the molecular structure of triglycerides is related to their functions.

  • possess hydrophobic tails of fatty acids;
  • which cause the molecule to be insoluble in water;
  • they are not so easily dissolved out of the cell;
  • this functions to provide the properties of the phospholipid bilayer in cell membranes;
  • acts as energy store for the cell;
  • due to their higher proportion of hydrogen compared to carbohydrates;
  • as a result the breakdown of triglycerides yields ore energy;
  • due to the lower proportion of oxygen to carbon that requires more oxygen for complete oxidation to occur.
  • triglycerides also float in water due to their lighter density;
  • this enables them to aid in the buoyancy of aquatic animals;



  • one glycerol molecule and two fatty acid molecules bonded to it from condensation reactions, producing these ester bonds.
  • a phosphate group is bonded covalently to the third hydroxyl group on the glycerol
  • A phospholipid therefore consists of two parts: the phosphate head and the fatty acid tails.
    The head is very hydrophilic (i.e. water-loving) and the tails are hydrophobic (i.e. water-hating)
  • In H2O, phospholipid molecules arrange into a bilayer: hydrophilic heads facing outwards into the water and hydrophobic tails facing inwards, avoiding water.
  • This is the basic structure of a cell membrane.


Lipids functions

  • energy storage
  • making biological membranes
  • insulation
  • protection eg protecting plants from drying up
  • boyancy
  • acting as hormones

Leave a Reply

Your email address will not be published. Required fields are marked *