Lipids

• Fatty acids consist of a hydrocarbon chain (known as R group) and a carboxyl group.
• The fatty acid chains can be:

  • saturated: contain only single covalent bonds between carbon atoms
  • unsaturated: contain 1 double bond (mono-unsaturated) or more than 1 double bond (poly-unsaturated)
• Triglycerides

  • formed from 1 molecule of glycerol and 3 molecules of fatty acids.
  • formed by esterification: an ester bond forms when a hydroxyl group (-OH) on glycerol bonds with the carboxyl group (-COOH) of the fatty acid.
  • condensation reaction: for each ester bond formed, a water molecule is released.
• Triglycerides are used for energy storage:

  • long hydrocarbon chains: many C-H bonds so lots of energy is released when fatty acids are broken down
  • low mass to energy ratio: lots of energy stored in a small volume
  • insoluble: doesn't affect water potential, so doesn't cause water to enter cells by osmosis, so no swelling
• Phospholipids:

  • formed from a glycerol molecule bonded to 2 fatty acid tails and a phosphate group.
  • the phosphate group is polar so hydrophilic, while the fatty acid tails are non-polar so hydrophobic.
• Phospholipids make up the bilayer of cell membranes:

  • in water, phospholipids form a double layer with the phosphate heads facing out towards the water on either side.
  • the centre of the bilayer is hydrophobic, so the membrane acts as a barrier to water-soluble substances.
• Emulsion test:

  1. Add ethanol to a sample in a test tube, then shake to dissolve any lipids.
  2. Add the solution to water
  3. If lipid is present, a milky emulsion will form.