AS-LEVEL AQA BIOLOGY NOTES 
TOPIC 3: lipids

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Triglycerides and phospholipids are examples of lipid macromolecules (large molecules composed of several smaller ones - but not necessarily a polymer).

Triglycerides
  • 1 glycerol + 3 fatty acid chains (F.A.s)
  • Bond in a condensation reaction which forms an ester bond between glycerol and each FA chain
    • also known as esterification
  • During breakdown (e.g. for metabolism), these ester bonds are broken down by hydrolysis
  • Can have one of several different fatty acid combinations
  • Unsaturated F.A. contains one or more carbon-carbon double bonds and could bond to more H atoms
  • Saturated F.A. contains no carbon-carbon double bonds and cannot bond to more H atoms
  • Fats are triglycerides that are solid at room temperature (often from animals - more saturated FAs)
  • Oils are triglycerides that are liquid at room temperature (often from plants - more unsaturated Fas)
  • Used as an energy store, thermal insulator, for buoyancy, and protection of vital organs
Phospholipids
  • Structure is similar to triglyceride, but one F.A. chain replaced with a phosphoric acid group
  • The ‘polar head’ refers to the glycerol and phosphoric acid. It is hydrophilic, meaning it attracts water
  • The F.A. ‘tails’ are hydrophobic, meaning they repel water
  • Phospholipids play a crucial role in forming the plasma membrane of cells. They form a bilayer with the FA tails facing into each other, and the polar heads facing outwards and dissolving in the aqueous intra- and extracellular fluids.
They are ideal for forming cell surface membranes as they enable integration of other molecules into the ‘mosaic’ and help to regulate the movement of molecules in and out of the cell.
Emulsion test for lipids
  1. Shake sample with ethanol
  2. Pour solution into water
  3. If the mixture turns cloudy, lipids are present
  4. This is due to lipids’ solubility in ethanol and insolubility in water