3.1.3 Lipids

• The two types of lipids are triglycerides and phospholipids
• The structure of a fatty acid is that there is a variable R group hydrocarbon chain which is either saturated or unsaturated and also a carboxyl group (-COOH)
• The difference between unsaturated and saturated fatty acid is that saturated has no C=C double bonds in the hydrocarbon chain as all carbons are fully saturated with hydrogen whereas unsaturated fatty acids have one or more C=C double bonds in the hydrocarbon chain which creates a bend
• Triglycerides form as 1 glycerol molecules and 3 fatty acids bind together through a condensation reaction removing 3 water molecules and 3 ester bonds
• Triglycerides are used as an energy store this is useful because:

  • It has a high ratio of C-H bonds to carbon atoms in hydrocarbon chains so it is used in respiration to release more energy than same mass of carbohydrates.
  • It has hydrophobic fatty acids so it's insoluble in water so there is no effect on water potential of cell.
• Phospholipids form a bilayer in the cell membrane, allowing diffusion of lipid soluble or small substances and restricting movement of water soluble or larger substances
• In a phospholipid there is a:

  • Phosphate head which is hydrophilic meaning it's attracted to water so it's positioned to point towards water at either side of the membrane
  • Fatty acid tail which is hydrophobic meaning it repels water so it's positioned to point away from water and towards the interior of the membrane.
• The test for lipids is to add ethanol and shake to dissolve the lipids and then add water. If its positive it'll show a milky white emulsion.