Lipids

• • Lipids are different from proteins and carbohydrates because they're not polymers formed from long chains of monomers
  • Lipids are made from a variety of different components but they all contain hydrocarbons
  • The components they're made from relates to the lipids function
  • There are two types of lipids triglycerides and phospholipids
• Triglycerides:

  • Triglycerides have one molecule of glycerol with three fatty acids attached to it
  • Fatty acid molecules have long tails made of hydrocarbons. The tails are hydrophobic. These tails make lipids insoluble in water
• Fatty acids:

  • Saturated fatty acids - don't have any double bonds between their carbon atoms. The fatty acid is saturated with hydrogen
  • Unsaturated fatty acids do have double bonds between carbon atoms, which cause the chains to kink
• Triglyceride formation:

  • Triglycerides are formed by condensation reactions
  • An ester bond is formed when a fatty acid joins to a glycerol molecule, releasing a molecule of water - this is a condensation reaction
• Phospholipids:

  • The lipids found in cell membranes aren't triglycerides - they're phospholipids
  • Phospholipids are pretty similar to triglycerides except one of the fatty acid molecules is replaced by a phosphate group
  • The phosphate group is hydrophilic while the fatty acid tails are hydrophobic
  • This important in the cell membrane
• Properties of triglycerides

  • Triglycerides are mainly used as energy storage molecules
  • They're good for this because the long hydrocarbon tails of the fatty acids contain lots of chemical energy - a load of energy is released when they're broken down
  • Because of these tails, lipids contain about twice as much energy per gram as carbohydrates
  • They are insoluble in water so.they do not affect the water potential of cells
  • The triglycerides bundle together as insoluble droplets in cells because the fatty acid tails are hydrophobic - the tails point inwards, shielding themselves from water with their glycerol heads
• Properties of phospholipids:

  • Phospholipids make up the bilayer of cell membranes
  • Cell membranes contro l what enters and leaves a cell
  • Phospholipid heads are hydrophilic and their tails are hydrophobic, so they form a double layer with their heads facing out towards the water on either side
  • The centre of the bilayer is hydrophobic, so water-soluble substances can't easily pass through it - the membrane acts as a barrier for those substances
• The emulsion test for lipids

  1. Shake the test substance with ethanol for about a minute, then pour the solution into water.
  2. Any lipid will show up as a milky emulsion
  3. The more lipid there is, the more noticeable the milky colour will be