lipids

• lipids are different from proteins and carbs - not polymers formed from long chains of monomers
• made from a variety of different components - but all contain hydrocarbons (molecules that only contain hydrogen and carbon atoms)
• components made from relates to the lipid's function
• two types of lipid - triglycerides and phospholipids
• triglycerides have one molecule of glycerol with three fatty acids attached to it. fatty acid molecules have long tails made of hydrocarbons - hydrophobic (repel water molecules) - tails make lipids insoluble in water
• all fatty acids consist of the same basic structure, but the hydrocarbon tail varies
• there are 2 kinds of fatty acids - saturated and unsaturated

  • 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 causes the chain to kink
• triglycerides are formed by condensation reactions - an ester bond forms between the two molecules, releasing a molecule of water - this is a condensation reaction - process happens 3x to form a triglyceride
• lipids found in cell membranes are not triglycerides - phospholipids
• phospholipids are pretty similar to triglycerides except one of the fatty acids is replaced with a phosphate group
• the phosphate group is hydrophilic (attracts water), the fatty acid tails are hydrophobic (repel water)
• triglycerides are mainly used as storage molecules good for this bc the long hydrocarbon tails of the fatty acids contain lots of chemical energy - means a lot of energy is released when they are broken down
• bc of their tails contain about 2x as much energy per gram than carbs
• triglycerides are insoluble in water - so don't affect the water potential of the cell and cause water to enter the cell by osmosis which would make the cell swell
• triglycerides bundle together in insoluble droplets in cells bc the fatty acid tails are hydrophobic - tails face inwards, shielding themselves from water with their glycerol heads
• phospholipids make up the bilayer of cell membranes - control what enters and leaves the 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 cannot easily pass through - membrane acts as a barrier
• to test for the presence of lipids in a sample: - emulsion test

  • shake the test substance with ethanol for 1 min then pour solution into water
  • any lipid will show up as a milky emulsion
  • the more lipid there is the more noticeable the milky colour will be
• ethanol flammable - test away from open flames