Lipids

• 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. All fatty acids have the same basic structure, but the hydrocarbon tail varies
• hydrophobic means that it repels water molecules
• The structure of a fatty acids:

  Carbon atom links fatty acids to glycerol

  Variable R group hydrocarbon tail

  A) hydrocarbon chain

  B) OH

  C) O

  D) R

  E) C

  
• Triglycerides are formed by condensation reactions
• When the ester bond is formed a molecule of water is released - this is also known as a condensation reaction. This happens twice more to form a triglyceride
• Fatty acids can be saturated or unsaturated
• the difference between unsaturated and saturated fatty acids is in their hydrocarbon tails - also known as the R group
• Saturated fatty acids don't have any double bonds between their carbon atoms. The fatty acid is saturated with hydrogen
• Unsaturated fatty acids have at least one double bond between carbon atoms, which cause the chains to kink.
• In cell membranes the lipids found there are not triglycerides they are 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. The fatty acid tails are hydrophobic.
• Triglycerides are mainly used as energy storage molecules
• The long hydrocarbon tails of the fatty acids contain lots of chemical energy - a load of energy is released when they're broken broken down. Because of these tails, lipids contain about twice as much energy per gram as carbohydrates
• Triglycerides are insoluble so they do not affect the water potential of the cell and cause water to enter the cells by osmosis (which would make them swell). The triglycerides clump together as insoluble droplets in cells because the fatty acids tails are hydrophobic - The tails fate inwards shielding themselves from water with their glycerol bonds
• Phospholipids make up the bilayer of cell membranes
• Cell membranes control what enters and leaves the cell
• Phospholipids 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 phospholipids bilayer is hydrophobic so water-soluble substances can't easily pass through it - the membrane acts as a barrier to those substances
• The emulsion test for lipids, tests for fats
• The emulsion test:

  1. Shake the test substance with ethanol for about a minute so that it dissolves, 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