Lipids

• Lipids contain carbon, hydrogen and oxygen, with only small amounts of oxygen. They are insoluble in water but soluble in organic solvents e.g. alcohol.
• Lipids are a source of energy as they provide twice the amount of energy supplied by carbohydrates once oxidised.
• Lipids have the role of waterproofing. This is because lipids are insoluble in water - for example, plants and insects have lipid waxy cuticles whilst mammals secrete oil from glands in the skin.
• Lipids have the role of insulation. This is because lipids are slow conductors of heat and they also insulate the myelin sheath.
• Lipids have the role of protection. This is because they absorb impact and are usually located around the major organs.
• Lipids are in membranes. This is because they are insoluble in water and tend to group together due to hydrophobic/hydrophilic interactions.
• Triglycerides

  Three fatty acids attached to one glycerol molecule via condensation reaction. Each fatty acid forms an ester bond with the glycerol molecule via condensation reaction.
• What are the bonds in triglycerides?

  Ester bonds
• Saturated vs unsaturated fatty acid

  These are the R groups of fatty acids

  Saturated = single bonds

  Unsaturated = double bonds

  • Monounsaturated = one double bond
  • Polyunsaturated = more than one double bond
• Why are unsaturated fats fluid at room temperature?

  The double bonds in the unsaturated fats cause the fatty acid molecules to bend, meaning they cannot pack together closely.
• Triglycerides are an excellent source of energy, good storage molecules, insoluble in water (they don't affect water potential or osmosis) and hydrophobic.
• Phospholipids

  Two fatty acids and one phosphate group attached to one glycerol molecule, joint by ester bonds.
• What does the hydrolysis of ester bonds release?

  Energy (in the form of ATP)
• What is the largest lipid?

  Triglyceride because lipids cannot form polymers.
• What is dehydration synthesis?

  Condensation reaction
• Essential fatty acids are needed in our diet because our body can't make them.
• Saturated fatty acids are solid in cold temperatures whilst unsaturated fatty acids are fluid in cold temperatures.
• What are the steps in the emulsion test for lipids?

  Take 2cm^3 of the sample (grind it in water if not already in liquid form) and add about 5cm^3 of ethanol. Add equal volume of water and shake gently. Control experiment = repeat experiment, replace sample with water to give the result of a clear solution.
• What would a positive result be in the emulsion test for lipids?

  A clear solution to a white emulsion
• Why is the positive result of an emulsion test for lipids cloudy?

  Light passes through the emulsion formed and is refracted as it passes from the oil droplets to the water droplets.