Lipids

• Triglycerides

  Type of lipid mainly used as energy storage molecules
• Phospholipids

  Type of lipid that forms a bilayer, main component of cell membranes and responsible for the flexibility of membranes and controlling what goes in and out of cells
• Formation of triglycerides

  1. Condensation of 1 molecule of glycerol and 3 molecules of fatty acid

  2. Ester bonds through condensation form between the glycerol and fatty acid chains

  3. One water molecule is released per ester bond so 3 molecules of water are released per triglyceride formed
• Triglyceride Structure & Properties

  • Source of energy (high ratio of energy storing carbon-hydrogen bonds to carbon atoms)
  • Storage molecules (low mass to energy ratio, especially useful for animals as reduces mass they carry)
  • Does not change water potential (large, non polar molecules -> insoluble in water)
  • Source of water (high ratio of hydrogen to oxygen atoms so releases water when oxidised)
• Roles of lipids

  • Source of energy (x2 energy as carbohydrates)
  • Waterproofing (insoluble in water-> waxy cuticles in plants, oily secretions in mammals)
  • Insulation (fats= slow conductors of heat so help retain body heat beneath body's surface & an electrical insulator in the myelin sheath)
  • Protection (fats often stored around delicate organs eg kidney)
• Function of triglycerides in energy release

  Chemical energy is stored in the fatty acid hydrocarbon tails so lots of energy is released when the triglycerides are broken down
• Lipids contain lots of energy, carbohydrates contain half the amount of energy per gram as lipids do when oxidised and release water
• Triglycerides repel water

  They are insoluble in water because the fatty acid tails are hydrophobic- the cells water potential is not affected by triglycerides (if they did not repel water -> water would enter the cell via osmosis -> the cell would swell up and potentially burst)
• Lipid droplets

  In cells the insoluble triglycerides crowd together as droplets because the hydrophobic fatty acid tails face inwards
• Structure of phospholipids

  Similar to triglycerides- both composed of fatty acid chains attached to glycerol. Main difference: 1 of the 3 fatty acid tails replaced by a hydrophilic phosphate group. Therefore a phospholipid molecule has a hydrophobic & hydrophilic part (so is an amphipathic molecule). Phospholipid head helps to hold at the surface of the cell membrane. The non polar hydrophobic tails and polar hydrophilic heads allow them to form phospholipid bilayers
• Hydrophobic tail of phospholipids

  Phospholipids have 2 fatty acid chains & they are hydrophobic (cannot interact with water). In membranes the hydrophobic fatty acid tails face inwards in the bilayer (so water soluble substances can't easily pass through)
• Hydrophilic head of phospholipids

  The phosphate group in phospholipids is hydrophilic and can interact with water. In membranes it faces the outside
• Structure of fatty acids

  Have long 'tails' made of a chain of hydrocarbons with 4-36 carbon atoms. The hydrocarbon tail is variable but most fatty acids contain 12-18 carbons. Glycerol links to the central carbon atom on fatty acids. The glycerol in all triglycerides are the same so differences in properties of fats and oils stem from variations in the 70 types of fatty acids
• Types of fatty acids

  • Saturated (only single bonds between carbon atoms in the hydrocarbon chain)
  • Unsaturated (have 1 or more double bonds between the carbon atoms in the hydrocarbon chain)
• Saturated fatty acids

  The number of hydrogen atoms attached to the carbon skeleton is MAXIMISED so saturated. Solid at room temp & usually of animal origin. Saturated fats are linked to increased cardiovascular disease in humans
• Unsaturated fats

  Double bonds 'kink' the carbon chain so unsaturated fats cannot pack together tightly, therefore liquid at room temp. Eg of unsaturated fatty acid= cooking oils
• Emulsion

  Cloudy emulsion due to any lipid in the sample being finely dispersed in the water to form an emulsion. Light passing through the emulsion is refracted as it passes from oil droplets to water droplets -> cloudy appearance
• Emulsion test

  1. Add 5cm cubed of ethanol to 2cm cubed of sample

  2. Shake solution and leave upright for 2-3 mins

  3. Transfer solution: Pour the solution into a boiling tube containing distilled water

  4. Observe the results: If lipid present= white + milky layer formed