lipids

• what are lipids

  Lipids are biological molecules that contain the elements carbon (C), hydrogen (H), and oxygen (O). However, lipids contain a much lower proportion of oxygen than carbohydrates.

  Lipids are not made up of long chains of monomers, meaning they are not considered as polymers.
• Roles of lipid

  The main functions of lipids

  • Energy supply - Lipids can be oxidised to provide energy to cells.
  • Structural components - Phospholipids are used in cell membranes.
  • Waterproofing Insoluble lipids are used to form water-resistant barriers.
  • Insulation Lipids can help retain heat or act as electrical insulators.
  • Protection Delicate organs are surrounded by a layer of fat
• fatty acids

  Most lipids are made up of fatty acids combined with an alcohol (usually glycerol). fatty acids consist of a carboxyl group (-COOH) attached to a hydrocarbon chain (R group).

  
• 2 types of fatty acids

  Saturated fatty acids and unsaturated fatty acids

  There are two types of fatty acid: saturated fatty acids and unsaturated fatty acids
• Saturated fatty acids:

  • These have hydrocarbon chains that are 'saturated' with hydrogen, meaning all carbon atoms are bonded to the maximum number of hydrogen atoms.
  • The hydrocarbon chain has no carbon-carbon double bonds. 
  • Lipids that contain saturated fatty acids have higher melting points and so are usually solid at room temperature (fats).

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• Unsaturated fatty acids:

  • These have hydrocarbon chains that do not contain the maximum number of hydrogen atoms bonded to the carbon atoms.
  • The hydrocarbon chain has at least one carbon-carbon double bond, which causes the chain to kink.
  • Lipids that contain unsaturated fatty acids have lower melting points and so are usually liquid at room temperature
• Unsaturated fatty acids may either be monounsaturated or polyunsaturated:

  • Monounsaturated One double bond.
  • Polyunsaturated Two or more double bonds.

  
• testing for lipids 

  To find out whether a sample contains lipids, you must carry out the emulsion testSteps to find out whether a sample contains lipids:

  1. Place your food sample in a test tube.
  2. Add 2 cm3 of ethanol.
  3. Shake.
  4. Add 2 cm3 of distilled water.
  5. If lipids are present, a milky white emulsion will appear.

  
• Triglycerides

  A triglyceride is a type of lipid used as a store of energy in animals, plants, and some bacteria.

  
• Features that allow triglycerides to store energy efficiently: 

  • Long hydrocarbon tails Their many carbon-hydrogen bonds can be broken to release energy.
  • Low mass to energy ratio - Lots of energy can be stored in a small volume.
  • Insoluble They do not affect the water potential of cells as they are large and non-polar.
  • High ratio of hydrogen to oxygen atoms - Triglycerides will release water when oxidised.
• Triglyceride formation and breakdown

  Triglycerides are synthesised via condensation reactions and broken down via hydrolysis reactions. These reactions involve the formation or the breakdown of covalent bonds known as ester bonds.
• • The hydroxyl groups (OH) on the glycerol and on the three fatty acids react together to release three water molecules (H2O).
  • This results in three ester bonds between the glycerol and the fatty acids.

  
• Hydrolysis reaction 

  Hydrolysis:

  • The addition of three water molecules (H2O) breaks the ester bonds.
  • This separates the glycerol and the fatty acids

  
• Phospholipids

  A phospholipid is a type of lipid used as a structural component of the cell membrane. 

  
• why are phospholipids polar

  A phospholipid is made up of two parts: 

  1. A hydrophilic 'head' - This contains glycerol and phosphate.
  2. A hydrophobic 'tail' - This contains fatty acids.

  The phosphate group is polar and so attracts water (hydrophilic) whereas the fatty acid tails repel water (hydrophobic)

  
• Phospholipid bilayer

  When phospholipids are placed in water, they arrange themselves into a double layer (bilayer) so that the hydrophilic heads are facing out (towards the water) and the hydrophobic tails are facing in (away from the water). this arrangement creates a hydrophobic centre in the bilayer so that water-soluble substances cannot pass throug

  
• Comparing triglycerides and phospholipids