4 - Biological Molecules

• Macromolecule:

  • made of small molecules (same/different types) joining together
  • there are 3 macromolecules that are important in our diets:
  • carbohydrates
  • proteins 
  • fat & oils (lipids)
• Carbohydrates:

  • elements present: carbon, hydrogen, oxygen
  • sub-units: glucose
  • examples: starch, glycogen, cellulose, sucrose
• Lipids (fats & oils):

  • elements = carbon, hydrogen, oxygen (lower oxygen content than carbohydrate)
  • sub-units = 3x fatty acids and 1x glycerol
  • examples = vegtable oils, animal fats
• protein:

  • elements = carbon, hydrogen, oxygen, nitrogen (sometimes also sulphur/phosphorus)
  • sub-units = amino acids (~20 different forms)
  • examples = enzymes, muscle, haemoglobin
• Starch test:

  • method:

  1. Add a few drops of iodine solution to a solution of the food

  • positive result = blue/black colour
• Benedict's test (reducing sugar):

  • method:

  1. Add an equal amount of Benedict’s solution to a solution of the food
  2. Boil carefully (thermostatically controlled water bath)

  • positive result = brick red colour
• Emulsion test (fats):

  • method:

  1. Dissolve the food in ethanol
  2. Pour the solution into a clean test tube of water

  • positive result = white emulsion
• Biuret test (protein):

  • method =

  1. Add an equal amount of sodium hydroxide to a solution of the food
  2. Mix carefully
  3. The add a few drops of 1% copper sulphate without shaking the mixture

  • positive result = violet halo
• DCPIP test (vitamin C):

  • method=

  1. DCPIP is a deep blue colour3 DCPIP solution into a test tube
  2. Measure 2.0cm
  3. Add fruit juice drop by drop, counting the drops used
  4. Shake the mixture after each drop

  • positive result = DCPIP decolourised