Tests for biological molecules

• Test for fats and oils - the emulsion test ...

  1. Sample is dissolved in ethanol
  2. Shake with an equal volume of water
  3. Dissolved lipids come out of solution because they are insoluble in water
  4. Form an emulsion - cloudy white
• Test for reducing sugars ...

  1. Equal volume of the solution being tested and Benedict's reagent are strongly heated
  2. If a reducing sugar is present, the solution will turn from blue to green, then orange, then brick red dependant on the concentration of the reducing sugar present
  3. Reducing sugars donate an electron to reduce copper ions in copper sulphate to red copper oxide
• Test for non-reducing sugars ...

  1. Sucrose is a non-reducing sugar
  2. Must be broken down into its component monosaccharides, by heating with hydrochloric acid
  3. Then the solution is neutralised with sodium hydroxide
  4. Benedict's reagent is then added and heated as before
  5. If the solution now turns red then a non-reducing sugar was initially present
• Test for protein - the biuret test ...

  1. Add a few drops of biuret to the sample
  2. The sodium hydroxide and copper sulphate react to make blue copper hydroxide which interacts with the peptide bonds present in the protein to make biuret which is purple
  3. At low concentration, the colour change is difficult to detect - use a colorimeter