Tests for Biological Molecules

Tests for Biological Molecules

1. Starch

   • Add a solution of iodine to the sample - if starch is present, the iodine solution changes colour from yellow/brown to blue/black

2. Reducing Sugars

   • When a reducing sugar is heated with Benedict's solution (alkaline copper sulfate), the solution changes colour from blue to orange-red
     • The orange-red is described as a precipitate because the substance comes oit of the solution and forms solid particles dispersed in the solution. This is Benedict's test

3. Non-reducing Suagrs

   • Boil sample with hydrochloric - this hyrolyses any sucrose present to form fructose and glucose monosaccharides
     • Cool the solution and neutralise it by adding sodium hydrogencarbonate solution
   • Carry out Benedict's test and if sucrose is present in the original sample, the test will give a positive result because glucose and fructose are present

4. Protein

   • Add biuret reagent to sample. The sodium hydroxide and copper sulfate will react with the peptide bonds found in protein, which results in a colour change from pale blue to lilac

5. Lipids

   • Add ethanol to dissolve any lipid present, then pour the liquid into water contained in another clean test tube. If lipid is present, a cloudy white emulsion will form near the top of the water