Biological Molecule Tests

Test for reducing and non-reducing sugars

• Reducing - add Benedict's reagents (alkaline copper sulphate) and place in boiling water bath. A reducing sugar will form a brick red precipitate.
• Non-reducing - negative test for reducing sugar. Acidify with hydrochloric acid. Neutralise with sodium hydrogen carbonate then add Benedict's solution. Place in a water bath and a non-reducing sugar will for a brick red precipitate.
• Non-reducing sugars can only be detected if first broken down into its constituent monosaccharides, e.g. by heating with HCl. Benedict's solution needs alkaline conditions to work which is why sodium hydrogen carbonate is added.

Test for lipids

• Add the sample to some ethanol in a test tube.
• Place on the lid and shake vigorously.
• Add cold water and if a lipid is present a white ring will form on top.

Test for protein

• Biurets agent - sodium hydroxide and copper (II) sulphate.
• Add biurets A which reacts to form blue copper hydroxide.
• Add biurets B which interacts with peptide bonds to make solution purple.
• Positive test turns lilac.