Biological Molecules

• Benedict’s, iodine, emulsion and biuret tests; semi-quantitative Benedict’s; the non-reducing sugar test.
• α- and β-glucose; monomers, polymers and sugar terms.
• Glycosidic bonds: condensation and hydrolysis.
• Starch, glycogen and cellulose: structure and function.
• Triglycerides and phospholipids.
• Amino acids, peptide bonds and the four levels of protein structure.
• Haemoglobin and collagen as globular and fibrous proteins.
• Hydrogen bonding in water: solvent, specific heat capacity, latent heat of vaporisation.

The four standard tests

Estimating the concentration of a reducing sugar

Standardise the test so the only variable is sugar concentration: the same volume of sample, the same volume of Benedict’s solution (in excess), the same temperature and the same heating time.

• Make a series of glucose solutions of known concentration (e.g. by serial or proportional dilution).
• Either record the time to the first colour change for each, or heat all for a fixed time and keep the tubes as colour standards.
• Test the unknown in the same way and match its time or colour to the standards. A colorimeter removes the subjectivity of judging colours by eye.

Acid hydrolysis then Benedict’s

1. Do a Benedict’s test on the sample first. If it stays blue, there is no reducing sugar.
2. Boil a fresh sample with dilute hydrochloric acid: this hydrolyses the glycosidic bonds, releasing reducing monosaccharides (sucrose → glucose + fructose).
3. Neutralise with sodium hydrogencarbonate (check with pH paper), because Benedict’s works only in alkaline conditions.
4. Heat with Benedict’s solution. An orange/brick-red precipitate now shows a non-reducing sugar was present.

α-glucose and β-glucose

carbon 1 α-glucose below β-glucose above

Monomers, polymers and sugars

• Monomer
• Polymer
• Macromolecule
• Monosaccharide _n
• Disaccharide