Biomolecules
Biomolecules covers carbohydrate classification and reactions, amino acids and proteins, enzymes, and nucleic acids.
Core rules
- Glucose is an aldohexose, fructose a ketohexose. Sucrose = glucose + fructose and is non-reducing; lactose = glucose + galactose; maltose = glucose + glucose.
- Glucose with bromine water gives gluconic acid (mild oxidation of the aldehyde), with nitric acid gives saccharic acid, and with excess phenylhydrazine gives an osazone.
- Glucose does not give the Schiff test or react with NaHSO₃ because the aldehyde carbon is locked as a cyclic hemiacetal.
- Fructose, although a ketose, reduces Tollens' and Fehling's reagents because the α-hydroxy ketone tautomerises under the basic conditions.
- Sucrose on hydrolysis with dilute acid or invertase gives glucose and fructose - invert sugar, which is laevorotatory.
- Amino acids exist as zwitterions; the peptide bond forms between –COOH and –NH₂ with loss of water. Primary structure is the sequence, secondary is α-helix or β-pleated sheet, tertiary is the folded 3-D shape; denaturation destroys secondary and tertiary structure.
- DNA contains deoxyribose with A, G, C, T; RNA contains ribose with A, G, C, U.
Worked example
Why does glucose fail to react with NaHSO₃ although it is an aldehyde? In solution glucose exists mainly as the six-membered pyranose hemiacetal, in which C-1 carries no free carbonyl group. Bisulphite addition requires an accessible C=O, so no product forms; the cyclic form accounts for the observation.
Traps that cost marks
- Calling sucrose a reducing sugar - it has no free anomeric hydroxyl.
- Claiming fructose gives no Tollens' test.
- Forgetting that glucose and fructose give the same osazone.
- Swapping thymine (DNA) with uracil (RNA).
- Writing sucrose hydrolysis as producing two glucose units.