Biomolecules
Biomolecules tests carbohydrates, proteins, enzymes, vitamins and nucleic acids, with emphasis on classification, structure and biological function.
Core rules
- Monosaccharides: glucose and fructose (C6H12O6). Sucrose is a non-reducing disaccharide; maltose and lactose are reducing.
- Glucose gives Tollens' and Fehling's tests; sucrose does not because its glycosidic bond locks the reducing groups.
- Proteins: primary, secondary, tertiary and quaternary structure; peptide bonds link amino acids.
- Enzymes are biological catalysts, highly specific and sensitive to pH and temperature.
- Vitamins are classified as fat-soluble (A, D, E, K) and water-soluble (B group, C).
- DNA contains deoxyribose and bases A, T, G, C; RNA contains ribose and uracil instead of thymine.
Worked example
Explain why sucrose is non-reducing. In sucrose, the anomeric carbon of glucose and the anomeric carbon of fructose are both involved in the glycosidic linkage, so neither ring can open to expose an aldehyde or ketone group. Reasoning: reducing behaviour requires a free anomeric carbon that can open to a carbonyl form.
Traps
Calling all disaccharides reducing, forgetting that sucrose is a non-reducing sugar, and confusing the bases of DNA and RNA. Do not say enzymes change the equilibrium constant; they only speed up attainment of equilibrium.