What are α-amino acids? Explain their classification and their amphoteric nature.

1. 1 · Define. Compounds with an amino and a carboxyl group on the same carbon: R-CH(NH₂)-COOH.
2. 2 · Classify by groups. Neutral (equal NH₂ and COOH, glycine), acidic (more COOH, glutamic acid), basic (more NH₂, lysine).
3. 3 · Classify by diet. Essential (must come from food: valine) and non-essential (made in the body: glycine).
4. 4 · Explain the zwitterion. In water: H₂N-CH(R)-COOH ⇌ H₃N⁺-CH(R)-COO⁻. COO⁻ takes H⁺ from acids; NH₃⁺ gives H⁺ to bases, so amino acids are amphoteric.
5. 5 · Link to real life. Dal with rice supplies a fuller set of essential amino acids.
6. 6 · Close in one line. So their dipolar nature makes amino acids salt-like, high-melting and water-soluble.

Scoring tip: Write the zwitterion equation; most marking schemes give a full mark for it.