Q.(i) What are essential and non-essential amino acids?

Amino acids are classed as essential (must be obtained from the diet) or non-essential (the body can synthesise them), and their amphoteric behaviour arises from the zwitterionic form, which carries both an acidic –NH3⁺ site and a basic –COO⁻ site.

(i) Essential vs non-essential amino acids

• Essential amino acids: cannot be synthesised by the human body in adequate amounts and must be obtained through the diet. Examples: valine, leucine, isoleucine.
• Non-essential amino acids: can be synthesised by the body itself and need not necessarily come from food. Examples: glycine, alanine, aspartic acid.

(ii) Amphoteric behaviour of amino acids

Each amino acid contains an acidic carboxyl group (–COOH) and a basic amino group (–NH2) on the same molecule. In aqueous/solid state, a proton is internally transferred from the –COOH to the –NH2 group, giving a dipolar ion called a zwitterion:

H2N–CHR–COOH ⇌ ⁺H3N–CHR–COO⁻ (zwitterion)

Because of this dual nature:

• In acidic solution, the –COO⁻ end accepts a proton, so the amino acid behaves as a base.
• In basic solution, the –NH3⁺ end donates a proton, so the amino acid behaves as an acid. …