Q.(a) Which of the following is a polysaccharide ?

Cellulose is the polysaccharide among the choices; amino acids are amphoteric because they exist as zwitterions carrying both an acidic and a basic site.

1. Identifying the polysaccharide Carbohydrates are classified by how many monosaccharide units they hydrolyse into: monosaccharides (1 unit, cannot be hydrolysed further, e.g. fructose), disaccharides (2 units, e.g. maltose and sucrose), and polysaccharides (many units, e.g. starch, cellulose, glycogen). Of the given options, only cellulose (iv) is a polysaccharide - it is a straight-chain polymer of many beta-D-glucose units joined by beta-1,4-glycosidic linkages.
2. Amphoteric behaviour of amino acids Every amino acid has, in the same molecule, an acidic carboxyl group (-COOH) and a basic amino group (-NH2). In aqueous (and solid) state, an internal acid-base reaction occurs: the -COOH group donates its proton to the more basic -NH2 group, generating a dipolar ion called a zwitterion: H3N+-CHR-COO-. This zwitterion can behave as either an acid or a base depending on the pH of the surrounding medium:

• In an acidic medium (low pH, excess H+), the -COO- end accepts a proton, and the amino acid behaves as a base, existing predominantly as the cation H3N+-CHR-COOH. …