Q.(i) What is the monomer of proteins?

At the isoelectric point an amino acid carries no net charge and exists as a zwitterion, with -COOH deprotonated and -NH2 protonated. Correct option (d).

In an amino acid the -COOH group loses a proton and the -NH2 group gains it, giving an internal salt (zwitterion). At the isoelectric point the positive and negative charges balance so the net charge is zero and the molecule does not migrate in an electric field.

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Insulin is a peptide hormone made of two polypeptide chains linked by disulfide bonds, making it a protein.

Reasoning: Insulin is a hormone secreted by the beta-cells of the pancreas that regulates blood glucose levels. Structurally, it consists of two polypeptide chains (the A-chain and B-chain, made of amino acids joined by peptide bonds) held together by disulfide (-S-S-) bridges. Since it is built from amino acids linked by peptide bonds, insulin is classified biochemically as a prote …

Optical activity requires a chiral (asymmetric) carbon; glycine's alpha-carbon carries two identical hydrogen substituents, so it has no chiral centre and is optically inactive.

Reasoning: An amino acid's alpha-carbon is chiral (and hence the amino acid is optically active) only if it bears four different groups: -NH2, -COOH, -H, and a side chain (R group) that is different from H.

• Aspartic acid, Lysine, and Cystine all have side chains different from hydrogen (a -CH2COOH, a -(CH2)4NH2, and a -CH2SSCH2- linked side chain respectively), so their alpha-carbons are genuinely chiral centres — these are optically active. …