CBSE Class 12 Chemistry: Biomolecules - Marking Scheme Solution
(a) (i) Peptide linkage: [1 Mark]
A peptide linkage is an amide linkage formed by the condensation reaction between the amino group ($-NH_2$) of one $\alpha$-amino acid and the carboxyl group ($-COOH$) of another $\alpha$-amino acid, with the elimination of a water molecule ($H_2O$).
$$R-CH(NH_2)-COOH + R'-CH(NH_2)-COOH \rightarrow R-CH(NH_2)-CO-NH-CH(R')-COOH + H_2O$$
(a) (ii) Denatured protein: [1 Mark]
When a protein in its native form is subjected to physical changes (like change in temperature) or chemical changes (like change in pH), the hydrogen bonds are disturbed. Due to this, globules unfold and helix gets uncoiled, and the protein loses its biological activity. This is called denaturation of protein.
(b) Amphoteric behaviour of amino acids: [1 Mark]
Amino acids contain both an acidic carboxyl group ($-COOH$) and a basic amino group ($-NH_2$) in the same molecule. In aqueous solution, the carboxyl group can lose a proton and the amino group can accept a proton, giving rise to a dipolar ion known as a zwitter ion. Because they can react with both acids and bases, they exhibit amphoteric behaviour.
$$H_2N-CH(R)-COOH \rightleftharpoons H_3N^+-CH(R)-COO^-$$
(c) (ii) Two different secondary structures of proteins: [1 Mark]
The two common types of secondary structures of proteins are:
- $\alpha$-helix structure: Formed by intramolecular hydrogen bonding between the $C=O$ and $N-H$ groups of the same polypeptide chain.
- $\beta$-pleated sheet structure: Formed when polypeptide chains are stretched out side-by-side and held together by intermolecular hydrogen bonds.
💡 EL10 Concept Insight:
Think of a protein like a complex origami sculpture; denaturation is like unfolding that sculpture until it's just a flat piece of paper—it loses its "shape" and its "job." Amphoteric behaviour is like a person who can speak two languages; because amino acids have both an "acid" part and a "base" part, they can talk (react) with both types of chemicals!