Biomolecules: NEET previous year questions

The alpha-helix is a right-handed coiling of the polypeptide backbone stabilised by hydrogen bonds and is the hallmark of the secondary structure of proteins. Primary structure is the linear amino-acid sequence, tertiary structure is the folded three-dimensional shape, and quaternary structure results from the assembly of more than one polypeptide chain.

Proteins are polypeptides (Statement II), polysaccharides are long chains of monosaccharides (Statement III), and almost all enzymes are proteins (Statement V), so these are correct. Statement I is wrong because lipids are generally hydrophobic and water-insoluble; Statement IV is wrong because adenine and guanine are substituted purines, not pyrimidines.

Amino acids are substituted methanes because the alpha carbon carries $\mathrm{H}$, $\mathrm{COOH}$, $\mathrm{NH}_{2}$ and an R group (I correct), and valine is a neutral (non-polar) amino acid (III correct). Statement II is wrong since serine has a hydroxyl side chain, making it an alcoholic (not aromatic) amino acid; Statement IV is wrong because lysine has an extra amino group, making it a basic (not acidic) amino acid. Hence I and III.

Trypsin is a proteolytic enzyme (P-III), morphine is an alkaloid secondary metabolite (Q-IV), concanavalin A is a lectin (R-II), and collagen acts as an intercellular ground substance (S-I). This gives P-III, Q-IV, R-II, S-I.

Lyases catalyse the removal of groups from substrates by mechanisms other than hydrolysis, leaving behind double bonds. The reaction shown removes the X and Y groups (as X-Y) and generates a C=C double bond, which is characteristic of lyase activity. Transferases move a group between substrates, isomerases interconvert isomers, and ligases join two molecules together, none of which fit this reaction.

Why the other options are wrong:

- (1) Transferases transfer a functional group between a pair of substrates, not create a double bond.
- (2) Isomerases interconvert optical, geometric or positional isomers.
- (4) Ligases link two compounds together, the opposite of this elimination.

Starch is the storage polysaccharide of plants, a reserve of energy (P-II). Antibodies are immunoglobulins that recognise and neutralise pathogens (Q-I). Concanavalin A is the standard textbook example of a lectin, a carbohydrate-binding protein (R-IV). GLUT-4 is the insulin-responsive transporter that carries glucose across the plasma membrane into muscle and fat cells (S-III). NCERT Class 11, Biomolecules (protein functions).

By WEIGHT the human body is roughly 65 per cent oxygen, 18 per cent carbon, 10 per cent hydrogen and 3 per cent nitrogen, so the order is oxygen, carbon, hydrogen, nitrogen. Oxygen dominates because most of the body is water and hydrogen contributes little mass despite being very abundant by number of atoms - a useful reminder that ranking by weight and ranking by atom count give different answers. NCERT Class 11, Biomolecules (elemental analysis).

Inulin contains a glycosidic bond, linking its fructose units. Insulin contains peptide bonds, linking its amino acid residues. Chapter: Biomolecules.

Collagen is the most abundant protein in animals, forming the structural framework of bones, skin, and connective tissues. Chapter: Biomolecules.

Lysine is a basic amino acid due to its positively charged amino group on the side chain. Chapter: Biomolecules.

Malonate inhibits catalytic activity competitively; collagen contains peptide bonds; fungal walls contain chitin; and ricin is a secondary metabolite. Thus P-II, Q-IV, R-III, S-I. Chapter: Biomolecules.