Biomolecules: JEE Main PYQ, 2020–2025

Answer: (A)

Proteins are classified by molecular shape. Fibrous proteins have polypeptide chains that are long and parallel, giving strong, insoluble, structural materials. Globular proteins have chains folded into compact spheres, which makes most of them soluble in water and suited to transport, catalysis and regulation. So the correct statement pairs fibrous with long insoluble strands and globular with compact soluble spheres.

Answer: (A)

Vitamins A, D, E and K are fat-soluble and are stored in the liver and adipose tissue. Vitamins of the B group and vitamin C are water-soluble and are excreted in urine, so they must be supplied regularly. Vitamin B₁₂ is the exception: it is water-soluble but is stored in the liver. The answer must meet both conditions, so vitamin A is out because it is fat-soluble, and vitamins C and B₆ are out because they are not stored.

Answer: (A)

Tollens' reagent is alkaline (ammoniacal silver nitrate). In base, fructose undergoes keto–enol tautomerism through an enediol intermediate and is converted into glucose and mannose, both aldoses, which reduce Ag⁺ to Ag. Bromine water is a mild, non-basic oxidant that oxidises the –CHO group of aldoses to –COOH but does not oxidise a ketone, and it does not cause the isomerisation. So glucose is oxidised to gluconic acid and fructose is not. The other options wrongly give fructose an aldehyde group, invoke C–C cleavage by a mild oxidant, or explain the difference by ring structure, which is the same for both reagents.

Answer: (C)

Vitamin D is unusual because the body can synthesise it. Under UV light, 7-dehydrocholesterol in the skin is converted to vitamin D. Dietary sources supply the rest. Vitamin C and vitamin B₁ must come from food, and vitamin K is mainly made by gut bacteria. Only vitamin D is made in the skin by sunlight.

Answer: (D)

Forming an oxime with hydroxylamine (and a cyanohydrin with HCN) shows a carbonyl group. Oxidation by a mild agent such as bromine water to gluconic acid, a six-carbon carboxylic acid, shows that the carbonyl group is an aldehyde group.

Answer: (A)

Hormones are grouped into three chemical classes. Steroids include testosterone, estradiol, progesterone and the adrenocortical hormones such as cortisol. Polypeptides include insulin, glucagon and oxytocin. Amino-acid derivatives include thyroxine, which is an iodinated tyrosine derivative, and adrenaline. For the order steroid, polypeptide, amino-acid derivative, only 'testosterone, glucagon, thyroxine' fits. The other sets either swap classes between positions or misclassify cortisol.

Answer: (A)

ADH (vasopressin) is a peptide made in the hypothalamus and released from the posterior pituitary. It acts on the kidney collecting ducts to increase water reabsorption and concentrate urine, which is the correct answer.

Answer: (A)

Principle: the oxidation product of glucose depends on the strength of the oxidant. Bromine water is mild and oxidises only the aldehyde group (C1) to –COOH, giving gluconic acid, HOOC–(CHOH)4–CH2OH, with one –COOH. Concentrated HNO3 is a strong oxidant and oxidises both the –CHO at C1 and the primary –CH2OH at C6, giving saccharic acid, HOOC–(CHOH)4–COOH, with two –COOH groups. The answer is therefore 1 and 2.

Answer: (A)

Principle: each vitamin has a characteristic deficiency disorder, and solubility (fat or water) is a separate property from where the vitamin is stored. Application: convulsions point to vitamin B₆ (pyridoxine). Pernicious anaemia points to vitamin B₁₂ (cobalamin). Increased blood clotting time points to vitamin K. Vitamins A, D, E and K are fat-soluble. The B-complex vitamins (B₆, B₁₂) and vitamin C are water-soluble. So only K is fat-soluble, even though B₁₂ is stored in the liver.

Answer: (A)

Insulin is the pancreatic hormone that lowers blood glucose. Its counterpart, glucagon, raises blood glucose. Insulin is produced by β-cells, while glucagon comes from α-cells. Remember that the hormone that lowers glucose is the one that is secreted when glucose is high.