Previous year questions: Biomolecules

The questions below come from official CBSE board papers and official Sample Question Papers. The year label in each heading is printed on that source, and the marks are the marks printed for that question or part.

Q1 (CBSE Board 2023, 1 mark)

Assertion (A): Proteins are polymers of alpha amino acids connected by a peptide bond. Reason (R): A tetrapeptide contains 4 amino acids linked by 4 peptide bonds.

Answer: A is true but R is false. A tetrapeptide does contain four amino acid residues, but joining four residues needs only three peptide bonds, because the bond count is one less than the residue count. The assertion about proteins is correct. The mark comes from counting the peptide bonds correctly, so state that a chain of n residues has n minus one bonds and apply it to four residues.

Q2 (CBSE Board 2023, 2 marks)

Give the reaction of glucose with hydrogen cyanide. The presence of which group is confirmed by this reaction?

Answer: Glucose reacts with hydrogen cyanide to give a cyanohydrin, in which the CN group adds to the aldehyde carbon and a new OH appears there. The reaction confirms the presence of the aldehyde group in glucose, because only a carbonyl group adds hydrogen cyanide in this way. The product is a chain of seven carbons, so the reaction also extends the carbon chain by one. The cyanohydrin structure and the group confirmed are the two points, with the chain extension as the extra detail that earns the second half.

Q3 (CBSE Board 2023, 3 marks)

Give reasons for any three of the following. (a) Penta-acetate of glucose does not react with hydroxylamine. (b) Amino acids behave like salts. (c) Water soluble vitamins must be taken regularly in the diet. (d) The two strands in DNA are complementary.

Answer: (a) In the penta-acetate all five hydroxyl groups are acetylated, including the one that has to open the ring, so the ring cannot open to the free aldehyde form and hydroxylamine cannot react with it. (b) An amino acid has an acidic carboxyl group and a basic amino group, so it exists as a zwitterion with a positive and a negative charge; it therefore behaves like an ionic salt and dissolves in water with a high melting point. (c) Water soluble vitamins are not stored in the body, since any excess is filtered out and leaves in urine, so a fresh supply is needed every day. (d) The bases on one strand hydrogen bond only with particular partners on the other strand, adenine with thymine and guanine with cytosine, so each strand carries the information needed to build the other. Three reasons carry three marks, so write each with its observation and its explanation rather than the name of the test alone.

Q4 (CBSE Board 2024, 1 mark)

Which functional groups of glucose interact to form the cyclic hemiacetal leading to the pyranose structure? (A) Aldehyde group and hydroxyl group at C4 (B) Aldehyde group and hydroxyl group at C5 (C) Ketone group and hydroxyl group at C4 (D) Ketone group and hydroxyl group at C5

Answer: (B) aldehyde group and hydroxyl group at C5. When the aldehyde carbon closes onto the hydroxyl at carbon 5, a six membered ring forms that contains five carbons and one oxygen, which is the pyranose form. Glucose is an aldohexose, so it is the aldehyde group that reacts. The aldehyde carbon closing on the hydroxyl at carbon 5 is the marking point, and the six membered ring that results is the pyranose form.

Q5 (CBSE Board 2024, 2 marks)

D-Glucose is heated with (a) hydrogen iodide and (b) concentrated nitric acid. Write the products formed in each case.

Answer: (a) Prolonged heating with hydrogen iodide reduces every carbon of glucose to a hydrocarbon skeleton and gives n-hexane, because the hydroxyl groups are replaced by iodine and then reduced; this proves the straight six carbon chain. (b) Concentrated nitric acid oxidises both ends of the sugar, so the aldehyde carbon and the primary alcohol at carbon 6 both become carboxyl groups, and the product is saccharic acid, HOOC-(CHOH)4-COOH. Each reagent carries one mark, so name n-hexane for hydrogen iodide and saccharic acid for nitric acid, and give the structure of the acid.

Q6 (CBSE Board 2025, 3 marks)

(a) Name the type of linkage responsible for the formation of proteins from alpha amino acids. (b) Write any two differences between DNA and RNA.

Answer: (a) Peptide bond, also called the amide linkage, formed with loss of water between the carboxyl group of one amino acid and the amino group of the next. (b) DNA contains the sugar 2-deoxyribose while RNA contains ribose; DNA has thymine as one of its bases while RNA has uracil in place of thymine. A third difference worth adding is that DNA is a double stranded helix while RNA is mostly single stranded. Part (a) is the peptide bond and part (b) needs two clear differences, with the sugar difference the strongest and the base difference the second.

Q7 (CBSE Board 2025, 2 marks)

Give chemical reactions to show the presence of an aldehydic group and of a straight chain in glucose.

Answer: The aldehydic group is shown by the reaction with hydrogen cyanide, which gives a cyanohydrin, and by the Tollens test, which gives a silver mirror with ammoniacal silver nitrate. The straight chain is shown by prolonged heating with hydrogen iodide, which gives n-hexane, and by oxidation with concentrated nitric acid to saccharic acid, a product that requires an unbranched six carbon chain with oxidisable ends. Two reactions are needed, one for the aldehyde group and one for the straight chain, so write the cyanohydrin or Tollens equation and then the hydrogen iodide route.

Q8 (CBSE SQP 2022-23, 2 marks)

D-Glucose is treated with (a) bromine water and (b) nitric acid. What are the products in the two cases?

Answer: (a) Bromine water is a mild oxidant and oxidises only the aldehyde group to a carboxyl group, so gluconic acid forms and the carbon chain length is unchanged. (b) Nitric acid is stronger and oxidises both the aldehyde end and the primary alcohol at carbon 6, so both ends become carboxyl groups and saccharic acid forms. The two results together locate the aldehyde at one end of a straight six carbon chain. State which reagent is mild and which is strong, then link each to its product and to the chain length that stays or changes.

Q9 (CBSE SQP 2023-24, 1 mark)

Assertion (A): An enzyme acts on one kind of reaction and one kind of substrate only. Reason (R): Enzymes are biological catalysts.

Answer: Both statements are true but the reason is not the correct explanation of the assertion. Being a biological catalyst does not by itself explain the narrow choice of substrate. The specificity comes from the active site, whose shape and chemical groups fit one substrate or one family of substrates, so the enzyme lowers the activation energy for that reaction only. The assertion and the reason are judged separately, and the active site explains the specificity, not the biocatalyst label alone.

Q10 (CBSE SQP 2025-26, 1 mark)

An amino acid has the formula HOOC-CH2CH(NH2)CH2COOH. Predict whether the pH of this amino acid will be greater than 7, equal to 7 or less than 7.

Answer: The pH will be less than 7, so the solution is acidic. The molecule is aspartic acid with two carboxyl groups and only one amino group, so the acidic groups outnumber the basic group and the zwitterion carries an extra negative charge that releases a proton to the solution, making the solution acidic. Count the acidic and basic groups, then state the effect on the zwitterion and the final pH in one line each.

How marks are usually split

Biomolecules questions are mostly short and factual, so the mark is lost by writing too little, not by writing too much. A two mark question needs two named facts, such as the two products or the two structural differences. A three mark question with any three parts means exactly three short answers, each with its reason.

For structure questions, draw the linkage rather than describing it in words. For tests, name the reagent and the observation together. For the peptide bond counting question, state the rule n minus one and then substitute the number.