How to Score Full Marks in the Board Exam
A complete bank of board-style questions with model answers for Biomolecules. In the exam: define terms precisely with an example, draw structures where asked (zwitterion, peptide bond, base pairing), and give the NCERT-canonical reasons (cyclic hemiacetal for glucose's missing aldehyde reactions; both anomeric carbons locked for sucrose being non-reducing; zwitterion for amino-acid properties; primary structure surviving denaturation). For deficiency-disease and DNA/RNA questions, state the specific fact the examiner wants.
1-Mark Questions (Definitions & Direct)
Q1. Define a carbohydrate. Answer: A polyhydroxy aldehyde or polyhydroxy ketone, or a compound that yields these on hydrolysis.
Q2. What is a glycosidic linkage? Answer: The C-O-C linkage formed (with loss of water) joining two monosaccharide units.
Q3. Name one reducing and one non-reducing disaccharide. Answer: Reducing: maltose (or lactose); non-reducing: sucrose.
Q4. What is the difference between a nucleoside and a nucleotide? Answer: A nucleoside is sugar + base; a nucleotide is sugar + base + phosphate.
Q5. Name the disease caused by deficiency of vitamin C. Answer: Scurvy.
Q6. Which base is present in RNA but not in DNA? Answer: Uracil.
2-Mark Questions
Q7. Why is glucose not able to react with sodium bisulphite and does not give the Schiff's test, although it has an aldehyde group? Answer: Because in aqueous solution glucose exists almost entirely in the cyclic hemiacetal (pyranose) form, with only a trace of the free open-chain aldehyde present. Hence the characteristic reactions of a free aldehyde group are not given.
Q8. What is meant by mutarotation? Answer: It is the gradual change in the optical rotation of a freshly prepared solution of a pure anomer (alpha or beta) of a sugar until it reaches a constant value, caused by the interconversion of the alpha and beta forms through the open-chain form.
Q9. Why is sucrose called a non-reducing sugar? Answer: In sucrose, the reducing groups (anomeric carbons) of both glucose and fructose are involved in the glycosidic linkage, so no free aldehyde or keto group is available to reduce Fehling's solution or Tollens' reagent.
Q10. Distinguish between fibrous and globular proteins. Answer: Fibrous proteins are thread-like, water-insoluble and structural (e.g. keratin). Globular proteins are roughly spherical, water-soluble and functional (e.g. insulin, enzymes).
2-Mark Questions (continued)
Q11. What is the zwitterion form of an amino acid? Why are amino acids amphoteric? Answer: A zwitterion is the dipolar ion +HN-CHR-COO-, formed when the -COOH transfers its proton to the -NH. Amino acids are amphoteric because they contain both an acidic group and a basic group, so they can react with both acids and bases.
Q12. Differentiate between amylose and amylopectin. Answer: Amylose is a linear, water-insoluble chain of alpha-glucose (about 15-20% of starch); amylopectin is a branched, water-insoluble chain of alpha-glucose (about 80-85% of starch).
Q13. What are essential and non-essential amino acids? Give one example of each. Answer: Essential amino acids cannot be synthesised by the body and must come from the diet (e.g. valine). Non-essential amino acids can be made by the body (e.g. glycine).
Q14. State the complementary base pairing in DNA. Answer: Adenine pairs with thymine (A-T) by two hydrogen bonds, and guanine pairs with cytosine (G-C) by three hydrogen bonds.
3-Mark Questions
Q15. Describe the structure of glucose, giving any two pieces of evidence for the functional groups present. Answer: Glucose is an aldohexose: a straight six-carbon chain with an aldehyde group at C-1 and five hydroxyl groups. Evidence: (i) it forms a pentaacetate with acetic anhydride, showing five -OH groups; (ii) on mild oxidation it gives gluconic acid (a six-carbon monocarboxylic acid), showing a terminal -CHO and an unbranched six-carbon chain. (It also forms an oxime and adds HCN, confirming a carbonyl group.)
Q16. Explain the four levels of structure of proteins. Answer: Primary structure is the sequence (order) of amino acids in the chain. Secondary structure is the regular local folding into an alpha-helix or beta-pleated sheet, stabilised by hydrogen bonds. Tertiary structure is the overall three-dimensional folding of the chain, stabilised by hydrogen bonds, disulphide bridges, ionic and hydrophobic interactions. Quaternary structure is the arrangement of two or more polypeptide subunits in the functional protein.
Q17. What is denaturation of proteins? Give two examples and state what happens to the structure. Answer: Denaturation is the loss of the natural three-dimensional shape and biological activity of a protein on treatment with heat, acids, bases, etc. Examples: coagulation of egg white on boiling and curdling of milk. During denaturation the secondary and tertiary structures are destroyed, but the primary structure (sequence of amino acids) remains intact.
3-Mark Questions (continued)
Q18. Classify vitamins on the basis of solubility and give two examples of each, with one deficiency disease. Answer: Fat-soluble vitamins (A, D, E, K) - e.g. vitamin A (deficiency: night blindness) and vitamin D (deficiency: rickets). Water-soluble vitamins (the B group and C) - e.g. vitamin C (deficiency: scurvy) and vitamin B (deficiency: beriberi). Fat-soluble vitamins are stored in the body; water-soluble vitamins must be supplied regularly.
Q19. Write the differences between DNA and RNA (any three). Answer: (i) DNA contains the sugar deoxyribose; RNA contains ribose. (ii) DNA contains the base thymine; RNA contains uracil instead. (iii) DNA is usually double-stranded (a double helix); RNA is usually single-stranded. (DNA stores genetic information; RNA is mainly involved in protein synthesis.)
Q20. Explain why amino acids have high melting points and are soluble in water. Answer: Amino acids exist as zwitterions (internal salts) with a positive and a negative end. The strong electrostatic forces between these charges (as in an ionic solid) give high melting points, and the charged zwitterion interacts strongly with polar water molecules, making them water-soluble.
3-Mark Questions (Reactions & Reasoning)
Q21. What happens when D-glucose is treated with (i) HI/red P, (ii) bromine water, (iii) HNO (conc.)? Answer: (i) With HI and red phosphorus, glucose is reduced to n-hexane (showing a straight six-carbon chain). (ii) With bromine water (a mild oxidant) the -CHO is oxidised to -COOH, giving gluconic acid. (iii) With concentrated nitric acid, both the -CHO and the terminal -CHOH are oxidised to -COOH, giving the dicarboxylic saccharic (glucaric) acid.
Q22. Why is sucrose dextrorotatory but the product of its hydrolysis laevorotatory? Answer: Sucrose is dextrorotatory (+66.5 degrees). On hydrolysis it gives glucose (+52.5 degrees) and fructose (-92.4 degrees). The laevorotation of fructose is greater in magnitude, so the mixture is overall laevorotatory; the sign of rotation inverts and the product is called invert sugar.
Q23. How is a peptide bond formed? Illustrate with two amino acids. Answer: The -COOH group of one amino acid reacts with the -NH group of another, eliminating a molecule of water and forming the amide (-CO-NH-) peptide bond. For example, glycine and alanine combine (losing water) to give the dipeptide glycylalanine.
5-Mark Questions (Long Answer)
Q24. (a) What are reducing and non-reducing sugars? (b) Why is maltose reducing but sucrose non-reducing? (c) Name the products of hydrolysis of sucrose, maltose and lactose. Answer: (a) Reducing sugars reduce Fehling's solution and Tollens' reagent (they have a free aldehyde/keto, i.e. a free anomeric group); non-reducing sugars do not. (b) In maltose one anomeric carbon is free, so it is reducing; in sucrose both anomeric carbons are involved in the glycosidic linkage, so there is no free reducing group and it is non-reducing. (c) Sucrose gives glucose + fructose; maltose gives glucose + glucose; lactose gives glucose + galactose.
Q25. Describe the structure and functions of nucleic acids. Answer: Nucleic acids (DNA and RNA) are polymers of nucleotides; each nucleotide consists of a pentose sugar, a nitrogenous base and a phosphate group. The sugar and phosphate form the backbone, with the bases projecting from it. DNA is a double helix of two complementary strands held by hydrogen bonds, with A pairing with T and G pairing with C. Functions: DNA replicates itself (transmitting genetic information to daughter cells during cell division) and directs protein synthesis - the information is transcribed into mRNA and then translated into the amino-acid sequence of proteins.
5-Mark Questions (continued)
Q26. (a) What are enzymes? (b) Give two characteristics of enzymes. (c) How does temperature and pH affect their activity? Answer: (a) Enzymes are biological catalysts (almost all are globular proteins) that speed up the reactions in living organisms by lowering the activation energy. (b) They are highly efficient (a small amount transforms a large amount of substrate) and highly specific (each catalyses one particular reaction of one substrate, by the lock-and-key fit at its active site). (c) Each enzyme has an optimum temperature and an optimum pH; above the optimum temperature or away from the optimum pH the enzyme is denatured and its activity falls sharply.
Q27. (a) Define vitamins. (b) Name the deficiency diseases of vitamins A, B, C, D and K. Answer: (a) Vitamins are organic compounds required in small amounts in the diet for normal growth and health, which the body cannot synthesise in sufficient amounts. (b) Vitamin A - night blindness; vitamin B - beriberi; vitamin C - scurvy; vitamin D - rickets/osteomalacia; vitamin K - poor blood clotting.
Quick-Fire Board Favourites
Q28. Name the sugar present in DNA and in RNA. Answer: Deoxyribose in DNA and ribose in RNA.
Q29. Why can humans not digest cellulose? Answer: Humans lack the enzyme needed to hydrolyse the beta-glycosidic linkages of cellulose, so it passes through undigested as fibre.
Q30. What is invert sugar? Answer: The equimolar mixture of glucose and fructose obtained on hydrolysis of sucrose, which is laevorotatory (the sign of rotation having inverted).
Q31. What type of bond joins amino acids in a protein? Answer: The peptide bond (an amide -CO-NH- linkage).
Q32. Name the storage carbohydrate of the human body and where it is stored. Answer: Glycogen, stored mainly in the liver and muscles.
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Q33. What are anomers? Give an example. Answer: Anomers are a pair of cyclic sugars that differ only in the configuration of the -OH at the anomeric carbon. Example: alpha-D-glucose and beta-D-glucose.
Q34. Which level of protein structure is destroyed during denaturation, and which is retained? Answer: The secondary and tertiary structures are destroyed; the primary structure (sequence of amino acids) is retained.
Q35. Name the purine and pyrimidine bases found in DNA. Answer: Purines: adenine and guanine. Pyrimidines: cytosine and thymine.
Q36. Give one biological function each of carbohydrates and proteins. Answer: Carbohydrates provide and store energy (e.g. glucose, glycogen/starch). Proteins build body tissues and act as enzymes, hormones and antibodies.