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CBSE Class 12 Chemistry Chapter 10 Biomolecules — 20 MCQs with Answers

Biomolecules—carbohydrates, proteins, vitamins, and nucleic acids—form the chemical foundation of life and occupy a crucial spot in the CBSE Class 12 Chemistry syllabus. Chapter 10 tests your ability to recall structures, apply concepts to new scenarios, and reason through assertion-based questions. The 20 MCQs below mirror the latest Board exam pattern, blending recall, application, and higher-order thinking. Each answer comes with a short explanation rooted in NCERT language, helping you self-assess and plug gaps before your Board paper.

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Key takeaways

  • ✓Carbohydrates are polyhydroxy aldehydes or ketones classified as monosaccharides, disaccharides, and polysaccharides based on chain length and glycosidic linkage.
  • ✓Proteins are polymers of amino acids linked by peptide bonds; their four levels of structure (primary, secondary, tertiary, quaternary) determine biological function.
  • ✓Enzymes are biological catalysts that lower activation energy; they are highly specific, reusable, and sensitive to temperature and pH changes.
  • ✓Vitamins are classified as fat-soluble (A, D, E, K) or water-soluble (B group, C); deficiencies lead to diseases like scurvy, rickets, and pernicious anemia.
  • ✓DNA is a double-stranded helix with A–T and G–C base pairing; RNA is single-stranded with uracil replacing thymine, playing roles in protein synthesis.
  • ✓Understanding denaturation, glycosidic bond types, and enzyme kinetics is critical for scoring in assertion-reason and case-based MCQs in CBSE Board exams.
  • ✓Regular MCQ practice sharpens recall, reduces silly errors, and improves time management under exam pressure.

Carbohydrates: Monosaccharides and Classification

Carbohydrates are polyhydroxy aldehydes or ketones with the general formula Cₙ(H₂O)ₘ. Monosaccharides—simple sugars like glucose (aldose) and fructose (ketose)—form the building blocks of all larger carbohydrates. Glucose exists in open-chain and cyclic pyranose forms; the cyclic structure arises through intramolecular hemiacetal formation when the aldehyde group reacts with a hydroxyl group on C5. Fructose, a ketohexose, is sweeter and more soluble. These structural differences impact metabolism: glucose enters glycolysis immediately, while fructose is phosphorylated in the liver first. Understanding stereochemistry—D versus L forms—and mutarotation (the shift between α and β anomers in solution) is essential for MCQs asking you to identify reducing sugars or predict product structures.
  • **MCQ 1:** Which of the following is a ketohexose? (A) Glucose (B) Fructose (C) Ribose (D) Galactose | **Answer: (B) Fructose** | Fructose is a six-carbon ketose sugar with a ketone group at C2, while glucose and galactose are aldohexoses and ribose is a pentose.
  • **MCQ 2:** In aqueous solution, glucose predominantly exists as: (A) Open-chain aldehyde (B) Pyranose ring (C) Furanose ring (D) Linear ketone | **Answer: (B) Pyranose ring** | At equilibrium, over 99% of glucose molecules adopt the six-membered pyranose ring via intramolecular hemiacetal formation.
  • **MCQ 3:** Mutarotation refers to: (A) Conversion of D-glucose to L-glucose (B) Interconversion of α and β anomers in solution (C) Ring-opening of all monosaccharides (D) Hydrolysis of disaccharides | **Answer: (B) Interconversion of α and β anomers in solution** | Mutarotation is the change in optical rotation as α and β anomers equilibrate via the open-chain form.

Disaccharides and Glycosidic Bonds

Disaccharides consist of two monosaccharide units joined by a glycosidic bond, formed when the anomeric OH of one sugar reacts with an OH of another, releasing water. Sucrose (glucose + fructose) has an α(1→2) glycosidic linkage; maltose (two glucose units) has α(1→4); lactose (glucose + galactose) has β(1→4). The position and configuration (α or β) of the bond determine the disaccharide's properties. Sucrose is non-reducing because both anomeric carbons are involved in the bond, so no free aldehyde or ketone group remains. Maltose and lactose are reducing sugars because one anomeric carbon is free to exist in open-chain form. This distinction is tested through Tollens' or Fehling's test scenarios in MCQs.
  • **MCQ 4:** Sucrose is a non-reducing sugar because: (A) It is a monosaccharide (B) Both anomeric carbons are locked in the glycosidic bond (C) It lacks hydroxyl groups (D) It is a polysaccharide | **Answer: (B) Both anomeric carbons are locked in the glycosidic bond** | The α(1→2) linkage means no free anomeric OH can form an aldehyde or ketone, so sucrose cannot reduce Fehling's solution.
  • **MCQ 5:** Lactose is composed of: (A) Glucose + Glucose (B) Glucose + Fructose (C) Glucose + Galactose (D) Fructose + Galactose | **Answer: (C) Glucose + Galactose** | Lactose, or milk sugar, is a disaccharide formed by β(1→4) glycosidic bond between glucose and galactose.
  • **MCQ 6:** Which enzyme hydrolyzes the glycosidic bond in maltose? (A) Lactase (B) Invertase (C) Maltase (D) Amylase | **Answer: (C) Maltase** | Maltase specifically cleaves the α(1→4) bond in maltose to yield two glucose molecules.

Polysaccharides: Starch, Cellulose, and Glycogen

Polysaccharides are polymers of monosaccharides linked by glycosidic bonds. Starch, the plant storage form of glucose, has two components: amylose (linear α(1→4) chains that coil into a helix) and amylopectin (branched chains with α(1→6) branch points every 20–25 glucose units). Cellulose, the structural component of plant cell walls, consists of linear β(1→4) linked glucose chains that form strong hydrogen-bonded microfibrils. Humans lack cellulase, so cellulose passes undigested as dietary fibre. Glycogen, the animal storage polysaccharide, resembles amylopectin but is more highly branched, allowing rapid glucose release from the liver and muscles. Understanding these structural differences explains why we can digest starch but not cellulose, and why glycogen is ideal for quick energy mobilization.
  • **MCQ 7:** Cellulose is indigestible to humans because: (A) It contains only pentose sugars (B) Humans lack the enzyme cellulase (C) It is a protein (D) It is water-insoluble | **Answer: (B) Humans lack the enzyme cellulase** | Cellulase breaks β(1→4) glycosidic bonds in cellulose; herbivores have gut bacteria producing cellulase, but humans do not.
  • **MCQ 8:** Glycogen differs from amylopectin in: (A) Type of monosaccharide (B) Degree of branching (C) Presence of β linkages (D) Solubility | **Answer: (B) Degree of branching** | Glycogen is more highly branched than amylopectin, with branch points every 8–12 glucose residues for faster glucose release.
  • **MCQ 9:** Amylose gives a blue colour with iodine due to: (A) Formation of a helical inclusion complex (B) Oxidation to carboxyl groups (C) Hydrolysis to glucose (D) Glycosidic bond cleavage | **Answer: (A) Formation of a helical inclusion complex** | Iodine molecules slip into the helical cavity of amylose, producing a characteristic blue colour.

Proteins: Amino Acids and Peptide Bonds

Proteins are polymers of amino acids, each with a central α-carbon bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen, and a variable R group (side chain). The 20 standard amino acids differ in their R groups, which can be nonpolar (valine, leucine), polar uncharged (serine, cysteine), acidic (aspartate, glutamate), or basic (lysine, arginine). Amino acids link via peptide bonds—covalent bonds between the carboxyl of one and the amino of the next, releasing water. The sequence of amino acids (primary structure) is determined by DNA and dictates how the protein folds into secondary structures (α-helix and β-sheet stabilized by hydrogen bonds), tertiary structure (overall 3D shape with disulfide bridges, hydrophobic interactions, and ionic bonds), and quaternary structure (multiple polypeptide chains). A single amino acid change can alter function drastically, as seen in sickle-cell anemia.
  • **MCQ 10:** The bond formed between two amino acids is called: (A) Glycosidic bond (B) Peptide bond (C) Ester bond (D) Hydrogen bond | **Answer: (B) Peptide bond** | A peptide bond is a covalent amide linkage between the carboxyl group of one amino acid and the amino group of another, with loss of H₂O.
  • **MCQ 11:** Which amino acid contains a disulfide linkage in proteins? (A) Serine (B) Cysteine (C) Methionine (D) Proline | **Answer: (B) Cysteine** | Two cysteine residues can form a disulfide bond (–S–S–) via oxidation, stabilizing tertiary and quaternary structures.
  • **MCQ 12:** The primary structure of a protein refers to: (A) α-helix and β-sheet (B) Sequence of amino acids (C) 3D folding (D) Subunit assembly | **Answer: (B) Sequence of amino acids** | Primary structure is the linear order of amino acids in the polypeptide chain, encoded by the gene sequence.

Protein Denaturation and Properties

Denaturation is the disruption of secondary, tertiary, and quaternary structures without breaking peptide bonds (primary structure remains intact). Causes include heat (breaks hydrogen bonds), pH extremes (protonates or deprotonates ionizable groups, disrupting ionic interactions), organic solvents (disrupt hydrophobic cores), heavy metals (bind to charged residues), and mechanical agitation. Denatured proteins lose biological activity but can sometimes refold (renaturation) if conditions are restored gently. Proteins are amphoteric—they can accept or donate protons depending on pH—and form colloidal solutions in water. They absorb UV light at 280 nm due to aromatic amino acids (tryptophan, tyrosine). Understanding denaturation explains cooking, digestion, and diagnostic tests like the Biuret test (detects peptide bonds) and xanthoproteic test (detects aromatic rings).
  • **MCQ 13:** Denaturation of a protein involves: (A) Breaking peptide bonds (B) Loss of secondary and tertiary structure (C) Conversion to amino acids (D) Formation of disulfide bonds | **Answer: (B) Loss of secondary and tertiary structure** | Denaturation unfolds the protein by disrupting weak interactions (H-bonds, hydrophobic forces) but leaves peptide bonds intact.
  • **MCQ 14:** Which reagent is used to detect the presence of peptide bonds in proteins? (A) Fehling's solution (B) Biuret reagent (C) Iodine solution (D) Benedict's reagent | **Answer: (B) Biuret reagent** | Biuret reagent (CuSO₄ in alkaline solution) forms a violet complex with peptide bonds, confirming the presence of proteins.
  • **MCQ 15:** Heating egg white causes it to solidify because: (A) Peptide bonds hydrolyze (B) Protein denatures and aggregates (C) New peptide bonds form (D) Amino acids polymerize | **Answer: (B) Protein denatures and aggregates** | Heat disrupts the albumin protein structure, causing it to unfold and clump together irreversibly.

Enzymes: Mechanism and Specificity

Enzymes are biological catalysts, mostly proteins, that accelerate reactions by lowering activation energy. The substrate binds to the enzyme's active site, forming an enzyme–substrate complex. The enzyme stabilizes the transition state, enabling the reaction to proceed faster, then releases the product and returns to its original state. Enzymes are highly specific (lock-and-key or induced-fit models), work under mild conditions (body temperature and pH), and are reusable. They are named by adding '-ase' to the substrate or reaction type (e.g., lactase hydrolyzes lactose, DNA polymerase synthesizes DNA). Enzyme activity is affected by temperature (optimal around 37°C for human enzymes; denatured above ~50°C), pH (each enzyme has an optimal pH; pepsin works at pH 2, trypsin at pH 8), and inhibitors (competitive block the active site, non-competitive bind elsewhere and change enzyme shape). Understanding these factors helps explain metabolic regulation and drug design.
  • **MCQ 16:** Enzymes speed up reactions by: (A) Increasing temperature (B) Lowering activation energy (C) Changing the equilibrium constant (D) Providing extra reactants | **Answer: (B) Lowering activation energy** | Enzymes stabilize the transition state, reducing the energy barrier and allowing more molecules to react at body temperature.
  • **MCQ 17:** The enzyme that breaks down starch in the mouth is: (A) Pepsin (B) Trypsin (C) Amylase (D) Lipase | **Answer: (C) Amylase** | Salivary amylase (ptyalin) hydrolyzes α(1→4) glycosidic bonds in starch, starting carbohydrate digestion.
  • **MCQ 18:** A competitive inhibitor: (A) Binds to the active site (B) Binds to an allosteric site (C) Denatures the enzyme (D) Changes substrate structure | **Answer: (A) Binds to the active site** | Competitive inhibitors resemble the substrate and compete for the active site, reducing enzyme activity; this can be overcome by increasing substrate concentration.

Vitamins: Classification and Functions

Vitamins are organic micronutrients essential for normal metabolism but not synthesized in sufficient amounts by the body. They are classified as fat-soluble (A, D, E, K—stored in fatty tissues, can accumulate to toxic levels) or water-soluble (B complex and C—not stored, excess excreted in urine, must be consumed regularly). Vitamin A (retinol) is crucial for vision (component of rhodopsin in retina), immune function, and skin health; deficiency causes night blindness. Vitamin D (calciferol) regulates calcium absorption and bone mineralization; deficiency leads to rickets in children and osteomalacia in adults. Vitamin E (tocopherol) is an antioxidant protecting cell membranes from free radical damage. Vitamin K is required for synthesis of clotting factors II, VII, IX, X; deficiency causes bleeding disorders. B vitamins act as coenzymes: B₁ (thiamine) in carbohydrate metabolism, B₂ (riboflavin) in redox reactions, B₃ (niacin) in NAD/NADP, B₁₂ (cobalamin) in DNA synthesis and myelin formation. Vitamin C (ascorbic acid) is a cofactor for collagen synthesis and enhances iron absorption; deficiency causes scurvy (bleeding gums, poor wound healing).
  • **MCQ 19:** Deficiency of Vitamin C causes: (A) Rickets (B) Scurvy (C) Night blindness (D) Beriberi | **Answer: (B) Scurvy** | Vitamin C is essential for collagen synthesis; deficiency leads to scurvy with symptoms like bleeding gums, loose teeth, and poor wound healing.
  • **MCQ 20:** Which vitamin is synthesized in the skin on exposure to sunlight? (A) Vitamin A (B) Vitamin D (C) Vitamin E (D) Vitamin K | **Answer: (B) Vitamin D** | UV-B radiation converts 7-dehydrocholesterol in the skin to cholecalciferol (Vitamin D₃), which is then activated in the liver and kidneys.

Nucleic Acids: DNA and RNA Structure

Nucleic acids—DNA and RNA—store and transmit genetic information. Each nucleotide consists of a pentose sugar (deoxyribose in DNA, ribose in RNA), a nitrogenous base (purines: adenine and guanine; pyrimidines: cytosine, thymine in DNA, uracil in RNA), and a phosphate group. Nucleotides link via 3'→5' phosphodiester bonds to form polynucleotide chains. DNA is a double helix (Watson-Crick model): two antiparallel strands held by hydrogen bonds between complementary bases (A pairs with T via 2 H-bonds; G pairs with C via 3 H-bonds). The helix has major and minor grooves where proteins bind. RNA is usually single-stranded and can fold into complex shapes (hairpins, loops). mRNA carries genetic code from DNA to ribosomes; tRNA brings amino acids to ribosomes; rRNA is a structural component of ribosomes. DNA replication is semiconservative: each new double helix has one old strand and one new strand. The base-pairing rules ensure faithful copying.

How to Attempt MCQs in the CBSE Chemistry Paper

CBSE Class 12 Chemistry papers typically include 5–10 MCQs from Biomolecules, each carrying 1 mark. Success in MCQs requires a blend of conceptual clarity, quick recall, and strategic time management. Start by reading the question stem carefully—identify keywords like 'not', 'except', 'always', or 'only', which can flip the meaning. Eliminate obviously wrong options first to narrow your choices. If two options seem close, recall NCERT definitions verbatim: examiners often test subtle differences (e.g., α versus β bonds, fat-soluble versus water-soluble vitamins). For assertion-reason MCQs, evaluate the assertion and reason independently, then check if the reason correctly explains the assertion—both can be true without the reason explaining the assertion. Practice under timed conditions: aim for 30–40 seconds per MCQ. If stuck, mark the question and move on—come back with fresh eyes. Review common traps: confusing maltose with lactose, mixing up enzyme names, or forgetting that sucrose is non-reducing. Use mnemonics for vitamin functions (e.g., 'A for eyes, D for bones, K for Klotting'). Finally, avoid second-guessing—change your answer only if you spot a clear factual error, not just a 'feeling'. Regular MCQ practice, like the 20 questions above, builds pattern recognition and confidence. CBSETUTOR.ai offers unlimited MCQ drills with instant photo-upload solving at just ₹999/month for all classes (6–12), with a 3-day free trial—think of it as a 24×7 tutor in your pocket, clarifying doubts the moment they arise.
  • Read each question twice, underline keywords like 'not', 'only', 'always' to avoid misinterpretation.
  • Eliminate obviously incorrect options first to improve your odds on educated guesses.
  • Recall NCERT definitions word-for-word—examiners often test subtle distinctions in terminology.
  • For assertion-reason MCQs, check: (1) Is the assertion true? (2) Is the reason true? (3) Does the reason explain the assertion?
  • Practice past-year CBSE MCQs to recognize recurring patterns and commonly tested concepts.
  • Manage time: allocate 30–40 seconds per MCQ; skip tough ones, return later with a clear mind.
  • Trust your first instinct unless you find a concrete factual mistake—overthinking leads to errors.

Frequently asked questions

How many MCQs from Biomolecules appear in the CBSE Class 12 Chemistry Board exam?+
Typically 4–6 MCQs from Biomolecules are included in Section A of the CBSE Class 12 Chemistry paper, each worth 1 mark. Recent years have also seen 1–2 assertion-reason questions from this chapter.
What is the difference between α-glucose and β-glucose?+
α-glucose and β-glucose are anomers differing in the position of the –OH group on the anomeric carbon (C1). In α-glucose, the –OH is below the plane of the ring; in β-glucose, it is above. They interconvert in solution via mutarotation.
Why is cellulose not digestible by humans but starch is?+
Starch has α(1→4) glycosidic bonds that human enzymes (amylase, maltase) can hydrolyze. Cellulose has β(1→4) bonds, and humans lack cellulase enzyme to break them. Herbivores have gut bacteria producing cellulase, so they can digest cellulose.
What causes protein denaturation, and is it reversible?+
Denaturation is caused by heat, extreme pH, organic solvents, heavy metals, or mechanical agitation—all disrupt hydrogen bonds and hydrophobic interactions. It is sometimes reversible (renaturation) if conditions are gently restored, but often irreversible if aggregation occurs.
How do I remember which vitamins are fat-soluble and which are water-soluble?+
Use the mnemonic 'All Dogs Eat Kibble' for fat-soluble vitamins A, D, E, K. All B vitamins and Vitamin C are water-soluble. Fat-soluble vitamins are stored in the body and can be toxic in excess; water-soluble ones are excreted and need regular intake.
What is the difference between DNA and RNA?+
DNA has deoxyribose sugar, thymine as a base, and is double-stranded; it stores genetic information. RNA has ribose sugar, uracil instead of thymine, and is usually single-stranded; it plays roles in protein synthesis (mRNA, tRNA, rRNA).
How does an enzyme lower the activation energy of a reaction?+
The enzyme binds the substrate at its active site, forming an enzyme–substrate complex that stabilizes the transition state. This lowers the energy barrier, allowing the reaction to proceed faster at body temperature without being consumed in the process.
What is a peptide bond and how is it formed?+
A peptide bond is a covalent amide linkage between the carboxyl group (–COOH) of one amino acid and the amino group (–NH₂) of another, with the release of one water molecule. This condensation reaction builds the polypeptide backbone.
Why is Vitamin B12 deficiency serious?+
Vitamin B12 (cobalamin) is essential for DNA synthesis and myelin formation around nerves. Deficiency causes pernicious anemia (low red blood cells) and neurological damage (tingling, memory loss). It is found mainly in animal products, so vegetarians may need supplements.
Can I use CBSETUTOR.ai to practice more Biomolecules MCQs?+
Yes, CBSETUTOR.ai offers unlimited chapter-wise MCQ drills, instant photo-upload doubt solving, and AI-powered explanations for Classes 6–12 at a flat ₹999/month. Start with a 3-day free trial to explore adaptive practice and track your weak topics in real time.

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