Biomolecules — Class 12 MCQs with Answers

Biomolecules — important questions & MCQs with answers (Class 12 Chemistry)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Glucose formula:

   ACH₂OBC₅H₁₀O₅CC₁₂H₂₂O₁₁DC₆H₁₂O₆
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   Answer: C₆H₁₂O₆

   Explanation: Glucose is a hexose, molecular formula C₆H₁₂O₆ — CH₂O is just its empirical (Cₙ(H₂O)ₙ) unit, C₅H₁₀O₅ fits a pentose, and C₁₂H₂₂O₁₁ is the disaccharide sucrose.

2. Q2 of 6Easy

   Vitamin C is chemically known as:

   ACalciferolBRetinolCThiamineDAscorbic acid
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   Answer: Ascorbic acid

   Explanation: Vitamin C is ascorbic acid, whose deficiency causes scurvy; calciferol is vitamin D, retinol is vitamin A, and thiamine is vitamin B₁ — don't mix up the chemical names.

3. Q3 of 6Medium

   Sucrose hydrolyses to:

   AGlucose onlyBTwo glucoseCGlucose + GalactoseDGlucose + Fructose
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   Answer: Glucose + Fructose

   Explanation: Hydrolysis breaks sucrose's glycosidic bond into one glucose and one fructose unit; the reaction is called inversion because the optical rotation flips from dextro (+) to laevo (−).

4. Q4 of 6Medium

   Fat-soluble vitamins:

   AA, D, E, KBB, CCAll B groupDNone
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   Answer: A, D, E, K

   Explanation: Vitamins A, D, E and K dissolve in fats, so they are absorbed with dietary lipids and stored in body fat and the liver, unlike water-soluble B and C vitamins, which are excreted.

5. Q5 of 6Hard

   Primary structure of protein refers to:

   ASequence of amino acidsB3D foldCSubunit assemblyDLocal helix
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   Answer: Sequence of amino acids

   Explanation: Primary structure is just the linear sequence of amino acids joined by peptide bonds; the 3D fold is tertiary, subunit assembly is quaternary, and α-helix/β-sheet local coiling is secondary.

6. Q6 of 6Hard

   DNA's sugar component:

   ARiboseBDeoxyriboseCGlucoseDFructose
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   Answer: Deoxyribose

   Explanation: DNA's backbone sugar is 2-deoxyribose, missing the C-2 -OH that ribose (RNA's sugar) has; glucose and fructose are hexoses, not the pentose used in nucleic acids.

Biomolecules notes: key concepts (Class 12 Chemistry)

Biomolecules covers the chemistry of carbohydrates, proteins, enzymes, vitamins and nucleic acids: the structure of glucose and its anomers, glycosidic and peptide bonds, reducing versus non-reducing sugars, the four levels of protein structure and denaturation, and how DNA and RNA store and transmit genetic information.

Classification of carbohydrates

Polyhydroxy aldehydes/ketones or compounds that yield them on hydrolysis: monosaccharides (glucose), disaccharides (sucrose) and polysaccharides (starch, cellulose).

Structure of glucose

An aldohexose (C₆H₁₂O₆); its cyclic pyranose form has a hemiacetal at C-1, and the open-chain form is confirmed by reactions with HCN, HI and acetic anhydride.

Anomers and mutarotation

α- and β-glucose differ only in the configuration at C-1 (anomeric carbon); in solution they interconvert, changing optical rotation — this is mutarotation.

Glycosidic linkage

The C–O–C bond joining two monosaccharide units (through the anomeric carbon), formed with loss of water; it links units in di- and polysaccharides.

Reducing vs non-reducing sugars

Sugars with a free aldehyde/ketone (a free anomeric –OH) — glucose, fructose, maltose, lactose — reduce Tollens'/Fehling's; sucrose does not (both anomeric carbons are locked).

Polysaccharides

Starch (amylose + branched amylopectin, α-links) is a plant store; glycogen is the animal store; cellulose (β-links) is a rigid structural fibre humans cannot digest.

Amino acids

Building blocks of proteins with –NH₂ and –COOH on the same carbon; they exist as dipolar zwitterions (H₃N⁺–CHR–COO⁻) and are amphoteric with an isoelectric point.

Peptide bond

An amide (–CO–NH–) link formed between the –COOH of one amino acid and the –NH₂ of the next, with loss of water; it is the backbone of every protein.

Structure of proteins

Primary = amino-acid sequence; secondary = α-helix/β-pleated sheet held by H-bonds; tertiary = overall 3-D fold; quaternary = assembly of subunits (e.g. haemoglobin).

Denaturation of proteins

Heat, acid or heavy metals break the H-bonds and disulphide links, unfolding the secondary/tertiary structure (the 1° sequence stays intact) and destroying biological activity.

Enzymes

Highly specific biological catalysts (mostly globular proteins) that lower activation energy; each acts on a particular substrate, e.g. maltase, invertase, zymase.

Vitamins

Micronutrients grouped as fat-soluble (A, D, E, K — stored in fat) and water-soluble (B group, C — excreted, needed regularly); deficiency causes specific diseases.

Nucleic acids

DNA and RNA are polymers of nucleotides (base + pentose sugar + phosphate); DNA has deoxyribose, thymine and a double helix, RNA has ribose, uracil and one strand.