Class 12 Chemistry · NCERT Chapter 10 (Part 3)

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Biomolecules Class 12 Chemistry Notes

Biomolecules are the molecules of life: carbohydrates, proteins, nucleic acids, vitamins, enzymes and hormones. This chapter covers the structure of glucose, reducing and non-reducing sugars, amino acids and peptide bonds, DNA double helix, enzymes and vitamins.

Class:12Subject:ChemistryCovers:Carbohydrates · Proteins · DNA/RNAExams:CBSE · CUET · NEET

6 Key Formulas2 Solved Examples40 Practice MCQs

DWritten byDeep Narayan

UpdatedOctober 2026

Key Concept Summary

Why is sucrose a non-reducing sugar while glucose is reducing?

Reducing sugars have a free anomeric –OH (hemiacetal/hemiketal) that opens to an aldehyde and reduces Fehling's/Tollen's. In sucrose the aldehyde of glucose and the ketone of fructose are BOTH locked in the glycosidic bond between their anomeric carbons, so neither can open — sucrose gives no silver mirror and no red Cu₂O.

Carbohydrates: Classification

Definition

Carbohydrates

Polyhydroxy aldehydes or ketones (or compounds that hydrolyse to them). They store and provide energy — glucose in blood, starch in plants, glycogen in animals — and build structure (cellulose).

Monosaccharides: simplest, e.g. glucose (C₆H₁₂O₆), fructose.
Disaccharides: two sugars joined, e.g. sucrose = glucose + fructose; maltose = glucose + glucose.
Polysaccharides: many units, e.g. starch, cellulose, glycogen.
Optical activity: all monosaccharides rotate plane-polarised light (D/L, α/β).

Know these formulas

Glucose/fructose C₆H₁₂O₆, sucrose/maltose/lactose C₁₂H₂₂O₁₁ — the exam leans on the water-loss count (disaccharide = monosaccharides − H₂O).

MCQ Questions

The monomer of starch is:

A disaccharide formed by condensation of two glucose units is:

Structure of Glucose and Sugars

Glucose exists as an open-chain aldohexose and, in water, largely as a six-membered cyclic hemiacetal (pyranose). The new chiral centre made when the ring closes is the anomeric carbon — the −OH there pointing down gives α-D-glucose, up gives β-D-glucose, and the two interconvert (mutarotation).

Open-chain form of glucose

\ce C H O - (C H O H) 4 - C H 2 O H

Open chain: aldehyde + 4 stereocentres + CH₂OH ⇒ D-glucose.
Ring: cyclic hemiacetal; Haworth projection shows pyran ring with the anomeric −OH.
Mutarotation: α (+112°) ⇌ open ⇌ β (+18.7°), equilibrium +52.7°.
Fructose is a ketohexose; the ring is furanose for fructose, pyranose for glucose.

Reducing sugar definition

ANY sugar whose ring can open to a free aldehyde (or keto) — glucose, fructose, maltose, lactose. Sucrose locks both anomeric carbons and cannot reduce Tollen's or Fehling's.

MCQ Questions

Mutarotation of glucose is due to:

Which of the following is a non-reducing sugar?

Proteins and Amino Acids

Definition

Amino acid

A compound with both −NH₂ and −COOH on the same α-carbon: H₂N–CHR–COOH. Zwitterionic (H₃N⁺–CHR–COO⁻) in solution; amphoteric — acts as acid or base.

Essential (cannot be made in the body: valine, leucine, isoleucine...) vs non-essential amino acids.
Peptide bond: −NH–CO− formed by condensation of the acid of one and amine of the next amino acid.
Primary structure = amino acid sequence; secondary = α-helix/β-sheet (H-bonding); tertiary = folding in 3D; quaternary = multiple chains.
Denaturation: heat, acid or urea unfolds the structure (boiled egg — irreversible).
Tests: biuret (violet, peptide bonds), ninhydrin (blue-purple, free amino groups).

10 essential

HemInVaLys... a mnemonic: phenylalanine, methionine, threonine, tryptophan, valine, isoleucine, leucine, lysine, histidine, arginine — ten essential amino acids for humans.

MCQ Questions

The bond joining two amino acids is a:

The zwitterion of glycine is:

Nucleic Acids: DNA and RNA

Nucleotide = nitrogenous base + sugar (ribose or deoxyribose) + phosphate; nucleoside = base + sugar only.
DNA: double helix (Watson–Crick), 2‑deoxyribose, bases A, T, G, C paired A‑T and G‑C via H-bonds.
RNA: single strand, ribose, U replaces T; messenger (mRNA), transfer (tRNA), ribosomal (rRNA) RNA.
Replication: each strand templates a complement — genetic information storage and transfer.

Polymer chain of a nucleic acid

\ce (B a se - S ug a r - P O 4 -)_{n}

Nucleoside vs nucleotide

One less phosphate = nucleoside (adenosine), one more = nucleotide (AMP, ATP). The exam distinguishes the −ine suffix (guanine, cytosine) from the −osine/otide ones.

MCQ Questions

DNA contains the sugar:

In RNA, uracil replaces:

Vitamins and Enzymes

Vitamins: essential organic micronutrients. Fat-soluble: A, D, E, K (stored in fat). Water-soluble: B‑complex, C (excreted daily, must be replenished).
Deficiency table: A → night blindness; B1 (thiamine) → beriberi; C (ascorbic acid) → scurvy; D → rickets; K → bleeding tendency.
Enzymes: proteins (some RNA = ribozymes) that catalyse a specific reaction at an active site; one enzyme per substrate; cofactors/coenzymes help activity; heat/pH denature them.

Enzyme naming

Usually the substrate + 'ase': sucrase splits sucrose, urease splits urea. Salivary amylase starts starch digestion in the mouth — a classic everyday example.

MCQ Questions

The vitamin whose deficiency causes scurvy is:

Q10

Enzymes act by:

Lipids, Hormones and Everyday Relevance

Definition

Lipids

Oils, fats and waxes — esters of long-chain acids with glycerol (triglycerides). They insulate, store energy and form membranes (phospholipids); oxidation of fats yields the most energy per gram among biomolecules.

Hormones: chemical messengers — insulin (lowers glucose), adrenaline (fight-or-flight), thyroxine (metabolism), testosterone/oestrogen (reproductive).
Vitamins flags, enzymes run metabolism, nucleic acids instruct, carbohydrates fuel — the same molecules you studied all connect.
Everyday: instant glucose drinks, sports electrolytes, vitamin tablets, insulin pens, protein supplements — all biomolecule chemistry in practice.

Chapter link

Peptides (Amines chapter) build proteins, aldol-type carbonyl chemistry builds sugars — Biomolecules is the payoff of the whole carbonyl section.

MCQ Questions

Q11

The chemical messengers of the body are:

Q12

A triglyceride is an ester of glycerol and:

NCERT Exercise Questions

From the textbook

NCERT questions for this chapter, in textbook order. Answers are given at a glance below each set.

Exercises (NCERT Q1–Q3)

1
What are the products of hydrolysis of lactose?
2
Write the structure of the zwitterionic form of alanine at pH 7.
3
Name the four bases present in DNA. Which one is not present in RNA?

Answers at a glance

1
D‑Glucose and D‑galactose.
2
+H3N‑CH(CH3)‑COO–
3
A, G, C, T; thymine is absent in RNA (uracil takes its place).

CBSE Pattern Questions

How CBSE asks this chapter

CBSE typically asks 2-3 questions from Biomolecules, focusing on carbohydrates (structures, reducing/non‑reducing sugars, polysaccharides), proteins (amino acids, peptide bond, denaturation), enzymes, vitamins and nucleic acids.

MCQ Questions

Q13

Which of the following is a non‑reducing sugar?

Q14

The purine bases present in DNA are:

Q15

Which vitamin is responsible for blood clotting?

Q16

The peptide linkage is formed between:

1 Mark Questions

1
What are monosaccharides?
2
Define glycosidic linkage.
3
Name the disease caused by deficiency of vitamin C.

2 Mark Questions

1
Differentiate between fibrous and globular proteins.
2
What is the biological significance of sucrose?

3 Mark Questions

1
Explain the mechanism of enzyme action.
2
Describe the primary, secondary, tertiary and quaternary structure of proteins.

5 Mark Questions

1
Describe the process of transcription in molecular biology.
2
Explain the following: (i) Why is cellulose not digested by humans? (ii) How does the structure of DNA help in its replication?

PDF: Assignments & Tests

Printable study material

Download topic-wise assignments and full CBSE-pattern tests with answer keys for this chapter.

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Class 12 Chemistry — downloadable assignments and CBSE-pattern tests (PDF).

Assignment 1 — Topic-wise Practice Set A

Topic-wise practice covering the first half of the chapter, with answer key.

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Assignment 2 — Topic-wise Practice Set B

Topic-wise practice covering the second half of the chapter, with answer key.

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CBSE Pattern Test 1

Full CBSE-pattern test (90 minutes, 40 marks) with answer key.

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CBSE Pattern Test 2

Second full CBSE-pattern test (90 minutes, 40 marks) with answer key.

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Quick Revision

Key formulas at a glance

Memorise these equations — direct application numericals and derivations in CBSE & JEE frequently hinge on these.

Glucose

\ce C 6 H 12 O 6

An aldohexose.

Sucrose

\ce C 12 H 22 O 11

α-D-glucose + β-D-fructose (glycosidic bond).

Peptide bond

\ce - C O - N H -

Links amino acids; formed by condensation.

Chargaff base pair

\ce A - T, G - C

DNA; A-U (uracil) replaces T in RNA.

Specific rotation

[α]_{D} = \frac{α}{l c}

Mutarotation of glucose is a change in [α]D.

Zwitterion

\ce H 3 N^{+} - C H R - C O O^{-}

Amino acids at the isoelectric point.

Exam Strategy

How this chapter is asked

High-yield question patterns observed across CBSE boards, JEE Main & Advanced, and NEET.

Carbohydrate classification: monosaccharides (glucose, fructose), disaccharides (sucrose, maltose, lactose), polysaccharides (starch, cellulose, glycogen).
Glucose structure: open chain (aldehyde) vs cyclic pyranose; α‑/β‑anomers; Haworth projection + mutarotation.
Reducing vs non-reducing sugars — sucrose non-reducing because both anomeric carbons are linked.
Tests: Tollen's/Benedict's for reducing sugars; invert sugar when sucrose hydrolyses.
Amino acids: α‑structure, essential vs non-essential, zwitterion/amphoteric, isoelectric point.
Protein structure levels: primary → secondary (α-helix, β-sheet) → tertiary → quaternary; denaturation.
Nucleic acids: nucleotide vs nucleoside; DNA double helix with A‑T/G‑C pairs; RNA types (mRNA, tRNA, rRNA).
Enzymes: specificity, active site, cofactors/coenzymes; denaturation by heat/pH.
Vitamins: fat- vs water-soluble; deficiency diseases (A–night blindness, B1–beriberi, C–scurvy, D–rickets).
Lipids (fats/oils/waxes, triglycerides), hormones and their functions.

Practice Numericals

Step-by-step worked examples

Standard exam-level problems solved with clear conceptual breakdowns.

Reducing vs non-reducing

Problem StatementMaltose and sucrose both have C₁₂H₂₂O₁₁. Which is reducing and why?

Step-by-Step Solution

1Maltose = two glucose units joined so one anomeric carbon is free.
2Its free hemiacetal opens to an aldehyde → reduces Fehling's.
3Sucrose joins C₁ of glucose to C₂ of fructose — both locked.

Final Answer

Maltose is reducing; sucrose is not.

Counting ATP from glucose

Problem StatementWrite the net equation of aerobic glucose oxidation in one line (concept only).

Step-by-Step Solution

1C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.
2This is why reducing sugar tests depend on the aldehyde, not the energy yield — keep them separate.

Final Answer

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ~38 ATP.

FAQ

Frequently asked questions

What is mutarotation and why does glucose show it?

Fresh crystals of α-D-glucose give a specific rotation of +112°, β-D-glucose +18.7°, but either solution in water slowly settles at +52.7°. The two anomers interconvert through the open-chain aldehyde form, changing rotation until equilibrium — that's mutarotation. The open form is also why glucose is a reducing sugar.

How do sugars and proteins give their colour tests?

Tollen's reagent (ammoniacal silver nitrate) and Benedict's/Fehling's (alkaline Cu²⁺) both oxidise the free aldehyde of a reducing sugar — a silver mirror or brick-red Cu₂O appears. Ninhydrin turns α-amino acids (and proteins) blue-purple by reacting with the free amino group; biuret's violet colour comes from peptide bonds. The two families are distinguished cleanly by these.

Why is an α-amino acid zwitterionic?

The molecule carries one acidic (–COOH) and one basic (–NH₂) group. In water the acid donates a proton to the base: –NH₃⁺ and –COO⁻ coexist. The zwitterion is neutral overall yet highly polar, which explains high melting points and water solubility. At the isoelectric pH, both forms are balanced and the amino acid does not migrate in an electric field.

What exactly differs between DNA and RNA?

Sugar: DNA has 2‑deoxyribose, RNA ribose (one extra –OH). Base: DNA uses thymine, RNA uses uracil. Shape: DNA is a double helix with A‑T and G‑C pairs; RNA is mostly single-stranded. Function: DNA stores genetic information; RNA transmits it — mRNA carries the message, tRNA ferries amino acids, rRNA builds the ribosome.

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