Chapters

Everything NCERT Class 12 Unit 10 (Biomolecules) says that can be asked, plus what JEE Main adds beyond the book (marked Beyond NCERT). The short note stays for the last week; this is for a proper revision pass.

Formula sheet

This chapter is facts, not formulas. These are the ones the questions turn on, on one screen.

Fact	What it is	Watch
Reducing sugar	free CHO or C=O (hemiacetal) reduces Tollens' / Fehling's	Every monosaccharide; maltose, lactose; not sucrose
Sucrose	α-D-glucose C1 — C2 β-D-fructose	Hydrolysis gives invert sugar (laevo)
Maltose	α-D-glucose C1 — C4 α-D-glucose	Reducing
Lactose	β-D-galactose C1 — C4 β-D-glucose	Reducing; milk sugar
Starch	amylose (15–20%, α 1→4, linear) + amylopectin (80–85%, α 1→4, branches α 1→6)	Glycogen is like amylopectin, more branched
Cellulose	β-D-glucose, 1→4, straight chain	The only β one
Peptide count	n amino acids in a chain → n − 1 peptide bonds	A ring of n has n
Sequences	n different amino acids, each once → n! peptides	With repeats allowed: kⁿ
DNA pairs	A=T (2 H-bonds), G≡C (3 H-bonds)	Count H-bonds per pair
DNA vs RNA	2-deoxyribose, T vs ribose, U	DNA double, RNA single strand
Vitamins	fat-soluble A, D, E, K; water-soluble B, C	B₁₂ is water-soluble but stored
Denaturation	2° and 3° structure lost	1° (sequence) stays

0 · What JEE Main asks from this chapter

180 questions in JEE Main 2019–2026 (151 MCQ, 29 numerical), about 1 per paper. Grouped by type (counts are approximate):

Type	Share	Typical question
Carbohydrates: glucose evidence, anomers, linkages, reducing sugars	~40%	Glycosidic linkage between C1 of α-glucose and C2 of β-fructose is in?
Amino acids and proteins: codes, essential ones, peptides, structure	~35%	Number of tripeptides from three different amino acids
Nucleic acids	~9%	Total H-bonds in a 13-base-pair double helix
Enzymes, hormones and tests	~9%	Iodine % in thyroxine
Vitamins	~7%	How many of A, B₁, B₆, B₁₂, C, D, E, K can be stored

Nearly all are recall. Learn the tables in this note exactly; the numericals are counting (chiral centres, peptide bonds, H-bonds) or a % composition.

1 · Carbohydrates: definition and classification

• Many have the formula Cₓ(H₂O)ᵧ, hence "hydrates of carbon", but the formula is not the test. Acetic acid C₂(H₂O)₂ fits and is not one; rhamnose C₆H₁₂O₅ is one and doesn't fit.
• Definition: optically active polyhydroxy aldehydes or ketones, or compounds that give these on hydrolysis.
• Sweet ones are sugars; also called saccharides (Greek sakcharon, sugar). Household sugar is sucrose; milk sugar is lactose.

By hydrolysis:

Class	On hydrolysis	Examples
Monosaccharide	cannot be hydrolysed further (about 20 in nature)	glucose, fructose, ribose
Oligosaccharide	2–10 monosaccharide units	disaccharides: sucrose (glucose + fructose), maltose (2 glucose)
Polysaccharide	a large number of units; not sweet (non-sugars)	starch, cellulose, glycogen, gums

Reducing sugars reduce Fehling's solution and Tollens' reagent. All monosaccharides are reducing, aldose or ketose.

Monosaccharides by carbons and group: an aldose has CHO, a ketose C=O. Triose (3 C), tetrose, pentose, hexose, heptose (7 C): e.g. aldohexose, ketopentose.

2 · Glucose

Preparation.

• From sucrose: boil with dilute HCl or H₂SO₄ in alcoholic solution; glucose and fructose in equal amounts.
• From starch (commercial): boil with dilute H₂SO₄ at 393 K under pressure (2–3 atm).
• Glucose (dextrose) is an aldohexose, the monomer of starch and cellulose, and probably the most abundant organic compound on earth. It occurs in sweet fruits, honey and ripe grapes.

The evidence for its open-chain structure, CHO–(CHOH)₄–CH₂OH:

1. Molecular formula C₆H₁₂O₆.
2. Prolonged heating with HI → n-hexane: six carbons in a straight chain.
3. Hydroxylamine → oxime, and HCN → cyanohydrin: a carbonyl group.
4. Bromine water (mild) → gluconic acid, a six-carbon acid: the carbonyl is an aldehyde.
5. Acetic anhydride → glucose pentaacetate: five –OH, on different carbons (the compound is stable).
6. Nitric acid turns glucose and gluconic acid alike into saccharic acid (a dicarboxylic acid): there is a 1° –OH.

Example (2026) · Match glucose + hydroxylamine → glucoxime; + Br₂ water → gluconic acid; + excess acetic anhydride → pentaacetate; + conc. HNO₃ → saccharic acid. Glucose + NaHCO₃: no reaction (it is not an acid; 2024).

Example (2022) · Fructose + HCN, then H₃O⁺ → C₇H₁₄O₈ (one carbon added as COOH). Fructose + NaBH₄, then HI/P → n-hexane C₆H₁₄.

D and L. Glucose is D-(+)-glucose. D/L gives the relative configuration; (+) means dextrorotatory. They are unrelated: a D sugar can be (−), like fructose, and D/L has nothing to do with d/l.

• The reference is glyceraldehyde: its (+) isomer is D (–OH on the right) and its (−) isomer is L.
• Write the most oxidised carbon (CHO) at the top. Compare the lowest asymmetric carbon (C-5 in glucose). –OH on the right → D. The other chiral carbons don't count.

What the open chain can't explain:

1. Glucose has a CHO yet gives no Schiff's test and no NaHSO₃ adduct.
2. Its pentaacetate doesn't react with hydroxylamine: no free CHO.
3. It exists in two crystalline forms. α-glucose, m.p. 419 K, crystallises from a concentrated solution at 303 K. β-glucose, m.p. 423 K, crystallises from a hot saturated solution at 371 K.

Cyclic structure. The –OH at C-5 adds to the CHO, giving a six-membered cyclic hemiacetal.

• The α and β forms differ only at C-1, the anomeric carbon (the old aldehyde carbon). They are anomers.
• The six-membered ring (5 C + 1 O) is a pyranose, after pyran. Haworth projections show it best.
• Both cyclic forms are in equilibrium with the open chain.

Example (2019) · Stereocentres: open-chain glucose 4 (C-2 to C-5); cyclic glucose 5 (C-1 becomes one).

Mutarotation and what else it explains · Beyond NCERT

• In water each anomer slowly changes to the equilibrium mixture, about 36% α and 64% β, through the open chain. The rotation changes until it settles at +52.5°: mutarotation.
• Glucose and galactose differ only at C-4 (2019).
• Glucose dissolves in water because of its five –OH, not its CHO (2024).

3 · Fructose

• A ketohexose, C₆H₁₂O₆, obtained with glucose from sucrose; found in fruits, honey and vegetables; used pure as a sweetener.
• The C=O is at C-2; six carbons in a straight chain.
• It is in the D-series and laevorotatory: D-(−)-fructose.
• Cyclic forms: the C-5 –OH adds to the C-2 carbonyl, giving a five-membered ring, a furanose (after furan: 4 C + 1 O). There are α and β anomers.

Fructose is a reducing sugar · Beyond NCERT · It has no CHO, yet it reduces Tollens' and Fehling's. In base it rearranges, through an enediol, to glucose and mannose (2025: both statements true).

4 · Disaccharides

Two monosaccharides join by an oxide linkage, losing water: the glycosidic linkage. If both reducing groups (CHO, C=O) are used in it, the sugar is non-reducing; if one is free, it is reducing.

Sugar	Units and link	Reducing?
Sucrose	C1 of α-D-glucose — C2 of β-D-fructose	No: both reducing groups are in the link
Maltose	C1 of α-D-glucose — C4 of α-D-glucose	Yes: C1 of the second glucose can open
Lactose (milk sugar)	C1 of β-D-galactose — C4 of β-D-glucose	Yes: C1 of the glucose can open

Invert sugar. Sucrose is dextrorotatory. Hydrolysis gives D-(+)-glucose (+52.5°) and D-(−)-fructose (−92.4°). Fructose's laevorotation is larger, so the mixture is laevorotatory. The sign of rotation inverts, so the product is invert sugar.

Example (2019, 2021) · "Sucrose's glycosidic link is C1 of α-glucose to C1 of β-fructose": false, it is C2 of fructose. A non-reducing sugar that hydrolyses to two reducing monosaccharides: sucrose.

Example (2020) · Maltose contains one acetal and one hemiacetal carbon. The hemiacetal makes it reducing.

Example (2020) · Chiral carbons in sucrose: 5 in the glucose ring + 4 in the fructose ring = 9.

5 · Polysaccharides

Many monosaccharide units joined by glycosidic links; mainly food stores or structural materials.

• Starch: the plants' main store and the main dietary carbohydrate (cereals, roots, tubers). A polymer of α-glucose, in two parts:
  • Amylose: water-soluble, about 15–20%. A long unbranched chain of 200–1000 α-D-(+)-glucose units, C1–C4 links.
  • Amylopectin: insoluble in water, about 80–85%. Branched: C1–C4 in the chain, C1–C6 at the branches.
• Cellulose: only in plants; the most abundant organic substance in the plant kingdom; the main part of plant cell walls. A straight chain of β-D-glucose, C1–C4 links.
• Glycogen ("animal starch"): the carbohydrate store in animals, in liver, muscles and brain; also in yeast and fungi. Built like amylopectin but more highly branched. Enzymes break it to glucose when the body needs it.

Example (2022) · A polysaccharide X, boiled with dilute H₂SO₄ at 393 K and 2–3 atm, gives Y; Y + Br₂ water → gluconic acid; X has only β links → cellulose.

Example (2022) · "Amylose is insoluble in water" is false in NCERT: amylose is the soluble part. The reason (a long linear chain of over 200 units) is true.

6 · Amino acids

Proteins are polymers of α-amino acids: –NH₂ and –COOH on the same carbon, RCH(NH₂)COOH. Only α-amino acids come from hydrolysing proteins. Names are trivial: glycine is sweet (glykos), tyrosine was first found in cheese (tyros).

Amino acid	Side chain R	3-letter, 1-letter
Glycine	–H	Gly, G
Alanine	–CH₃	Ala, A
Valine*	(CH₃)₂CH–	Val, V
Leucine*	(CH₃)₂CHCH₂–	Leu, L
Isoleucine*	CH₃CH₂CH(CH₃)–	Ile, I
Arginine*	HN=C(NH₂)–NH–(CH₂)₃–	Arg, R
Lysine*	H₂N–(CH₂)₄–	Lys, K
Glutamic acid	HOOC–CH₂CH₂–	Glu, E
Aspartic acid	HOOC–CH₂–	Asp, D
Glutamine	H₂NCO–CH₂CH₂–	Gln, Q
Asparagine	H₂NCO–CH₂–	Asn, N
Threonine*	CH₃CH(OH)–	Thr, T
Serine	HOCH₂–	Ser, S
Cysteine	HSCH₂–	Cys, C
Methionine*	CH₃SCH₂CH₂–	Met, M
Phenylalanine*	C₆H₅CH₂–	Phe, F
Tyrosine	p-HOC₆H₄CH₂–	Tyr, Y
Tryptophan*	indolyl–CH₂–	Trp, W
Histidine*	imidazolyl–CH₂–	His, H
Proline	ring (whole structure)	Pro, P

* Essential (10): valine, leucine, isoleucine, arginine, lysine, threonine, methionine, phenylalanine, tryptophan, histidine. They can't be made in the body and must come from the diet. The other ten are non-essential. Memory line: the codes that don't match the initial letter are R (Arg), K (Lys), E (Glu), D (Asp), Q (Gln), N (Asn), F (Phe), Y (Tyr), W (Trp).

Classification: equal –NH₂ and –COOH → neutral; more –NH₂ → basic (lysine, arginine); more –COOH → acidic (aspartic, glutamic acid).

Properties.

• Colourless, crystalline, water-soluble, high-melting solids that behave like salts, not like simple amines or acids.
• In water the –COOH loses a proton and the –NH₂ gains one, giving a zwitter ion H₃N⁺–CHR–COO⁻: neutral overall but with both charges. It is amphoteric: it reacts with acids and with bases.
• All except glycine are optically active (the α-carbon is asymmetric). They exist as D and L forms; most natural ones are L (–NH₂ written on the left).

Example (2024, 2025) · Essential among Arg, Phe, Asp, Cys, His, Val, Pro: 4 (Arg, Phe, His, Val). Among Val, Pro, Lys, Thr, Tyr: Val, Lys, Thr.

Example (2023) · Sulphur-containing amino acids: cysteine and methionine. Cysteine pairs up through its –SH (–S–S–, a disulphide bridge; 2025).

Example (2025) · Valine at pH 2 is H₃N⁺–CH(R)–COOH; at pH 10 it is H₂N–CH(R)–COO⁻.

Counting chiral carbons · Beyond NCERT · Threonine and isoleucine each have 2 (the side chain has one too). Ile-Arg-Pro has 2 + 1 + 1 = 4 (2020).

7 · Peptides and proteins

Peptide bond. The –COOH of one amino acid and the –NH₂ of the next lose water and form an amide, –CO–NH–.

• Two amino acids give a dipeptide: glycine's COOH + alanine's NH₂ → glycylalanine (Gly-Ala).
• Three give a tripeptide (two peptide bonds), and so on: tetra-, penta-, hexapeptide.
• More than ten: a polypeptide. More than a hundred residues and M > 10 000 u: a protein. The line is not sharp: insulin, with 51 amino acids, counts as a protein because it has a protein's defined shape.

Example (2023) · A peptide giving 3 Gly, 2 Leu and 2 Val per mole has 7 residues and 6 peptide bonds. In a linear tetrapeptide, residues − peptide bonds = 1 (2022).

Example (2024, 2025, 2023) · Three different amino acids, each once: 3! = 6 tripeptides. Four, each once: 4! = 24 tetrapeptides. Tripeptides from two amino acids with repeats: 2³ = 8.

Example (2022) · A protein has 0.30% glycine (75 g mol⁻¹). At least one glycine per molecule, so M ≥ 75/0.0030 = 25 000 → 25 (× 10³ g mol⁻¹).

Shape.

• Fibrous proteins: chains lie parallel, held by H-bonds and disulphide bonds; fibre-like; generally insoluble in water. Keratin (hair, wool, silk), myosin (muscles).
• Globular proteins: chains coil into a sphere; usually soluble. Insulin, albumins.

Four levels of structure:

Level	What it is	Held by
Primary	the sequence of amino acids in each chain; a change makes a different protein	peptide bonds
Secondary	the backbone's regular folding: α-helix or β-pleated sheet	H-bonds between C=O and –NH– of peptide bonds
Tertiary	overall folding of the chain: fibrous or globular	H-bonds, disulphide links, van der Waals, electrostatic forces
Quaternary	arrangement of two or more chains (sub-units), e.g. haemoglobin	the same forces

• α-Helix: the chain twists into a right-handed screw; the –NH of each residue H-bonds to the C=O of an adjacent turn.
• β-Pleated sheet: chains stretched almost fully, laid side by side, held by intermolecular H-bonds; it looks like pleated drapery.

Denaturation. A native protein has its unique 3-D shape and biological activity. Heat or a pH change disturbs the H-bonds: globules unfold, helices uncoil, and activity is lost. Secondary and tertiary structure are destroyed; primary stays intact. Examples: egg white coagulating on boiling; milk curdling (lactic acid from bacteria).

Tests for proteins and sugars · Beyond NCERT

Test	Positive for
Biuret (alkaline CuSO₄, violet)	two or more peptide bonds: tripeptides and proteins, not glycine or a dipeptide (2022)
Ninhydrin (purple, Ruhemann's purple)	amino acids and proteins (egg albumin)
Xanthoproteic (conc. HNO₃, yellow)	proteins with aromatic rings
Neutral FeCl₃ (violet)	tyrosine (a phenol; 2026)
Molisch's	all carbohydrates
Barfoed's	monosaccharides, not disaccharides (in a short time)
Seliwanoff's (red)	ketoses (fructose) faster than aldoses
Benedict's (red Cu₂O)	reducing sugars; not sucrose or amylose (2023)

8 · Enzymes

• Biocatalysts that run the body's reactions under mild conditions. Almost all are globular proteins.
• Very specific to one reaction and one substrate.
• Named after the substrate (maltase hydrolyses maltose to 2 glucose) or after the reaction (oxidoreductases oxidise one substrate while reducing another). The name ends in -ase.
• Needed only in small amounts; like other catalysts they lower the activation energy. NCERT's figures: acid hydrolysis of sucrose 6.22 kJ mol⁻¹; with the enzyme sucrase, 2.15 kJ mol⁻¹.

Enzymes JEE pairs · Beyond NCERT · Invertase: sucrose → glucose + fructose. Zymase (yeast): glucose → ethanol + CO₂. Diastase (malt): starch → maltose. Maltase: maltose → glucose. Urease: urea → NH₃ + CO₂. Pepsin (stomach): proteins → peptides.

Example (2022) · Glucose → (zymase) ethanol → (NaOI) iodoform + formate. Zymase is the step that makes the alcohol.

9 · Vitamins

Organic compounds needed in small amounts in the diet; their deficiency causes specific diseases. Most can't be made in the body (plants make almost all; gut bacteria make some). Named A, B, C, D …, with sub-groups B₁, B₂, B₆, B₁₂. The name came from "vital amine"; the e was dropped when most proved to have no amino group. An excess is also harmful.

• Fat-soluble: A, D, E, K. Stored in the liver and adipose tissue.
• Water-soluble: B group and C. Excreted in urine, so they must be taken regularly. Exception: B₁₂ is stored.

Vitamin	Sources	Deficiency
A	fish liver oil, carrots, butter, milk	xerophthalmia (hardened cornea), night blindness
B₁ (thiamine)	yeast, milk, green vegetables, cereals	beri beri
B₂ (riboflavin)	milk, egg white, liver, kidney	cheilosis (cracked lips), digestive disorders, burning skin
B₆ (pyridoxine)	yeast, milk, egg yolk, cereals, grams	convulsions
B₁₂	meat, fish, egg, curd	pernicious anaemia
C (ascorbic acid)	citrus fruits, amla, green leafy vegetables	scurvy (bleeding gums)
D	sunlight, fish, egg yolk	rickets (children), osteomalacia (adults)
E	wheat germ oil, sunflower oil	fragile RBCs, muscular weakness
K	green leafy vegetables	longer blood-clotting time

Example (2024) · Vitamins that can be stored, from A, B₁, B₆, B₁₂, C, D, E, K: A, D, E, K and B₁₂ = 5.

10 · Nucleic acids

Chromosomes in the nucleus carry heredity; they are made of proteins and nucleic acids: DNA and RNA, long polymers of nucleotides (polynucleotides).

Composition. Complete hydrolysis gives a pentose sugar, phosphoric acid and nitrogenous bases.

	DNA	RNA
Sugar	β-D-2-deoxyribose	β-D-ribose
Bases	A, G, C, T	A, G, C, U
Structure	double helix	single strand, may fold back

• Nucleoside = base joined at C-1′ of the sugar (sugar carbons are primed to tell them from the base's).
• Nucleotide = nucleoside + phosphate at C-5′.
• Nucleotides join by phosphodiester links between the 5′ and 3′ carbons of neighbouring sugars.

• Primary structure: the sequence of nucleotides.
• Secondary structure (Watson–Crick): two strands wound around each other, held by H-bonds between base pairs. The strands are complementary, not identical: A pairs with T, C with G.
• RNA comes in three kinds: messenger (m-RNA), ribosomal (r-RNA), transfer (t-RNA).

Functions. DNA is the chemical basis of heredity and the store of genetic information; it duplicates itself at cell division, so each daughter cell gets an identical copy. Proteins are made by the RNAs; the message for each protein is in the DNA.

Example (2024) · Strand ATGCTTCA → complementary strand TACGAAGT.

Example (2025) · 5′-GGCAAATCGGCTA-3′: 7 G/C pairs × 3 + 6 A/T pairs × 2 = 21 + 12 = 33 H-bonds.

Example (2023) · Uracil C₄H₄N₂O₂ (112): %N = 28/112 × 100 = 25%. A nucleotide of uracil (with ribose and phosphate) has 9 O atoms (2022).

Base facts · Beyond NCERT · A=T: 2 H-bonds; G≡C: 3. A and G are purines (two rings); C, T, U are pyrimidines (one ring). Their chirality comes from the D-sugar (2026).

11 · Hormones

Intercellular messengers made by endocrine glands and poured straight into the blood.

Chemical type	Examples
Steroids	estrogens, androgens
Polypeptides	insulin, endorphins
Amino-acid derivatives	epinephrine, norepinephrine, thyroxine

• Insulin lowers blood glucose (released when it rises fast); glucagon raises it. Together they keep it in a narrow range.
• Epinephrine and norepinephrine mediate responses to external stimuli.
• Thyroxine (thyroid) is an iodinated derivative of tyrosine. Too little: hypothyroidism (lethargy, obesity); too much: hyperthyroidism. Low iodine in the diet causes hypothyroidism and an enlarged thyroid, prevented by adding sodium iodide to table salt.
• Adrenal cortex: glucocorticoids control carbohydrate metabolism, modulate inflammation and respond to stress; mineralocorticoids control the kidney's excretion of water and salt. Failure causes Addison's disease (hypoglycaemia, weakness, poor stress response).
• Gonads: testosterone (male characteristics), estradiol (female characteristics, menstrual cycle), progesterone (prepares the uterus for the fertilised egg).

Example (2025) · Thyroxine C₁₅H₁₁I₄NO₄ (777): %I = 4 × 127 / 777 × 100 = 508/777 × 100 = 65%.

12 · Traps

Trap	What goes wrong	Do this
Sucrose link	C1–C1 or C1–C4 (2019, 2021)	C1 of α-glucose to C2 of β-fructose
Lactose link	α link, or galactose C4 (2020, 2021)	β, C1 of galactose to C4 of glucose
Maltose	Calling it non-reducing (2026)	Reducing: one glucose's C1 is free
Amylose	"Insoluble, branched" (2021, 2022)	Soluble, unbranched; amylopectin is the branched, insoluble one
Glycogen	"Straight chain like amylose" (2019)	More branched than amylopectin
Invert sugar	Giving fructose's rotation to glucose (2026)	Glucose +52.5°, fructose −92.4°: the mix is laevo
D-(+)-fructose	Assuming D means (+) (2025)	Fructose is D-(−); D/L is configuration only
Glucose and Schiff's	"Glucose gives Schiff's test" (2020)	It doesn't: mostly cyclic
Br₂ water vs HNO₃	Dicarboxylic acid from Br₂ water (2024)	Br₂ water → gluconic (mono); HNO₃ → saccharic (di)
α vs β glucose m.p.	Putting α higher (2026)	α 419 K, β 423 K
Tyrosine	Calling it essential (2020, 2025)	Not essential; phenylalanine is
B₁₂	Excreted like other B vitamins (2022)	Water-soluble but stored
Denaturation	Losing the primary structure (2022, 2024)	Sequence stays; 2° and 3° go
Biuret	Counting glycine or a dipeptide (2022)	Needs two peptide bonds

13 · Numbers worth knowing

• α-glucose m.p. 419 K (from 303 K); β-glucose m.p. 423 K (from 371 K).
• Rotation: glucose +52.5°, fructose −92.4°.
• Starch: amylose 15–20% (200–1000 units), amylopectin 80–85%.
• Protein: > 100 residues and > 10 000 u; insulin 51 amino acids. 20 amino acids, 10 essential.

Compound	g mol⁻¹
glucose, fructose	180
sucrose, maltose, lactose	342
glycine	75
alanine	89
uracil	112
thyroxine	777

14 · For the boards only

JEE Main doesn't ask this, but the CBSE paper can.

• Carbohydrates are most of our food. Honey has long been an instant energy source for Vaids in Ayurveda.
• Cellulose: cell walls of bacteria and plants; wood for furniture; cotton for clothes. Carbohydrates supply textiles, paper, lacquers and breweries.
• D-ribose and 2-deoxy-D-ribose are in nucleic acids; carbohydrates also combine with proteins and lipids.
• Proteins: milk, cheese, pulses, peanuts, fish, meat; needed for growth and maintenance. "Protein" is from Greek proteios, of prime importance.
• DNA fingerprinting: a person's base sequence is unique, the same in every cell, and can't be altered. Used in forensics, paternity tests, identifying bodies, and studying racial groups and evolution.
• Watson, Crick and Wilkins: Nobel Prize 1962 for the double helix. Har Gobind Khorana (born 1922): Nobel Prize 1968, with Nirenberg and Holley, for cracking the genetic code.