JEE Mains Chemistry · Teaching notes
Biomolecules — JEE Mains Chemistry
Biomolecules has 135 past-year questions from 2021 to 2026, and 16 of them ask for a number rather than an option. It is a recall chapter: most questions name a sugar, an amino acid, an enzyme or a vitamin and ask for one exact fact about it, often as a match-the-list where one swapped pair costs the mark. The reasoning that remains is short and repeatable: read D or L from a Fischer projection, decide whether a sugar still has a free anomeric carbon, and count peptide bonds, sequences or hydrogen bonds. Carbohydrates, amino acids and proteins carry nearly four questions in five. NCERT's text is the reference throughout, and where a key rests on a fact outside it, the page says so.
Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.
Subtopic notes
Monosaccharides: Classification and Glucose Reactions
17 PYQsCarbohydrates are sorted by how many units hydrolysis gives and by their carbonyl group and chain length; every monosaccharide is a reducing sugar, and each reaction of glucose (HI, NH₂OH, HCN, bromine water, acetic anhydride, nitric acid) proves one feature of its open-chain structure.
Cyclic Structures, D/L Configuration and Anomers
13 PYQsA sugar is D or L by the OH on its last stereocentre in the Fischer projection, not by its sign of rotation; glucose closes into a six-membered ring and fructose into a five-membered one, and the new stereocentre at the carbonyl carbon gives the α and β anomers.
Disaccharides, Polysaccharides and Reducing Sugars
26 PYQsA glycosidic link joins the anomeric carbon of one sugar to an OH of the next; a sugar still reduces Tollens' or Fehling's reagent only if some anomeric carbon keeps its free OH, which is why maltose and lactose reduce and sucrose does not; starch, glycogen and cellulose differ in the anomer and branching of their glucose chains.
Amino Acids: Essential, Codes and Properties
19 PYQsThe twenty α-amino acids differ only in their side chain, which fixes each one's one-letter code, its class (acidic, basic or neutral) and the test that picks it out; ten are essential, all but glycine are chiral, and as dipolar ions they are high-melting, water-soluble solids.
Peptides and Protein Structure
30 PYQsAmino acids join by peptide (amide) bonds into chains read from the free NH₂ end to the free COOH end; a chain of n residues has n − 1 peptide bonds, and a protein's shape is built in four levels, of which denaturation destroys all but the primary sequence.
Enzymes and Vitamins
18 PYQsEnzymes are globular-protein catalysts, each specific to one reaction and usually named after its substrate; vitamins split into fat-soluble A, D, E and K, which the body stores, and the water-soluble B group and C, which it excretes (except B12), and each vitamin has one deficiency disease to remember.
Nucleic Acids
12 PYQsA nucleotide is a base on C-1′ of a pentose sugar with a phosphate on C-5′, and nucleotides join through phosphodiester links; DNA uses 2-deoxyribose and the bases A, G, C, T, RNA uses ribose and A, G, C, U, and in the DNA double helix A pairs with T by two hydrogen bonds and G with C by three.
Formula & revision sheet
6 formulas · 10 reference tables · 47 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
6 formulas · 10 reference tables · 47 gotchas across all subtopics — the exam-eve cheat-sheet
Reference tables (1)
Classes of carbohydrates and their colour tests8 rows
| Test | Reagent | Positive for | What you see |
|---|---|---|---|
| Fehling's | Copper(II) sulphate with sodium potassium tartrate in NaOH | Every reducing sugar | Red precipitate of Cu₂O |
| Benedict's | Copper(II) sulphate with sodium citrate and sodium carbonate | Every reducing sugar | Orange-red precipitate of Cu₂O |
| Tollens' | in ammonia | Every reducing sugar, fructose included | Silver mirror |
| Barfoed's | Copper(II) acetate in dilute acetic acid | Reducing monosaccharides, within about two minutes | Red Cu₂O; reducing disaccharides react only after long heating |
| Seliwanoff's | Resorcinol in hydrochloric acid | Ketoses quickly; aldoses and pentoses only slowly | Cherry-red colour |
| Iodine | Iodine in potassium iodide solution | Starch | Blue-black colour |
| Biuret | Dilute copper(II) sulphate in NaOH | Two or more peptide bonds: proteins, tripeptides, biuret itself | Violet colour |
| Xanthoproteic | Concentrated nitric acid | Proteins with aromatic side chains | Yellow colour, orange with ammonia |
Watch out for (6)
- Oligosaccharide units need not be identical→ Classes of carbohydrates and their colour tests
- Fructose reduces Tollens' reagent without a CHO group→ Classes of carbohydrates and their colour tests
- Seliwanoff's test uses no copper→ Classes of carbohydrates and their colour tests
- Bromine water stops at one acid group→ Reactions that prove the open-chain structure of glucose
- Starch is hydrolysed by dilute acid→ Reactions that prove the open-chain structure of glucose
- The acetate count is the OH count→ Reactions that prove the open-chain structure of glucose
Formulas (1)
Reference tables (1)
Cyclic structures, anomers and epimers of glucose and fructose6 rows
| Pair | Relationship | Where they differ |
|---|---|---|
| α-D-glucose and β-D-glucose | Anomers | Configuration at C-1 only |
| D-glucose and D-galactose | Epimers | Configuration at C-4 only |
| D-glucose and D-mannose | Epimers | Configuration at C-2 only |
| D-glucose and D-fructose | Functional isomers, both C₆H₁₂O₆ | Aldehyde at C-1 against ketone at C-2 |
| D-glucose and L-glucose | Enantiomers | Every stereocentre inverted |
| Glucose and ribose | Called homologous in some papers | Ribose has one CHOH unit fewer, C₅ against C₆ They differ by CH₂O, not by CH₂, so this is a paper's label, not a true homologous series. |
Watch out for (6)
- D and L are not the sign of rotation→ D/L configuration from the Fischer projection
- The terminal CH₂OH carbon is not a stereocentre→ D/L configuration from the Fischer projection
- L-glucose flips every OH→ D/L configuration from the Fischer projection
- The α anomer has the lower melting point and the higher rotation→ Cyclic structures, anomers and epimers of glucose and fructose
- Fructose forms a five-membered ring→ Cyclic structures, anomers and epimers of glucose and fructose
- Anomers and epimers are different pairs→ Cyclic structures, anomers and epimers of glucose and fructose
Reference tables (2)
Glycosidic linkages in sucrose, maltose and lactose3 rows
| Disaccharide | Units on hydrolysis | Glycosidic link | Reducing? |
|---|---|---|---|
| Sucrose (cane sugar) | α-D-(+)-glucose and β-D-(−)-fructose | C-1 of glucose to C-2 of fructose, α1–β2 | No |
| Maltose (malt sugar) | Two α-D-glucose units | C-1 of one glucose to C-4 of the next, α1–4 | Yes |
| Lactose (milk sugar) | β-D-galactose and β-D-glucose | C-1 of galactose to C-4 of glucose, β1–4 | Yes |
Starch, glycogen and cellulose: linkages and sources4 rows
| Polysaccharide | Unit and linkage | Shape and solubility | Found in |
|---|---|---|---|
| Amylose | α-D-glucose, C-1 to C-4 only | Unbranched chain of 200 to 1000 units; soluble in water | Starch of plants, 15–20% |
| Amylopectin | α-D-glucose, C-1 to C-4 in chains, C-1 to C-6 at branches | Branched; insoluble in water | Starch of plants, 80–85% |
| Glycogen | α-D-glucose, C-1 to C-4 with C-1 to C-6 branches | More highly branched than amylopectin | Liver, muscles and brain of animals; yeast and fungi |
| Cellulose | β-D-glucose, C-1 to C-4 only | Straight chains packed side by side; insoluble in water | Plant cell walls; not digested by humans |
Watch out for (7)
- A C-1 to C-4 link leaves one C-1 free→ Reducing and non-reducing sugars
- Polysaccharides count as non-reducing→ Reducing and non-reducing sugars
- Fructose, not glucose, is laevorotatory→ Glycosidic linkages in sucrose, maltose and lactose
- Sucrose joins α-glucose to β-fructose→ Glycosidic linkages in sucrose, maltose and lactose
- Lactose uses C-1 of galactose→ Glycosidic linkages in sucrose, maltose and lactose
- Amylose is the water-soluble fraction→ Starch, glycogen and cellulose: linkages and sources
- Cellulose and starch differ in the anomer, not the carbons→ Starch, glycogen and cellulose: linkages and sources
Formulas (1)
Reference tables (2)
The twenty amino acids: codes, side chains and the essential ones20 rows
| Amino acid | Code | Side chain R | Class | Essential? |
|---|---|---|---|---|
| Glycine | G | Neutral | No | |
| Alanine | A | Neutral | No | |
| Valine | V | Neutral | Yes | |
| Leucine | L | Neutral | Yes | |
| Isoleucine | I | Neutral | Yes | |
| Arginine | R | Basic | Yes | |
| Lysine | K | Basic | Yes | |
| Glutamic acid | E | Acidic | No | |
| Aspartic acid | D | Acidic | No | |
| Glutamine | Q | Neutral | No | |
| Asparagine | N | Neutral | No Its amide N is not basic, so asparagine has only one basic group, the α-NH₂. | |
| Threonine | T | Neutral | Yes | |
| Serine | S | Neutral | No | |
| Cysteine | C | Neutral | No | |
| Methionine | M | Neutral | Yes | |
| Phenylalanine | F | Neutral | Yes | |
| Tyrosine | Y | (para) | Neutral | No |
| Tryptophan | W | -indolyl, two fused rings, one with N | Neutral | Yes |
| Histidine | H | -imidazolyl, a five-membered ring with two N | Basic | Yes |
| Proline | P | joined back to the α-N, a five-membered ring | Neutral | No |
Tests for amino-acid side chains and the ninhydrin test6 rows
| Amino acid | Side-chain group | Test | Result |
|---|---|---|---|
| Tyrosine | Phenolic OH | Neutral FeCl₃ | Violet colour |
| Serine, threonine | Alcoholic OH | Ceric ammonium nitrate | Red colour |
| Lysine | Primary amine, | Hinsberg's reagent, | Sulphonamide that dissolves in alkali |
| Glutamine, asparagine | Primary amide, | Hoffmann bromamide, with NaOH | Amine with one carbon fewer |
| Tyrosine, tryptophan, phenylalanine | Benzene ring | Xanthoproteic, concentrated | Yellow colour |
| Every α-amino acid and protein | Free α-amino group | Ninhydrin | Purple colour |
Watch out for (8)
- Aspartic acid is D, asparagine is N→ The twenty amino acids: codes, side chains and the essential ones
- Tyrosine is not essential→ The twenty amino acids: codes, side chains and the essential ones
- Proline's ring is five-membered, and histidine has a ring→ The twenty amino acids: codes, side chains and the essential ones
- Not every chiral amino acid has one stereocentre→ Structure, dipolar ion and chirality of α-amino acids
- Amino acids are salts, not organic liquids→ Structure, dipolar ion and chirality of α-amino acids
- Protein hydrolysis gives α-amino acids→ Structure, dipolar ion and chirality of α-amino acids
- Lysine carries an amine, glutamine an amide→ Tests for amino-acid side chains and the ninhydrin test
- Ferric chloride needs a phenol→ Tests for amino-acid side chains and the ninhydrin test
Formulas (2)
Reference tables (1)
Levels of protein structure and denaturation5 rows
| Level | What it describes | Held by | After denaturation |
|---|---|---|---|
| Primary | The sequence of amino acids in each chain | Peptide (covalent amide) bonds | Intact |
| Secondary, α-helix | The chain coiled into a right-handed spiral | Hydrogen bonds between the C=O and N–H of peptide bonds on neighbouring turns | Lost; the helix uncoils |
| Secondary, β-pleated sheet | Chains stretched out and laid side by side | Hydrogen bonds between the C=O and N–H of neighbouring chains | Lost |
| Tertiary | The overall folding of the chain, which gives the fibrous or globular shape | Hydrogen bonds, disulphide links, van der Waals and electrostatic forces | Lost; globules unfold |
| Quaternary | The spatial arrangement of two or more polypeptide subunits | The same weak forces acting between the subunits | Lost |
Watch out for (8)
- A chain has a direction→ Counting peptide bonds and peptide sequences
- n residues make n − 1 bonds→ Counting peptide bonds and peptide sequences
- The biuret test needs two peptide bonds→ Counting peptide bonds and peptide sequences
- Read from the free NH₂ end→ Reading and writing peptide sequences
- The acid chloride belongs to the first residue→ Reading and writing peptide sequences
- Quaternary structure is not the overall fold→ Levels of protein structure and denaturation
- Denaturation keeps the peptide bonds→ Levels of protein structure and denaturation
- Fibrous proteins are the insoluble ones→ Levels of protein structure and denaturation
Reference tables (2)
Enzymes and the reaction each one catalyses7 rows
| Enzyme | Converts | Into | Source or site |
|---|---|---|---|
| Invertase | Sucrose (cane sugar) | Glucose and fructose | Yeast |
| Zymase | Glucose | Ethanol and carbon dioxide | Yeast |
| Diastase | Starch | Maltose | Malt (germinating barley) |
| Maltase | Maltose | Glucose | Yeast |
| Urease | Urea | Ammonia and carbon dioxide | Soya bean |
| Pepsin | Proteins | Peptides | Stomach |
| Trypsin | Peptides and proteins | Amino acids | Pancreas, acting in the intestine |
Vitamins: chemical names, deficiency diseases and sources9 rows
| Vitamin | Chemical name | Deficiency disease | Main sources |
|---|---|---|---|
| A | Retinol | Xerophthalmia (hardening of the cornea) and night blindness | Fish liver oil, carrots, butter, milk |
| B1 | Thiamine | Beri-beri (loss of appetite, retarded growth) | Yeast, milk, green vegetables, cereals |
| B2 | Riboflavin | Cheilosis (fissures at the corners of the mouth and lips), digestive disorders | Milk, egg white, liver, kidney |
| B6 | Pyridoxine | Convulsions | Yeast, milk, egg yolk, cereals, grams |
| B12 | Cyanocobalamin | Pernicious anaemia | Meat, fish, egg, curd |
| C | Ascorbic acid | Scurvy (bleeding gums) | Citrus fruits, amla, green leafy vegetables |
| D | Calciferol | Rickets in children, osteomalacia in adults | Sunlight, fish, egg yolk |
| E | Tocopherol | Increased fragility of red blood cells, muscular weakness | Vegetable oils such as wheat germ and sunflower oil |
| K | Phylloquinone | Increased blood clotting time | Green leafy vegetables |
Watch out for (7)
- Maltase, not oxidase, hydrolyses maltose→ Enzymes and the reaction each one catalyses
- Diastase stops at maltose→ Enzymes and the reaction each one catalyses
- Enzymes are specific→ Enzymes and the reaction each one catalyses
- B12 is water soluble but stored→ Fat-soluble and water-soluble vitamins: which are stored
- Thiamine and ascorbic acid are not stored→ Fat-soluble and water-soluble vitamins: which are stored
- Thiamine, riboflavin, pyridoxine in order→ Vitamins: chemical names, deficiency diseases and sources
- Vitamin K deficiency slows clotting→ Vitamins: chemical names, deficiency diseases and sources
Reference tables (1)
Nucleosides, nucleotides and the bases and sugars of DNA and RNA8 rows
| Component | Kind | In DNA | In RNA |
|---|---|---|---|
| Adenine (A) | Purine base, two rings | Yes | Yes |
| Guanine (G) | Purine base, two rings | Yes | Yes |
| Cytosine (C) | Pyrimidine base, one ring | Yes | Yes |
| Thymine (T) | Pyrimidine base, 5-methyluracil | Yes | No |
| Uracil (U) | Pyrimidine base | No | Yes |
| β-D-2-Deoxyribose | Pentose sugar, furanose ring | Yes | No |
| β-D-Ribose | Pentose sugar, furanose ring | No | Yes |
| Phosphate | Joins C-5′ of one sugar to C-3′ of the next | Yes | Yes |
Watch out for (5)
- The nucleic-acid sugar is a β furanose→ Nucleosides, nucleotides and the bases and sugars of DNA and RNA
- Nucleotides are joined by phosphodiester links→ Nucleosides, nucleotides and the bases and sugars of DNA and RNA
- Chirality comes from the sugar→ Nucleosides, nucleotides and the bases and sugars of DNA and RNA
- Count one strand, not both→ Base pairing and counting hydrogen bonds in DNA
- RNA makes the proteins→ Base pairing and counting hydrogen bonds in DNA
PYQ weightage by concept
18 concepts · 135 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
18 concepts · 135 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| Reactions that prove the open-chain structure of glucose | 9 | 7% |
| Classes of carbohydrates and their colour tests | 8 | 6% |
| Concept | PYQs | Share |
|---|---|---|
| D/L configuration from the Fischer projection | 7 | 5% |
| Cyclic structures, anomers and epimers of glucose and fructose | 6 | 4% |
| Concept | PYQs | Share |
|---|---|---|
| Glycosidic linkages in sucrose, maltose and lactose | 13 | 10% |
| Reducing and non-reducing sugars | 8 | 6% |
| Starch, glycogen and cellulose: linkages and sources | 5 | 4% |
| Concept | PYQs | Share |
|---|---|---|
| The twenty amino acids: codes, side chains and the essential ones | 8 | 6% |
| Structure, dipolar ion and chirality of α-amino acids | 7 | 5% |
| Tests for amino-acid side chains and the ninhydrin test | 4 | 3% |
| Concept | PYQs | Share |
|---|---|---|
| Levels of protein structure and denaturation | 14 | 10% |
| Counting peptide bonds and peptide sequences | 12 | 9% |
| Reading and writing peptide sequences | 4 | 3% |
| Concept | PYQs | Share |
|---|---|---|
| Vitamins: chemical names, deficiency diseases and sources | 8 | 6% |
| Enzymes and the reaction each one catalyses | 6 | 4% |
| Fat-soluble and water-soluble vitamins: which are stored | 4 | 3% |
| Concept | PYQs | Share |
|---|---|---|
| Nucleosides, nucleotides and the bases and sugars of DNA and RNA | 8 | 6% |
| Base pairing and counting hydrogen bonds in DNA | 4 | 3% |
Test yourself on Biomolecules
20 past JEE Mains questions from this chapter, timed at 48 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.