ChemistryModerate Weightage ★★★Class 12
Biomolecules & Polymers
Carbohydrates (classification, reducing sugars), proteins (amino acids, peptide bond, denaturation), nucleic acids (DNA/RNA), enzymes, and polymers — expect 2–3 EAPCET questions.
2–3Questions in EAPCET
~2%Paper Weightage
5Key Concepts
3Mistake Traps
Concept Core
Carbohydrates, proteins, nucleic acids, enzymes, and polymers — the essential biomolecule framework.
Carbohydrates — Classification and Properties
| Class | Examples | Hydrolysis |
|---|
| Monosaccharides | Glucose (C₆H₁₂O₆), Fructose, Ribose | Cannot be hydrolysed further |
| Disaccharides | Sucrose, Maltose, Lactose | Gives 2 monosaccharides |
| Polysaccharides | Starch, Cellulose, Glycogen | Gives many monosaccharides |
Reducing sugars: Glucose, fructose, maltose, lactose (give +ve Fehling/Tollens test). Non-reducing: Sucrose (no free anomeric OH).
Glucose is an aldohexose (6C, CHO group). Fructose is a ketohexose (6C, C=O at C-2). Both are isomers.
Proteins — Structure and Classification
Proteins = polymers of α-amino acids linked by peptide bonds (−CO−NH−).
Structure levels:
1° = amino acid sequence (peptide bonds)
2° = α-helix or β-pleated sheet (H-bonds)
3° = 3D folding (disulfide, H-bonds, hydrophobic)
4° = multiple polypeptide chains (haemoglobin)
Denaturation: Disruption of 3D structure by heat, pH, chemicals. Primary structure intact; biological activity lost. Cooking an egg white = denaturation.
Amino Acids
General structure: H₂N−CHR−COOH (R = side chain). At physiological pH, exist as zwitterions (NH₃⁺−CHR−COO⁻).
Essential amino acids: Cannot be synthesised by the body (must be obtained from diet). Examples: valine, leucine, lysine, tryptophan, methionine, phenylalanine, threonine, isoleucine.
Isoelectric point (pI): pH at which amino acid carries no net charge. At pI, amino acid does not migrate in electric field.
Nucleic Acids — DNA & RNA
| Feature | DNA | RNA |
|---|
| Sugar | Deoxyribose (2'-deoxy) | Ribose |
| Bases | A, G, C, T (thymine) | A, G, C, U (uracil) |
| Structure | Double helix (Watson-Crick) | Single stranded (mostly) |
| Function | Genetic information storage | Protein synthesis |
Base pairing: A=T (2 H-bonds), G≡C (3 H-bonds). C-G pairs are stronger.
Vitamins and Hormones
Fat-soluble vitamins: A, D, E, K (stored in body; excess causes toxicity)
Water-soluble vitamins: B-complex, C (excreted when in excess; need regular intake)
Deficiency diseases: Vitamin A → night blindness; B₁ → beriberi; B₁₂ → pernicious anaemia; C → scurvy; D → rickets; K → poor blood clotting.
Polymers — Addition and Condensation
| Polymer | Type | Monomer(s) |
|---|
| Polyethylene | Addition | Ethene (CH₂=CH₂) |
| PVC | Addition | Vinyl chloride (CH₂=CHCl) |
| Teflon | Addition | Tetrafluoroethylene (CF₂=CF₂) |
| Nylon-6,6 | Condensation | Hexamethylene diamine + adipic acid |
| Nylon-6 | Condensation | Caprolactam (ring opening) |
| Dacron (Terylene) | Condensation | Ethylene glycol + terephthalic acid |
| Bakelite | Condensation | Phenol + formaldehyde |
Formula Vault
Biomolecule key facts for EAPCET.
Glucose Formula
C₆H₁₂O₆ (aldohexose)
Open chain: CHO−(CHOH)₄−CH₂OH
Reducing Sugars
Glucose, fructose, maltose, lactose
Give +ve Fehling's and Tollens' test
Non-reducing Sugar
Sucrose (no free anomeric OH)
Negative Fehling's/Tollens' test
DNA Base Pairing
A=T (2 H-bonds); G≡C (3 H-bonds)
More G-C pairs → higher melting point
DNA vs RNA Sugar
DNA: 2'-deoxyribose; RNA: ribose
Key structural difference
Protein Levels
1°=sequence; 2°=helix/sheet; 3°=3D; 4°=quaternary
H-bonds maintain 2° structure
Nylon-6,6 Monomers
Hexamethylene diamine + adipic acid
Both have 6 carbons → Nylon-6,6
Vitamins: Fat vs Water soluble
Fat: A, D, E, K | Water: B, C
Fat-soluble: stored (toxic in excess)
Worked Examples
5 problems — reducing sugars, base pairing, protein structure, polymer classification, and a vitamins trap.
EasyWhich of these is a reducing sugar: sucrose, glucose, or fructose?▾
Identify the reducing sugars among sucrose, glucose, and fructose.
1
Reducing sugars have a free aldehyde (−CHO) or ketone group that can reduce Fehling's solution or Tollens' reagent.
2
Glucose: has free −CHO → reducing sugar ✓
3
Fructose: has free ketone group → reducing sugar ✓ (despite being a ketone; undergoes tautomerism)
4
Sucrose: no free anomeric OH (C1 of glucose + C2 of fructose are bonded together) → non-reducing ✗
✓ Glucose and fructose are reducing sugars; sucrose is non-reducing
EasyHow many hydrogen bonds form between A-T and G-C base pairs?▾
State the number of hydrogen bonds in A-T and G-C base pairs in DNA.
1
A-T (adenine-thymine): 2 hydrogen bonds
2
G-C (guanine-cytosine): 3 hydrogen bonds
3
Consequence: DNA strands with more G-C pairs require more energy to separate (higher melting temperature).
✓ A-T: 2 H-bonds; G-C: 3 H-bonds
MediumClassify Nylon-6,6, Teflon, and Bakelite as addition or condensation polymers.▾
Identify each as addition or condensation polymer: Nylon-6,6, Teflon, Bakelite.
1
Teflon (polytetrafluoroethylene): from CF₂=CF₂ monomer — no small molecule eliminated → addition polymer.
2
Nylon-6,6: from hexamethylene diamine + adipic acid — water eliminated at each amide bond → condensation polymer.
3
Bakelite: from phenol + formaldehyde — water eliminated → condensation polymer.
✓ Addition: Teflon. Condensation: Nylon-6,6 and Bakelite
EAPCET LevelExplain why denaturation destroys protein function but hydrolysis does not necessarily.▾
Compare denaturation and hydrolysis of proteins in terms of their effect on structure and function.
1
Denaturation: 3D folding is disrupted (H-bonds, disulfide bonds broken) by heat/pH/chemicals. Primary structure (peptide bond sequence) is INTACT. The protein is unfolded but amino acids are still connected. Biological activity is lost (active site shape is destroyed).
2
Hydrolysis: Peptide bonds are cleaved (by acid, base, or protease enzymes). Individual amino acids are released. This is complete breakdown of the polypeptide chain.
3
Key distinction: Denaturation preserves the primary structure; hydrolysis destroys it.
✓ Denaturation: 3D structure lost, primary structure intact; Hydrolysis: peptide bonds cleaved, amino acids released
Trap QuestionVitamin C is fat-soluble and stored in the body — True or False?▾
A student claims vitamin C is fat-soluble and can be stored in the body for months. Evaluate.
1
The trap: Vitamin C (ascorbic acid) is a WATER-SOLUBLE vitamin.
2
Water-soluble vitamins (B-complex, C): excreted in urine when in excess. Must be consumed regularly. NOT stored for long periods.
3
Fat-soluble vitamins (A, D, E, K): dissolved in and stored in fat tissues. Can accumulate — excess causes toxicity (hypervitaminosis).
4
Vitamin C deficiency → scurvy (bleeding gums, poor wound healing). Regular daily intake needed (not stored like fat-soluble vitamins).
✓ False — Vitamin C is water-soluble; it is excreted in urine and cannot be stored in body
Mistake DNA
3 biomolecule errors from EAPCET distractor analysis.
🍬
Sucrose is a Reducing Sugar (It's Sweet and Must Reduce!)
Sweetness has nothing to do with reducing ability. Sucrose is non-reducing because no free anomeric −OH is available.
❌ Wrong
Sucrose: reducing sugar ✗
(sweet and very common
— must be reducing!)
✓ Correct
Sucrose: NON-reducing ✓
No free anomeric OH ✓
Both anomeric C are linked ✓
(C1-glucose + C2-fructose bond)
Reducing sugars: glucose, fructose, maltose, lactose (free aldehyde or ketone form available). Non-reducing: sucrose (anomeric carbons of both monomers are tied up in the glycosidic bond).
🧬
RNA Uses Thymine; DNA Uses Uracil
DNA has thymine (T); RNA has uracil (U). Students frequently reverse this.
❌ Wrong
RNA contains thymine (T) ✗
DNA contains uracil (U) ✗
(completely reversed!)
✓ Correct
DNA: A, T, G, C ✓
(thymine has methyl group)
RNA: A, U, G, C ✓
(uracil replaces thymine)
Memory: DNA has Thymine (T for Two-stranded helix). RNA has Uracil (U for Universal messenger). Uracil = thymine without the CH₃ group at position 5.
🔥
Denaturation Breaks Peptide Bonds
Denaturation disrupts only the SECONDARY, TERTIARY, and QUATERNARY structures (H-bonds, disulfide bonds, etc.). Primary structure (peptide bonds) remains intact.
❌ Wrong
Heating protein:
peptide bonds broken
→ amino acids released ✗
(that's hydrolysis!)
✓ Correct
Denaturation: 3D unfolding ✓
Primary structure intact ✓
Peptide bonds NOT broken ✓
Hydrolysis breaks peptide bonds
Denaturation = conformational change only. The polypeptide backbone (primary structure) is unchanged. Only the 3D arrangement is disrupted. Boiled egg white is denatured — not hydrolysed.
Chapter Intelligence
Biomolecules is a memorisation chapter — systematic learning of classifications and key facts pays off.
EAPCET Weightage (2019–2024)
Reducing vs non-reducing sugars~8 Polymer classification (addition/condensation)~6 Protein structure levels~5 Vitamins: fat vs water soluble~4 High-Yield PYQ Patterns
Sucrose is non-reducing; glucose/fructose areDNA: A-T (2H), G-C (3H) base pairsNylon-6,6 vs Nylon-6 monomersAddition vs condensation polymer IDDenaturation: 3D lost, 1° intactFat-soluble: A, D, E, KDeficiency: vitamin A → night blindness
Exam Strategy
- Reducing sugars: glucose, fructose, maltose, lactose → all give Fehling's/Tollens' positive. Sucrose → negative. This distinction is tested directly almost every year.
- DNA base pairing: A=T (2 H-bonds), G≡C (3 H-bonds). DNA has thymine; RNA has uracil. These are direct fact questions.
- Polymers: addition = no small molecule byproduct (Teflon, polyethylene, PVC). Condensation = water or HCl eliminated (Nylon, Dacron, Bakelite).
- Vitamins: Fat-soluble = A, D, E, K (stored, toxic in excess). Water-soluble = B and C (excreted, need regular intake). Link deficiency diseases to specific vitamins.