Chapter 14 — Biomolecules — is where chemistry meets biology! This chapter covers the four major classes of biomolecules: carbohydrates, proteins, nucleic acids, and vitamins. It carries 4-6 marks in Board exams and is mostly factual. Focus on the structures of glucose, classification of carbohydrates, and amino acid structures.
Key Concepts
Carbohydrates
General formula: Cₙ(H₂O)ₙ — “hydrates of carbon”
Classification
| Type | Hydrolysis | Examples |
|---|---|---|
| Monosaccharides | Cannot be hydrolysed further | Glucose (C₆H₁₂O₆), Fructose, Ribose, Galactose |
| Disaccharides | Give 2 monosaccharides | Sucrose (Glu + Fru), Maltose (Glu + Glu), Lactose (Glu + Gal) |
| Polysaccharides | Give many monosaccharides | Starch, Cellulose, Glycogen |
Glucose — Structure Determination
Molecular formula: C₆H₁₂O₆
1. Reacts with HI → n-hexane (6 C chain)
2. Reacts with NH₂OH → oxime (has C=O)
3. Reacts with HCN → cyanohydrin (confirms aldehyde)
4. Acetylation with acetic anhydride → pentaacetate (5 −OH groups)
5. Mild oxidation with Br₂/H₂O → gluconic acid (confirms −CHO)
Cyclic structure (Haworth):
Glucose exists as cyclic hemiacetal (C1 aldehyde + C5 hydroxyl form a ring)
Two anomers: α-D-glucose (−OH at C1 below ring) and β-D-glucose (−OH at C1 above ring)
Mutarotation: Change in optical rotation when α or β form is dissolved in water (equilibrium mixture forms)
Reducing vs Non-reducing Sugars
| Reducing Sugars | Non-reducing Sugars |
|---|---|
| Have free aldehyde/ketone group (or hemiacetal) | No free anomeric carbon — both involved in glycosidic bond |
| Give positive Tollen’s/Fehling’s test | No reaction with Tollen’s/Fehling’s |
| Glucose, Fructose, Maltose, Lactose | Sucrose (the only common non-reducing disaccharide) |
Polysaccharides
| Polysaccharide | Monomer | Linkage | Function |
|---|---|---|---|
| Starch (Amylose) | α-D-glucose | α-1,4 glycosidic | Energy storage (plants) |
| Starch (Amylopectin) | α-D-glucose | α-1,4 + α-1,6 (branching) | Energy storage (plants) |
| Cellulose | β-D-glucose | β-1,4 glycosidic | Structural (plant cell walls) |
| Glycogen | α-D-glucose | α-1,4 + α-1,6 (more branching) | Energy storage (animals) |
Amino Acids and Proteins
Amino Acids
General structure: H₂N−CHR−COOH (α-amino acids). All except glycine are optically active.
In solution, they exist as zwitterions: ⁺H₃N−CHR−COO⁻ (internal salt)
Isoelectric point (pI): pH at which amino acid exists as zwitterion with no net charge.
Essential Amino Acids
10 amino acids that the body cannot synthesise — must be obtained from diet:
Valine, Leucine, Isoleucine, Phenylalanine, Tryptophan, Threonine, Methionine, Lysine, Arginine, Histidine
Structure of Proteins
| Level | Description | Forces |
|---|---|---|
| Primary | Linear sequence of amino acids | Peptide bonds (−CO−NH−) |
| Secondary | α-helix or β-pleated sheet | Hydrogen bonds between C=O and N−H |
| Tertiary | 3D folding of polypeptide | H-bonds, disulphide bridges, ionic, hydrophobic |
| Quaternary | Multiple polypeptide chains together | Same as tertiary |
Denaturation
Loss of 2°, 3°, 4° structure (primary remains intact). Caused by heat, pH change, organic solvents. Example: boiling an egg — albumin denatures.
Enzymes
Biological catalysts (proteins). Highly specific — lock and key model.
- Optimum temperature: 25-37°C (human enzymes)
- Optimum pH: varies (pepsin = 2, trypsin = 8)
- Naming: usually end in “-ase” (lipase, urease, maltase)
Vitamins
| Vitamin | Chemical Name | Deficiency Disease | Source |
|---|---|---|---|
| A | Retinol | Night blindness, Xerophthalmia | Carrots, liver, fish oil |
| B₁ | Thiamine | Beri-beri | Cereals, pulses, yeast |
| B₂ | Riboflavin | Cheilosis (cracked lips) | Milk, eggs, liver |
| B₆ | Pyridoxine | Convulsions | Cereals, fish, meat |
| B₁₂ | Cyanocobalamin | Pernicious anaemia | Meat, fish, eggs |
| C | Ascorbic acid | Scurvy | Citrus fruits, amla |
| D | Calciferol | Rickets (children), Osteomalacia (adults) | Sunlight, fish oil, eggs |
| E | Tocopherol | Infertility | Vegetable oils, nuts |
| K | Phylloquinone | Slow blood clotting | Green vegetables, liver |
Fat-soluble vitamins: A, D, E, K (stored in liver and fatty tissues — can accumulate)
Nucleic Acids
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, G, C, T (Thymine) | A, G, C, U (Uracil) |
| Structure | Double helix | Single strand |
| Function | Stores genetic info | Protein synthesis |
| Location | Nucleus | Nucleus + cytoplasm |
A = T (2 hydrogen bonds) in DNA
A = U (2 hydrogen bonds) in RNA
G ≡ C (3 hydrogen bonds) in both
Chargaff’s Rule: In DNA, [A] = [T] and [G] = [C]
Important Definitions
| Term | Definition |
|---|---|
| Carbohydrate | Polyhydroxy aldehyde or ketone, or compounds that give these on hydrolysis |
| Glycosidic Linkage | Bond between anomeric carbon of one sugar and −OH of another |
| Peptide Bond | −CO−NH− bond linking two amino acids |
| Denaturation | Loss of biological activity of protein due to disruption of higher-order structure |
| Nucleotide | Sugar + nitrogenous base + phosphate group |
| Nucleoside | Sugar + nitrogenous base (no phosphate) |
Solved Examples — NCERT Based
Example 1: Reducing vs Non-reducing Sugar
Q: Why is maltose a reducing sugar but sucrose is not?
Solution: In maltose, the glycosidic bond is between C1 of one glucose and C4 of the other. The C1 of the second glucose is free → can open to form free aldehyde → gives Tollen’s/Fehling’s test → reducing sugar.
In sucrose, the bond is between C1 of glucose and C2 of fructose — both anomeric carbons are locked → no free aldehyde/ketone → non-reducing.
Example 2: Protein Structure
Q: What happens when a protein is denatured? Is it reversible?
Solution: Denaturation disrupts the secondary, tertiary, and quaternary structures (H-bonds, disulphide bridges, hydrophobic interactions break), but the primary structure (peptide bonds) remains intact. The protein loses its biological activity. It is sometimes reversible (if conditions are restored) but often irreversible (like cooking an egg).
Example 3: Vitamin Classification
Q: A person has night blindness and slow blood clotting. Which vitamins are deficient?
Solution: Night blindness → Vitamin A (Retinol) deficiency. Slow blood clotting → Vitamin K (Phylloquinone) deficiency. Both are fat-soluble vitamins.
Example 4: DNA Base Pairing
Q: A DNA strand has the sequence: ATGCCTA. Write the complementary strand.
Solution: Using base pairing rules (A↔T, G↔C):
Given: A T G C C T A
Complement: T A C G G A T
Important Questions for Board Exams
1 Mark Questions
- What is the difference between a nucleotide and a nucleoside?
- Name the deficiency disease caused by Vitamin C.
- What is a glycosidic linkage?
- Which polysaccharide is stored in the human liver?
2 Mark Questions
- What is mutarotation? Which sugar shows it?
- Distinguish between reducing and non-reducing sugars with examples.
- What are essential amino acids? Name any four.
- What is the difference between DNA and RNA (any 4 points)?
3 Mark Questions
- Describe the four levels of protein structure.
- Write the Haworth structure of α-D-glucose and β-D-glucose. What is the difference?
- Classify vitamins as water-soluble and fat-soluble. Give one example and deficiency disease for each.
5 Mark Questions
- What are carbohydrates? How are they classified? Discuss the structure of glucose with evidence.
- What are proteins? Discuss their primary, secondary, tertiary and quaternary structures. What is denaturation?
Quick Revision Points
- Carbohydrates: mono (glucose, fructose), di (sucrose, maltose, lactose), poly (starch, cellulose)
- Glucose: aldohexose, exists as cyclic hemiacetal (α and β anomers)
- Sucrose = only common non-reducing disaccharide (both anomeric C involved)
- Starch: amylose (linear α-1,4) + amylopectin (branched α-1,4 + α-1,6)
- Cellulose: β-1,4 linked (humans can’t digest — no enzyme for β-linkage)
- Amino acids: zwitterionic, all optically active except glycine
- Protein structure: 1° (sequence) → 2° (helix/sheet) → 3° (3D fold) → 4° (multi-chain)
- Fat-soluble vitamins: A, D, E, K; Water-soluble: B, C
- DNA: double helix, A=T (2 H-bonds), G≡C (3 H-bonds)
- Chargaff’s rule: [A]=[T], [G]=[C]
Chapter Navigation
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Next: Polymers Class 12 Notes
Related Chapters in Class 12 Chemistry
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