CBSE Class 12 Chemistry: Biomolecules — Notes 2026

Carbohydrates

• Polyhydroxy aldehydes/ketones or substances that hydrolyse to give them; formula Cₙ(H₂O)ₘ
• Monosaccharides: glucose, fructose (C₆H₁₂O₆); disaccharides: sucrose, lactose, maltose; polysaccharides: starch, cellulose, glycogen
• Reducing sugars (Fehling's/Tollens test): all monosaccharides; maltose, lactose; sucrose is non-reducing

Glucose — Structure and Properties

• Open chain: CHO − (CHOH)₄ − CH₂OH; D-glucose (−OH on right at C5 in Fischer projection)
• Cyclic form: glucopyranose (6-membered ring); α and β anomers differ at C1
• Reactions: oxidation (gluconic acid), reduction (sorbitol), fermentation (ethanol)

Proteins — Amino Acids

• α-amino acids: −NH₂ and −COOH groups on same carbon; 20 natural amino acids
• Essential amino acids (cannot be synthesised by body; must come from diet): Val, Leu, Ile, Phe, Trp, Met, Thr, Lys
• Peptide bond: −CO−NH− formed by condensation between −COOH and −NH₂

Protein Structure

• Primary: sequence of amino acids in polypeptide chain
• Secondary: α-helix (H-bonds within chain) or β-pleated sheet (H-bonds between chains)
• Tertiary: 3D folding; disulfide bonds, hydrophobic interactions; quaternary: multiple subunits

Denaturation and Enzymes

• Denaturation: disruption of secondary/tertiary structure by heat, acids, bases; protein loses biological activity
• Enzymes: biological catalysts; protein in nature; highly specific (lock-and-key)
• Factors affecting enzyme activity: pH (optimum ~7), temperature (optimum ~37°C), substrate concentration

Nucleic Acids

• DNA: deoxyribose sugar, phosphate, four bases (A, T, G, C); double helix (Watson-Crick)
• RNA: ribose sugar, uracil instead of thymine; single stranded; mRNA, tRNA, rRNA
• Base pairing: A-T (2 H-bonds), G-C (3 H-bonds); antiparallel strands

CBSE Exam — Biomolecules

• What is a reducing sugar? Give two examples and state the test used
• Explain primary and secondary structure of proteins with diagrams
• How is DNA different from RNA? (tabular comparison)