Biomolecules - Class 12 Chemistry (CBSE)
1. Carbohydrates
• Definition: Organic compounds made up of carbon, hydrogen, and oxygen, generally in the ratio
Cn(H2O)m.
• Classification:
• - Monosaccharides: Simple sugars like glucose, fructose, galactose.
• - Disaccharides: Formed by two monosaccharides, e.g., sucrose, lactose, maltose.
• - Polysaccharides: Long chains of monosaccharides, e.g., starch, glycogen, cellulose.
• Reducing sugars: Carbohydrates that can reduce Fehling’s or Tollen’s reagent due to presence
of free aldehyde/ketone group.
• Non-reducing sugars: Carbohydrates without free aldehyde/ketone group (e.g., sucrose).
• Glucose structure: Exists in open-chain and cyclic (α, β forms) structures. Shows mutarotation.
• Fructose structure: A ketohexose, also shows open-chain and cyclic forms.
• Mutarotation: Change in optical rotation due to interconversion between α and β anomers in
aqueous solution.
• Glycosidic linkage: C–O–C bond between two monosaccharide units.
• Important disaccharides:
• - Maltose: glucose + glucose (α-1,4 linkage).
• - Lactose: glucose + galactose (β-1,4 linkage).
• - Sucrose: glucose + fructose (α-1,2 linkage, non-reducing).
• Polysaccharides:
• - Starch: Storage form of glucose in plants (amylose + amylopectin).
• - Glycogen: Storage form in animals, highly branched.
• - Cellulose: Structural polysaccharide in plants, β-1,4 linkages.
2. Proteins
• Definition: Natural polymers made up of amino acids linked by peptide bonds.
• Amino acids: Contain amino group (–NH2) and carboxyl group (–COOH) attached to same
carbon (α-carbon).
• Zwitter ion: Amino acids exist as dipolar ions with both +ve (–NH3+) and –ve (–COO–) charges.
• Amphoteric nature: Can act as both acid and base.
• Isoelectric point: pH at which amino acid has no net charge.
• Peptide bond: –CO–NH– linkage formed between –COOH of one amino acid and –NH2 of
another.
• Levels of protein structure:
• - Primary: Linear sequence of amino acids.
• - Secondary: Coiling/folding (α-helix, β-sheet) due to hydrogen bonding.
• - Tertiary: 3D folding stabilized by hydrogen, disulfide, ionic, and hydrophobic interactions.
• - Quaternary: Arrangement of more than one polypeptide chain.
• Denaturation: Loss of native structure of protein due to heat, pH, or chemicals (e.g., boiling egg
white).
3. Vitamins
• Definition: Organic compounds required in small amounts for normal metabolism.
• Classification:
• - Water-soluble: Vitamin B-complex and C.
• - Fat-soluble: Vitamins A, D, E, K.
• Deficiency diseases:
• - Vitamin A: Night blindness.
• - Vitamin B1: Beriberi.
• - Vitamin B12: Pernicious anemia.
• - Vitamin C: Scurvy.
• - Vitamin D: Rickets.
• - Vitamin K: Excessive bleeding due to delayed clotting.
• - Vitamin E: Fertility disorders.
4. Nucleic Acids
• Definition: Biomolecules that store and transmit genetic information.
• Components:
• - Nucleoside = Sugar + Nitrogenous base.
• - Nucleotide = Nucleoside + Phosphate group.
• Types:
• - DNA (Deoxyribonucleic acid): Double-stranded helix, sugar is deoxyribose, bases are A, T, G,
C.
• - RNA (Ribonucleic acid): Single-stranded, sugar is ribose, bases are A, U, G, C.
• Base pairing: A–T (two H-bonds), G–C (three H-bonds).
• Chargaff’s rule: [A] = [T], [G] = [C] in DNA.
• Functions:
• - DNA: Genetic material, replication, protein synthesis.
• - RNA: mRNA, tRNA, rRNA involved in protein synthesis.
5. Enzymes
• Definition: Biological catalysts made of proteins that speed up biochemical reactions.
• Characteristics: Highly specific, efficient, work under mild conditions.
• Mechanism of action:
• - Lock and Key Model: Enzyme active site is rigid and complementary to substrate.
• - Induced Fit Model: Active site undergoes conformational change upon substrate binding.
• Factors affecting enzyme activity:
• - Temperature: Optimum temperature required.
• - pH: Optimum pH required.
• - Inhibitors: Substances that decrease enzyme activity (competitive/non-competitive).
