Biomolecules
'The chemistry of life is ORCHESTRATED by a handful of molecular types — carbohydrates for energy, proteins for function, nucleic acids for information, and enzymes for catalysis.'
1. Chapter Overview
Biomolecules are ORGANIC compounds that constitute the LIVING ORGANISM. This chapter covers: CARBOHYDRATES (monosaccharides, disaccharides, polysaccharides — their structure and properties), PROTEINS (amino acids, peptide bond, primary to quaternary structure), VITAMINS (classification into fat-soluble and water-soluble), NUCLEIC ACIDS (DNA and RNA — their components, structure, and differences), and ENZYMES (mechanism of action, factors affecting enzyme activity).
2. Carbohydrates
Classification
- Monosaccharides: Simplest sugars. Cannot be hydrolysed further. Empirical formula (CH₂O)ₙ.
- Glucose (C₆H₁₂O₆), Fructose (C₆H₁₂O₆), Ribose (C₅H₁₀O₅).
- Disaccharides: Two monosaccharides joined by GLYCOSIDIC LINKAGE.
- Sucrose (glucose + fructose), Maltose (glucose + glucose), Lactose (glucose + galactose).
- Polysaccharides: Many monosaccharides. Cellulose, Starch (amylose + amylopectin), Glycogen.
Glucose
- Open chain (Fischer) : Aldohexose — CHO at C1, −OH groups at C2-C5, CH₂OH at C6.
- Cyclic (Haworth) : Glucose forms a PYRANOSE ring (six-membered, including O). α and β anomers differ at C1.
- Reducing sugar: Glucose has a FREE ALDEHYDE group (or hemiacetal form) that can REDUCE Tollen's reagent, Fehling's solution.
Important Reactions of Glucose
- Oxidation: Glucose + Br₂ → Gluconic acid (CHO → COOH). Glucose + HNO₃ → Saccharic acid (both ends oxidised).
- Reduction: Glucose + NaBH₄/H₂ → Sorbitol (CHO → CH₂OH).
- Osazone formation: Glucose + phenylhydrazine → GLUCOSAZONE (yellow crystals — characteristic of glucose).
Sucrose
- 'Sucrose is a NON-REDUCING sugar — the glycosidic linkage is between C1 of glucose and C2 of fructose, involving BOTH anomeric carbons. No free aldehyde or ketone group remains.'
- Inversion of sucrose: Sucrose + H₂O (H⁺) → Glucose + Fructose. The mixture is called INVERT SUGAR.
Polysaccharides
| Polysaccharide | Monomer | Linkage | Function |
|---|---|---|---|
| Starch | α-D-Glucose | α-1,4 and α-1,6 | ENERGY STORAGE in plants |
| Cellulose | β-D-Glucose | β-1,4 | STRUCTURAL — plant cell walls |
| Glycogen | α-D-Glucose | α-1,4 and α-1,6 | ENERGY STORAGE in animals |
3. Proteins
Amino Acids
- BUILDING BLOCKS of proteins. General formula: H₂N−CH(R)−COOH.
- Zwitterion: In aqueous solution, the −NH₂ gets protonated (−NH₃⁺) and −COOH gets deprotonated (−COO⁻). 'Amino acids are DIPOLAR IONS.'
- Classification: Essential (cannot be synthesised by the body — must be obtained from diet) vs Non-essential.
- Isoelectric point: pH at which the amino acid has NO net charge.
Peptide Bond
- −COOH of one amino acid + −NH₂ of another → −CO−NH− (peptide bond) + H₂O.
- 'A peptide bond is an AMIDE bond — it has PARTIAL DOUBLE BOND character, making the peptide backbone RIGID.'
Structure of Proteins
| Level | Description | Bonds Involved |
|---|---|---|
| Primary | LINEAR SEQUENCE of amino acids | Peptide bonds |
| Secondary | Local folding: α-HELIX or β-SHEET | Hydrogen bonds (N−H···O=C) |
| Tertiary | 3D folding of a SINGLE polypeptide | H-bonds, disulphide bridges, ionic bonds, hydrophobic interactions |
| Quaternary | Assembly of MULTIPLE polypeptide chains | Same as tertiary + subunit interactions |
| Denaturation | LOSS of native 3D structure (due to heat, pH change, etc.) | Disruption of ALL bonds except peptide bonds |
4. Vitamins
| Vitamin | Chemical Name | Classification | Deficiency Disease | Source |
|---|---|---|---|---|
| A | Retinol | FAT-SOLUBLE | Night blindness, Xerophthalmia | Carrots, milk, liver |
| B₁ | Thiamine | WATER-SOLUBLE | Beri-Beri | Rice bran, yeast |
| B₂ | Riboflavin | WATER-SOLUBLE | Glossitis, skin lesions | Milk, eggs |
| B₆ | Pyridoxine | WATER-SOLUBLE | Anaemia, skin disorders | Cereals, meat |
| B₁₂ | Cyanocobalamin | WATER-SOLUBLE | Pernicious anaemia | Meat, liver |
| C | Ascorbic acid | WATER-SOLUBLE | Scurvy (bleeding gums) | Citrus fruits |
| D | Calciferol | FAT-SOLUBLE | Rickets (children), Osteomalacia | Sunlight, fish oil |
| E | Tocopherol | FAT-SOLUBLE | Fertility issues | Vegetable oils |
| K | Phylloquinone | FAT-SOLUBLE | Bleeding (blood clotting affected) | Green leafy vegetables |
5. Nucleic Acids
DNA (Deoxyribonucleic Acid)
- Nucleotides: Deoxyribose sugar + Phosphate group + Nitrogenous base (A, G, C, T).
- Double helix (Watson-Crick model): Two antiparallel strands held together by H-bonds between COMPLEMENTARY base pairs: A=T (2 H-bonds), G≡C (3 H-bonds).
- 'DNA is the MOLECULE OF HEREDITY — it stores GENETIC information.'
- Chargaff's rule: [A] = [T] and [G] = [C] in double-stranded DNA.
RNA (Ribonucleic Acid)
- Nucleotides: Ribose sugar + Phosphate + Base (A, G, C, U — URACIL replaces thymine).
- Types:
- mRNA (messenger): Carries genetic code from DNA to ribosomes.
- tRNA (transfer): Brings amino acids to ribosome during protein synthesis.
- rRNA (ribosomal): Structural component of ribosomes.
DNA vs RNA
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, G, C, T | A, G, C, U |
| Strands | DOUBLE (double helix) | SINGLE (generally) |
| Function | Stores genetic information | Protein synthesis, regulation |
| Location | Nucleus (eukaryotes) | Nucleus + cytoplasm |
6. Enzymes
- 'Enzymes are BIOLOGICAL CATALYSTS — they LOWER the activation energy of biochemical reactions.'
- Lock and key model: The substrate fits EXACTLY into the enzyme's ACTIVE SITE.
- Induced fit model: The active site CHANGES SHAPE to accommodate the substrate.
Factors Affecting Enzyme Activity
- Temperature: Activity increases up to an OPTIMUM (~37°C for human enzymes), then decreases (denaturation).
- pH: Each enzyme has an OPTIMUM pH (e.g., pepsin: pH 2, trypsin: pH 8).
- Substrate concentration: Rate increases initially, then PLATEAUS (enzyme saturation).
- Inhibitors: COMPETITIVE (binds active site) and NON-COMPETITIVE (binds elsewhere).
7. Common Mistakes
- Glucose vs fructose: Both are C₆H₁₂O₆. Glucose is an ALDOHEXOSE (has aldehyde group). Fructose is a KETOHEXOSE (has ketone group). Both are REDUCING sugars.
- Sucrose is NON-REDUCING: This is because the glycosidic bond involves BOTH anomeric carbons.
- Peptide bond: It is an AMIDE bond between −CO−NH− — NOT an ester bond.
- Denaturation vs hydrolysis: Denaturation BREAKS the 3D structure (disrupts H-bonds, etc.) but does NOT break peptide bonds. Hydrolysis BREAKS peptide bonds.
- RNA has URACIL, not thymine: DNA has thymine (T). RNA has uracil (U). This is a critical difference.
8. CBSE Exam Focus
- Carbohydrates — classification, structure of glucose (open and cyclic), reducing vs non-reducing sugars, polysaccharides
- Proteins — amino acids (zwitterion, isoelectric point), peptide bond, levels of protein structure, denaturation
- Vitamins — fat-soluble vs water-soluble, deficiency diseases
- Nucleic acids — DNA structure (double helix, base pairing), RNA types, differences between DNA and RNA
- Enzymes — mechanism, factors affecting activity
9. Self-Test
Q1: Why is sucrose a non-reducing sugar but maltose is a reducing sugar? A1: In sucrose, the glycosidic bond is between C1 of glucose and C2 of fructose — BOTH anomeric carbons are involved, so no free aldehyde/ketone group exists. In maltose, the bond is between C1 of one glucose and C4 of another — one anomeric carbon is FREE, making it a reducing sugar.
Q2: What is a zwitterion? Give an example. A2: A zwitterion is a DIPOLAR ION with both positive and negative charges but NET ZERO charge. In aqueous solution, amino acids exist as H₃N⁺−CH(R)−COO⁻ — the zwitterionic form.
Q3: Distinguish between DNA and RNA. A3: DNA has deoxyribose sugar, bases A/T/G/C, double-stranded. RNA has ribose sugar, bases A/U/G/C, single-stranded.
Q4: Name two water-soluble and two fat-soluble vitamins. A4: Water-soluble: B-complex and C. Fat-soluble: A, D, E, K (any two).
Q5: What is the difference between α-helix and β-sheet in protein secondary structure? A5: α-helix is a RIGHT-HANDED coiled structure (like a spring), stabilised by H-bonds between N−H of one residue and C=O of the residue 4 positions ahead. β-sheet is a PLEATED sheet structure stabilised by H-bonds between adjacent polypeptide chains (or segments of the same chain).
10. Conclusion
Biomolecules are the MOLECULES THAT MAKE LIFE POSSIBLE:
- CARBOHYDRATES: 'Energy storage (starch, glycogen) and structural support (cellulose). Simple sugars power cellular metabolism.'
- PROTEINS: 'The MACHINES of the cell — enzymes catalyse reactions, structural proteins provide support, antibodies defend against pathogens.'
- VITAMINS: 'Essential organic compounds — we need SMALL amounts, but deficiency causes serious disease.'
- NUCLEIC ACIDS: 'DNA stores the BLUEPRINT of life. RNA reads and executes the blueprint.'
- ENZYMES: 'The CATALYSTS of life — without them, biochemical reactions would be FAR too slow to sustain life.'
'Biomolecules chemistry is the BRIDGE between chemistry and biology — understanding the molecules of life is the first step to understanding LIFE itself.'
