Biomolecules
'Life is a series of chemical reactions catalysed by enzymes.' — Biochemistry
1. Chapter Overview
Living organisms are made of CHEMICALS — both INORGANIC (water, minerals) and ORGANIC (carbohydrates, lipids, proteins, nucleic acids). This chapter explores the STRUCTURE and FUNCTION of these BIOMOLECULES, their POLYMERISATION into MACROMOLECULES, the METABOLIC PATHWAYS that transform them, and the ENZYMES that CATALYSE these reactions.
2. Chemical Composition of Living Organisms
Wet Weight vs Dry Weight
- Water constitutes 70-90% of living cells
- In DRY weight: Proteins (50%), Nucleic acids (15-20%), Carbohydrates (12-15%), Lipids (8-10%)
Micromolecules vs Macromolecules
| Feature | Micromolecules | Macromolecules |
|---|---|---|
| Size | < 1000 Da (Dalton) | > 1000 Da |
| Examples | Monosaccharides, amino acids, nucleotides, water, minerals | Proteins, polysaccharides, nucleic acids, lipids (some) |
| Property | SIMPLE compounds | POLYMERS (most are) |
3. Carbohydrates
Classification
| Class | Subunits | Examples | Functions |
|---|---|---|---|
| Monosaccharides | Single sugar unit | Glucose, Fructose, Galactose | QUICK energy source |
| Oligosaccharides | 2-9 units | Sucrose (glucose+fructose), Lactose, Maltose | TRANSPORT, storage |
| Polysaccharides | >10 units | Starch, Glycogen, Cellulose, Chitin | ENERGY storage, structural |
Key Carbohydrates
- Glucose: C₆H₁₂O₆ — PRIMARY energy source for cells
- Sucrose: Cane sugar — transport form in plants
- Starch: AMYLOSE (unbranched) + AMYLOPECTIN (branched) — plant storage
- Glycogen: Animal storage (liver, muscle) — MORE branched than starch
- Cellulose: β-1,4 linkages — structural (plant cell wall) — humans CANNOT digest
- Chitin: N-acetylglucosamine — exoskeleton of arthropods, fungal cell walls
4. Lipids
Characteristics
- HYDROPHOBIC (insoluble in water, soluble in organic solvents)
- NOT polymers (mostly — some aggregate non-covalently)
- Higher C,H content → MORE energy per gram than carbohydrates
Types
| Type | Structure | Examples | Function |
|---|---|---|---|
| Simple lipids | FATTY ACID + ALCOHOL (ester) | Fats, oils, waxes | ENERGY storage, insulation |
| Compound lipids | Lipid + other group | Phospholipids, Glycolipids | MEMBRANE structure |
| Derived lipids | From simple lipids | Steroids (cholesterol, hormones) | SIGNALLING, membrane fluidity |
Fatty Acids
- Saturated: NO double bonds (solid at room temp — ghee, butter)
- Unsaturated: One+ double bonds (liquid — oils)
- Essential fatty acids: Must be obtained from diet (linoleic acid)
Phospholipids
- Glycerol + 2 fatty acids + phosphate group
- AMPHIPATHIC (hydrophilic head + hydrophobic tails)
- FORM the CELL MEMBRANE (lipid bilayer)
5. Proteins
Amino Acids — Building Blocks
- 20 STANDARD amino acids
- Structure: NH₂—CH(R)—COOH (amino group, R group, carboxyl group)
- Essential: 9 amino acids that humans CANNOT synthesise (must be in diet)
- Isoelectric point: pH at which net charge = 0
Peptide Bond Formation
- Amino group of one amino acid reacts with CARBOXYL group of another → H₂O released (CONDENSATION)
- Dipeptide → Tripeptide → Polypeptide → PROTEIN
Levels of Protein Structure
| Level | Description | Bonds |
|---|---|---|
| Primary | LINEAR sequence of amino acids | PEPTIDE bonds |
| Secondary | HELIX or SHEET folding | HYDROGEN bonds (between backbone groups) |
| Tertiary | 3D folding of ONE polypeptide | H-bonds, ionic, disulphide, hydrophobic |
| Quaternary | MULTIPLE polypeptide chains ASSOCIATED | Same as tertiary + INTERCHAIN bonding |
Protein Types and Functions
| Function | Examples |
|---|---|
| Enzymatic | Amylase, Trypsin, DNA polymerase |
| Structural | Collagen (bone), Keratin (hair), Elastin |
| Transport | Haemoglobin (O₂), Myoglobin |
| Defence | Antibodies (immunoglobulins) |
| Contractile | Actin, Myosin (muscle) |
| Hormonal | Insulin, Growth hormone |
6. Nucleic Acids
Nucleotides — Building Blocks
- Structure: NITROGENOUS BASE + PENTOSE SUGAR + PHOSPHATE
- Purines: Adenine (A), Guanine (G)
- Pyrimidines: Cytosine (C), Thymine (T — DNA), Uracil (U — RNA)
DNA vs RNA
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, G, C, T | A, G, C, U |
| Strands | DOUBLE (antiparallel) | SINGLE (usually) |
| Function | Genetic STORAGE | Protein SYNTHESIS |
| Types | One type mainly | mRNA, tRNA, rRNA |
Central Dogma
- DNA → (transcription) → RNA → (translation) → PROTEIN
7. Enzymes
Characteristics
- PROTEIN catalysts (most are proteins; some RNA = RIBOZYMES)
- HIGH specificity (lock and key)
- ENORMOUS catalytic power (10¹⁰ times faster)
- NOT consumed in the reaction
Mechanism (Induced Fit Model)
- Enzyme + Substrate → Enzyme-Substrate complex → Enzyme + Product
- Active site: Where substrate binds
- Activation energy barrier: Enzymes LOWER it
Factors Affecting Enzyme Activity
| Factor | Effect |
|---|---|
| Temperature | INCREASES up to OPTIMUM (37°C), then DENATURES |
| pH | Each enzyme has OPTIMAL pH (pepsin pH 2, trypsin pH 8) |
| Substrate concentration | Rate ↑ with [S] up to V_max (Michaels-Menten kinetics) |
| Inhibitors | COMPETITIVE (binds active site) vs NON-COMPETITIVE (binds elsewhere) |
8. Common Mistakes
- All proteins are enzymes, but NOT all enzymes are proteins: Some RNA molecules (ribozymes) catalyse reactions
- Lipids are NOT polymers: True fats are triglycerides, NOT repeating subunits
- Cellulose is indigestible by humans: We lack the ENZYME to break β-1,4 linkages
- Enzymes are NOT consumed in reactions: They are REUSED — that is why small amounts can catalyse large amounts of substrate
- DNA is NOT the only nucleic acid that carries information: mRNA, tRNA, and rRNA all carry information or function in gene expression
9. CBSE Exam Focus
- Carbohydrate classification — monosaccharides to polysaccharides (3-mark)
- Protein structure — primary to quaternary (5-mark)
- DNA vs RNA — differences (3-mark)
- Enzyme characteristics — lock and key vs induced fit (3/5-mark)
- Amino acids — structure, peptide bond (3-mark)
10. Self-Test (5+ Q&A)
Q1: What is the difference between starch and cellulose? A: Starch: α-1,4 linkages (DIGESTIBLE), branched (amylopectin). Cellulose: β-1,4 linkages (NOT digestible by humans), LINEAR, structural polysaccharide.
Q2: Describe the four levels of protein structure. A: Primary: Linear amino acid sequence. Secondary: α-helix or β-pleated sheet (H-bonds). Tertiary: 3D folding of one polypeptide. Quaternary: Multiple polypeptide chains associate.
Q3: What is induced fit model of enzyme action? A: Substrate binding INDUCES a SHAPE CHANGE in the enzyme's active site, leading to a BETTER fit. The ES complex then forms products.
Q4: Name the bases in DNA and RNA. A: DNA: Adenine (A), Guanine (G), Cytosine (C), Thymine (T). RNA: A, G, C, Uracil (U).
Q5: Why is glycogen called 'animal starch'? A: Glycogen is the STORAGE polysaccharide in animals (like starch in plants). It is stored in LIVER and MUSCLE and broken down to glucose when ENERGY is needed.
11. Conclusion
Biomolecules are the MOLECULAR machinery of life. Carbohydrates STORE and provide energy. Lipids form MEMBRANES and store concentrated energy. Proteins SERVE as enzymes, structures, transporters, and defenders. Nucleic acids STORE and TRANSMIT genetic information. Enzymes CATALYSE all metabolic reactions with high specificity and efficiency. Together, these biomolecules create the MOLECULAR BASIS of life — understanding them is ESSENTIAL for biochemistry, molecular biology, and medicine.
