Biomolecules
Carbohydrates
Organic compounds composed of C, H, O (ratio approximately 1:2:1).
Monosaccharides: Simple sugars, cannot be hydrolysed.
- Glucose (C6H12O6), Fructose, Galactose.
- Ribose (C5H10O5), Deoxyribose (C5H10O4) — in nucleic acids.
Disaccharides: Two monosaccharides joined by glycosidic bond.
- Sucrose (glucose + fructose) — cane sugar.
- Maltose (glucose + glucose) — malt sugar.
- Lactose (glucose + galactose) — milk sugar.
Polysaccharides: Many monosaccharides.
- Starch (amylose + amylopectin) — plant storage.
- Glycogen — animal storage (liver, muscles).
- Cellulose — structural (plant cell wall), beta-1,4 linkages.
- Chitin — exoskeleton of arthropods, fungal cell wall.
Lipids
Hydrophobic or amphipathic molecules. Mostly C, H with less O.
Simple lipids: Fats and oils (triacylglycerols = glycerol + 3 fatty acids).
- Saturated: Solid at room temp (butter, ghee).
- Unsaturated: Liquid at room temp (oils).
Compound lipids: Phospholipids (membrane component), Glycolipids.
Derived lipids: Steroids (cholesterol, hormones).
Functions: Energy storage (9 kcal/g), insulation, cell membrane structure, hormone precursors.
Proteins
Polymers of amino acids linked by peptide bonds.
Amino acids:
- General structure: NH2-C(RH)-COOH.
- 20 standard amino acids (9 essential, 11 non-essential).
- Zwitterions (both + and - charges at neutral pH).
Levels of protein structure:
- Primary: Linear sequence of amino acids.
- Secondary: Alpha-helix or beta-pleated sheet (H-bonds).
- Tertiary: 3D folding (disulphide bonds, ionic bonds, hydrophobic interactions).
- Quaternary: Multiple polypeptide subunits (Haemoglobin - 4 subunits).
Functions: Enzymes, hormones, structural (keratin, collagen), transport (Hb), antibodies.
Nucleic Acids
DNA (Deoxyribonucleic Acid):
- Double helix (Watson and Crick, 1953).
- Nucleotides: phosphate + deoxyribose sugar + nitrogenous base (A, T, G, C).
- Base pairing: A=T (2 H-bonds), G(triple)C (3 H-bonds).
- Stores genetic information.
RNA (Ribonucleic Acid):
- Single-stranded (usually).
- Ribose sugar, U instead of T.
- Types: mRNA, tRNA, rRNA.
Enzymes
Biological catalysts (mostly proteins, some RNA = ribozymes).
Mechanism
- Lock and key model (Emil Fischer): Active site is complementary to substrate.
- Induced fit model (Koshland): Active site moulds around substrate.
Factors Affecting Enzyme Activity
- Temperature: Optimum temperature (usually 37 C in humans). Denatures at high T.
- pH: Each enzyme has optimum pH. Pepsin (pH 2), Trypsin (pH 8).
- Substrate concentration: Initial rate increases then plateaus (enzyme saturation).
- Enzyme concentration: Rate increases proportionally.
Enzyme Inhibition
- Competitive: Inhibitor competes for active site (can be overcome by more substrate).
- Non-competitive: Inhibitor binds elsewhere (allosteric site), alters active site shape.
Cell Cycle
The series of events a cell goes through to divide.
Phases
Interphase (90% of cell cycle):
- G1 phase: Cell growth, protein synthesis.
- S phase: DNA replication (chromosomes duplicated).
- G2 phase: Preparation for division, organelle duplication.
M phase (Mitosis): Cell division.
G0 (Quiescent stage): Differentiated cells that do not divide (neurons, muscle cells).
Mitosis (Equational Division)
Stages
Prophase: Chromosomes condense, nuclear membrane dissolves, spindle fibres form.
Metaphase: Chromosomes align at equatorial plate (metaphase plate). Spindle fibres attach to kinetochores.
Anaphase: Sister chromatids separate and move to opposite poles.
Telophase: Chromosomes decondense, nuclear membranes reform, spindle disappears.
Cytokinesis: Division of cytoplasm. Plant cells: cell plate formation. Animal cells: cleavage furrow.
Significance of Mitosis
- Growth and development.
- Replacement of damaged/dead cells.
- Asexual reproduction (in some organisms).
Meiosis (Reduction Division)
Occurs in germ cells to produce gametes. Reduces chromosome number from 2n to n.
Meiosis I (Reductional)
Prophase I: Subdivided into:
- Leptotene, Zygotene (synapsis), Pachytene (crossing over), Diplotene, Diakinesis.
Metaphase I: Homologous pairs align at equator.
Anaphase I: Homologous chromosomes separate (not sister chromatids).
Telophase I: Two haploid cells formed.
Meiosis II (Equational)
Similar to mitosis (prophase II, metaphase II, anaphase II, telophase II). Four haploid daughter cells formed.
Significance of Meiosis
- Maintains chromosome number across generations.
- Genetic variation through crossing over and independent assortment.
Comparison: Mitosis vs Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | 1 | 2 |
| Daughter cells | 2 | 4 |
| Chromosome number | Same (2n) | Half (n) |
| Genetic variation | None | Yes (crossing over) |
| Occurs in | Somatic cells | Germ cells |
| Purpose | Growth, repair | Gamete formation |
Worked Examples
Example 1: A cell with 20 chromosomes undergoes mitosis. How many chromosomes in each daughter cell? If it undergoes meiosis? Solution: Mitosis: 20 chromosomes (diploid maintained). Meiosis: 10 chromosomes (haploid).
Example 2: What happens to enzyme activity at very high temperature? Solution: Enzyme denatures (loses 3D structure), active site changes shape, activity drops irreversibly.
Common Mistakes
- DNA replication occurs in S phase: Not in G1 or G2.
- Sister chromatids vs homologous chromosomes: Homologous pairs separate in Meiosis I, sister chromatids separate in Mitosis and Meiosis II.
- Crossing over occurs in pachytene: Not in leptotene or zygotene.
- All enzymes are proteins: Some RNA molecules (ribozymes) also catalyse reactions.
ISC Exam Focus
- Theory (70%): Biomolecule structure, enzyme mechanism, cell cycle phases, mitosis and meiosis.
- Application (30%): Numerical problems on chromosome number, distinguishing mitosis/meiosis.
- ISC frequently asks: "Distinguish between mitosis and meiosis" and "Explain the stages of ...".
Self-Test Questions
Q1: What are the four levels of protein structure? Answer: Primary (sequence), Secondary (alpha-helix/beta-sheet), Tertiary (3D folding), Quaternary (multiple subunits).
Q2: Write the three phases of interphase. Answer: G1 (growth), S (DNA replication), G2 (preparation for division).
Q3: Distinguish between mitosis and meiosis. Answer: Mitosis: 2 daughter cells, same chromosome number, somatic. Meiosis: 4 daughter cells, half chromosome number, germ cells.
Q4: What is crossing over and when does it occur? Answer: Exchange of genetic material between homologous chromosomes during pachytene of prophase I.
Q5: Define enzyme. Explain the lock and key model. Answer: Enzyme is a biological catalyst. Lock and key: active site (lock) is complementary to substrate (key).
Q6: If a cell has 10 pairs of chromosomes, how many chromosomes in each cell after mitosis? After meiosis? Answer: After mitosis: 20 (diploid). After meiosis: 10 (haploid).
