Biology Unit 3
Cell: Structure and Functions
The fundamental unit of life, biomolecules, and the mechanics of cellular division for IAT.
1. Cell Theory & Basics
Tenets of Cell Theory
Expanded by Rudolf Virchow (1855) with the concept of "Omnis cellula-e cellula".
- All living organisms are composed of cells and products of cells.
- All cells arise from pre-existing cells.
- The cell is the structural and functional unit of life.
2. Prokaryotic Insights
Exam Gold: Mesosomes
Mesosomes are special membranous infoldings (vesicles, tubules, lamellae) of the plasma membrane.
- Functions: Cell wall formation, DNA replication, distribution to daughter cells, respiration, and secretion.
- Note: In some modern datasets, they are considered artifacts, but for IAT/NCERT, focus on their functional roles listed above.
3. Cell Membrane & Transport
Structural Model
Fluid Mosaic Model (Singer & Nicolson, 1972): Lipid bilayer with proteins (integral and peripheral). Cholesterol provides stability.
Transport Mechanisms
-
1. Passive Transport: No ATP required. Moves along gradient.
- Simple Diffusion: Neutral solutes (O2, CO2).
- Osmosis: Water movement via semi-permeable membrane.
- Facilitated Diffusion: For polar molecules via carrier proteins.
-
2. Active Transport: Requires ATP. Moves against gradient.
- Example: Na+/K+ Pump.
4. Organelle Identification Strategy
Professor's Note: ID Logic
Use functional markers to identify organelles in MCQs:
| Marker / Function | Organelle |
|---|---|
| ATP Production / Oxidative Phosphorylation | Mitochondria |
| Packaging / Glycosylation of Proteins | Golgi Apparatus |
| Hydrolytic Enzymes / Suicidal Bag | Lysosome |
| Protein synthesis / Ribosome attachment | RER |
| Steroidal hormone synthesis / Detoxification | SER |
| Turgidity / Waste storage (in plants) | Vacuole |
5. Enzyme Kinetics (IAT High-Yield)
Michaelis-Menten Logic
- Vmax: Saturation point where all enzyme active sites are occupied.
- Km (Michaelis Constant): Substrate concentration at 1/2 Vmax.
- Low Km = High affinity (binds substrate tightly).
- High Km = Low affinity.
Inhibition Patterns
- Competitive Inhibition: Mimics substrate. Km increases (needs more substrate to reach 1/2 Vmax), Vmax remains constant.
- Non-Competitive Inhibition: Binds elsewhere. Vmax decreases, Km remains constant.
6. Cell Cycle & Checkpoints
The G1 Checkpoint
The G1 Checkpoint (Restriction Point) is the most critical. It assesses:
- Cell size and nutrient availability.
- DNA integrity (checking for damage).
- Growth factors.
If conditions are not met, the cell enters **G0 (Quiescent) phase** instead of S phase.
Mitosis vs Meiosis Gold Table
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth / Repair | Gamete formation |
| Daughter Cells | 2 Diploid (2n) | 4 Haploid (n) |
| Variation | Identical clones | Genetic recombination |
| Crossing Over | Absent | Present (Prophase I - Pachytene) |
| Synapsis | Absent | Present (Zygotene) |