Cell Biology – Study Notes (INI CET)
Purpose of these notes – A concise, self‑contained guide to the essentials of cell biology, ready for quick revision before the INI CET exam.
1. Simple Explanation
| Term | What it Means |
|---|---|
| Cell | The smallest unit of life; the “building block” of all living organisms. |
| Prokaryote | Cell without a true nucleus (e.g., bacteria). |
| Eukaryote | Cell with a true nucleus and membrane‑bound organelles (plants, animals, fungi, protists). |
| Biomembrane | Phospholipid bilayer that separates the cell interior from the exterior and controls traffic in and out of the cell. |
Key concept – All cells share a common structure (cytoplasm, plasma membrane, nucleus in eukaryotes) but differ in organelles, size, and functions.
2. Key Points
2.1 Cell Structure & Organelles
| Organelle | Location | Main Function |
|---|---|---|
| Nucleus | Cytoplasm, enveloped by nuclear envelope | Stores DNA, controls gene expression |
| Nucleolus | Inside nucleus | Ribosomal RNA synthesis |
| Mitochondria | Cytoplasm | ATP production (oxidative phosphorylation) |
| Chloroplast | Plant cells | Photosynthesis (chlorophyll) |
| Endoplasmic Reticulum (ER) | Network surrounding nucleus | Rough ER – protein synthesis; Smooth ER – lipid synthesis |
| Golgi Apparatus | Post‑ER | Protein modification, sorting & shipping |
| Lysosomes | Cytoplasm | Intracellular digestion (hydrolytic enzymes) |
| Peroxisomes | Cytoplasm | Fatty‑acid oxidation, detoxification |
| Ribosomes | Free or ER‑bound | Protein synthesis |
| Cytoskeleton | Cytoplasm | Structural support, motility, intracellular transport |
Special notes for prokaryotes:
- Peptidoglycan cell wall (Gram‑positive/negative).
- Plasmids – extra‑chromosomal DNA.
- Flagella / Pili – motility & attachment.
2.2 Cell Membrane Dynamics
| Process | Mechanism | Energy Requirement |
|---|---|---|
| Passive diffusion | Small, non‑polar molecules | None |
| Facilitated diffusion | Protein channels/transporters | None |
| Active transport | ATP‑driven pumps (e.g., Na⁺/K⁺‑ATPase) | Yes |
| Endocytosis | Vesicle formation (phagocytosis, pinocytosis) | Yes |
| Exocytosis | Vesicle fusion with plasma membrane | Yes |
Key terms – Fluid mosaic model, lipid rafts, signal transduction, receptor‑ligand binding.
2.3 Cell Cycle & Division
| Phase | What Happens | Key Proteins/Events |
|---|---|---|
| G₁ (Gap 1) | Cell grows; RNA & protein synthesis | Cyclin‑dependent kinases (CDKs) |
| S (Synthesis) | DNA replication | DNA polymerase |
| G₂ (Gap 2) | Preparation for mitosis | Cyclin‑B |
| M (Mitosis) | Nuclear division | Chromosomes condense → metaphase → anaphase → telophase |
| Cytokinesis | Cytoplasm separates | Actomyosin contractile ring |
Meiosis – Two rounds of division, producing 4 haploid gametes (meiosis I & II).
2.4 Energy & Metabolism
| Pathway | Organism | Main Substrate | Output | Key Enzymes |
|---|---|---|---|---|
| Aerobic Respiration | Eukaryotes, many prokaryotes | Glucose (O₂) | 36‑38 ATP | Pyruvate dehydrogenase, Krebs cycle, ETC |
| Anaerobic Glycolysis | All | Glucose | 2 ATP + lactic acid | Phosphofructokinase |
| Fermentation | Yeast, some bacteria | Glucose | 2 ATP + ethanol | Alcohol dehydrogenase |
| Photosynthesis | Plants, cyanobacteria | CO₂ + H₂O | Glucose + O₂ | Rubisco, PSII/PSI |
3. Worked Example
Estimating Diffusion Time Across a Typical Cell
Problem:
A small molecule (D ≈ 1 × 10⁻⁶ cm² s⁻¹) diffuses across a spherical cell with radius ( r = 5 µm ). Estimate the time required for diffusion.
Solution Steps
-
Convert radius to cm
( 5 µm = 5 × 10^{-4};cm ) -
Use the diffusion time formula
For a spherical cell, the average diffusion time ( t ) ≈ ( \frac{r^2}{2D} )