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Cell Biology
2.1 Prokaryotic vs. Eukaryotic Cells
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | Absent (nucleoid region) | Present (membrane-bound) |
| Membrane organelles | Absent | Present (mitochondria, ER, Golgi) |
| Chromosome | Single circular DNA | Multiple linear chromosomes |
| Cell size | 1–10 μm | 10–100 μm |
| Cell wall | Present (peptidoglycan in bacteria) | Present in plants/fungi (cellulose/chitin); absent in animal cells |
| Examples | Bacteria, archaea | Plants, animals, fungi, protists |
2.2 Cell Organelles and Their Functions
| Organelle | Function | Mnemonic |
|---|---|---|
| Nucleus | DNA storage, gene expression, cell division | "Control centre" |
| Mitochondria | ATP synthesis (aerobic respiration) — "powerhouse of the cell" | Krebs cycle, oxidative phosphorylation |
| Chloroplast | Photosynthesis — light energy to chemical energy | Only in plant/algae cells |
| Ribosome | Protein synthesis (translation of mRNA) | Present in all living cells |
| Endoplasmic Reticulum (ER) | Rough ER: protein processing; Smooth ER: lipid synthesis | Interconnected membrane system |
| Golgi apparatus | Protein packaging, modification, sorting, secretion | "Post office of the cell" |
| Lysosome | Intracellular digestion — contains hydrolytic enzymes | "Suicide bags" |
| Vacuole | Storage — water, nutrients, waste (large in plant cells) | Maintains turgidity in plants |
| Cell membrane | Selective permeability; communication; material exchange | Fluid-mosaic model (Singer & Nicolson 1972) |
2.3 Cell Division
Mitosis produces 2 identical daughter cells with the same chromosome number. It is used for growth, repair, and asexual reproduction. Stages proceed as: Prophase → Metaphase → Anaphase → Telophase → Cytokinesis.
Meiosis produces 4 gametes with half the chromosome number (haploid) and is used in sexual reproduction. It involves two divisions (Meiosis I and II). Crossing over in Prophase I creates genetic variation.