- Biology is the largest part of general science in SSC and railway papers, and a large share of those questions come from cell biology, genetics and plants.
- Most questions are one-liners: "Which organelle is the powerhouse of the cell?", "Which hormone ripens fruit?", "Down syndrome is caused by…?" — so a clean set of matched pairs goes a long way.
- This page complements our Human Body & Health notes, which cover body systems, vitamins and diseases.
Discovery of the Cell
- Robert Hooke first observed (dead) cells in cork in 1665; Antonie van Leeuwenhoek first saw living cells.
- Robert Brown discovered the nucleus in 1831.
- Cell theory — Schleiden (plants, 1838) and Schwann (animals, 1839); Rudolf Virchow added that all cells come from pre-existing cells (1855).
Cell Organelles
| Organelle | Function / nickname |
|---|---|
| Mitochondria | Make energy (ATP) by respiration — the "powerhouse of the cell"; have their own DNA |
| Ribosomes | Protein synthesis; have no membrane; found in all cells |
| Lysosomes | Digestive enzymes — the "suicide bags" |
| Golgi apparatus | Packages and dispatches materials — the cell's "post office" |
| Endoplasmic reticulum | Transport network; rough ER has ribosomes, smooth ER makes lipids |
| Chloroplasts | Photosynthesis — plants only; contain chlorophyll and their own DNA |
| Vacuole | Storage; one large vacuole in plant cells, small ones in animal cells |
| Centrioles | Help in cell division — usually in animal cells |
| Cell wall | Rigid outer layer — cellulose in plants, chitin in fungi, peptidoglycan in bacteria |
Prokaryotic cells (bacteria) have no true nucleus or membrane-bound organelles; eukaryotic cells (plants, animals, fungi) do.
Cell discoveredRobert Hooke, 1665
PowerhouseMitochondria
Suicide bagsLysosomes
Human chromosomes46 (23 pairs)
DNA double helixWatson & Crick, 1953
Fruit-ripening hormoneEthylene
Cell Division
| Mitosis | Meiosis | |
|---|---|---|
| Where | Body (somatic) cells | Reproductive cells — forms gametes |
| Daughter cells | 2, identical to the parent | 4, genetically different |
| Chromosome number | Stays the same (diploid) | Halved (haploid) |
| Purpose | Growth and repair | Sexual reproduction and variation |
DNA, Genes and Chromosomes
- The double-helix structure of DNA was described by Watson and Crick in 1953, using X-ray images by Rosalind Franklin.
- DNA bases pair as A–T and G–C; RNA has uracil instead of thymine.
- Humans have 46 chromosomes (23 pairs) — 22 pairs of autosomes and one pair of sex chromosomes. Females are XX and males XY, so the father's sperm decides the sex of the child.
Mendel's Laws
Gregor Mendel, the "father of genetics", experimented on pea plants. His three laws are dominance, segregation and independent assortment. A monohybrid cross gives a 3:1 phenotypic ratio (1:2:1 genotypic); a dihybrid cross gives 9:3:3:1.
Genetic Disorders
| Disorder | Cause |
|---|---|
| Down syndrome | An extra chromosome 21 (trisomy 21) |
| Turner syndrome | Female with one X chromosome (XO) |
| Klinefelter syndrome | Male with an extra X (XXY) |
| Haemophilia, colour blindness | X-linked recessive — far more common in males |
| Sickle-cell anaemia, thalassaemia | Autosomal recessive disorders of haemoglobin |
Classification of Living Things
Carolus Linnaeus introduced binomial nomenclature (genus + species, e.g., Homo sapiens) and is the father of taxonomy. R.H. Whittaker proposed the five-kingdom system (1969): Monera, Protista, Fungi, Plantae and Animalia.
Plants
- Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, in the presence of sunlight and chlorophyll. Light reactions take place in the thylakoids; the Calvin cycle in the stroma. The oxygen released comes from water.
- Xylem carries water and minerals upward; phloem carries food. Water is lost through stomata by transpiration.
- Nitrogen fixation: Rhizobium bacteria in the root nodules of legumes convert atmospheric nitrogen into usable forms.
| Plant hormone | Main role |
|---|---|
| Auxin | Cell elongation, bending towards light (phototropism), root formation |
| Gibberellin | Stem elongation, seed germination, bolting |
| Cytokinin | Cell division; delays ageing of leaves |
| Abscisic acid | The "stress hormone" — closes stomata, causes dormancy |
| Ethylene | The only gaseous hormone — ripens fruit |
| Plant group | Feature | Examples |
|---|---|---|
| Thallophyta | No true root, stem or leaf | Algae such as Spirogyra |
| Bryophyta | The "amphibians of the plant kingdom"; no vascular tissue | Mosses, liverworts |
| Pteridophyta | First plants with vascular tissue; no seeds | Ferns |
| Gymnosperms | Naked seeds | Pine, cycas |
| Angiosperms | Flowering plants; seeds inside fruits; monocots and dicots | Mango, wheat |
- Nickname questions: powerhouse (mitochondria), suicide bag (lysosome), amphibians of the plant kingdom (bryophytes), stress hormone (abscisic acid).
- Disorder ↔ cause: trisomy 21, XO, XXY, X-linked recessive.
- Comparisons: mitosis vs meiosis, prokaryote vs eukaryote, xylem vs phloem.
Q1. Which organelle is known as the "suicide bag" of the cell?
✅ Answer: (c) Lysosome
💡 Explanation: Lysosomes contain digestive enzymes that can break down the cell itself.
Q2. Down syndrome is caused by:
✅ Answer: (a) An extra chromosome 21
💡 Explanation: Down syndrome is trisomy 21; Turner is XO and Klinefelter is XXY.
Q3. Which plant hormone is a gas and helps ripen fruit?
✅ Answer: (d) Ethylene
💡 Explanation: Ethylene is the only gaseous plant hormone.
Q4. Meiosis produces:
✅ Answer: (b) Four haploid cells
💡 Explanation: Meiosis halves the chromosome number and forms four genetically different gametes.
Q5. The five-kingdom classification was proposed by:
✅ Answer: (c) R.H. Whittaker
💡 Explanation: Whittaker proposed Monera, Protista, Fungi, Plantae and Animalia in 1969.
🔁 Revise in 60 Seconds
- Hooke 1665 (cell), Brown 1831 (nucleus), Schleiden & Schwann (cell theory), Virchow (cells from cells).
- Mitochondria — powerhouse; lysosome — suicide bag; ribosome — protein; chloroplast & cell wall — plants.
- Mitosis: 2 identical; meiosis: 4 haploid. 46 chromosomes; father decides sex. Mendel 3:1, 9:3:3:1.
- Down 21 trisomy, Turner XO, Klinefelter XXY. Ethylene ripens fruit; ABA stress; bryophytes = amphibians.
📌 Prelims Traps to Avoid
- ✘ Ribosomes are found in both prokaryotes and eukaryotes, and have no membrane.
- ✘ The oxygen released in photosynthesis comes from water, not carbon dioxide.
- ✘ Fungal cell walls are made of chitin, not cellulose.
What is the difference between mitosis and meiosis?
Mitosis takes place in body cells and produces two identical diploid daughter cells for growth and repair. Meiosis takes place in reproductive cells and produces four genetically different haploid cells (gametes) for sexual reproduction.
Which organelle is called the powerhouse of the cell?
The mitochondrion, because it produces ATP — the cell's energy currency — through cellular respiration. Mitochondria also have their own DNA.