Biology
🔒 Log in to trackCell, organelles and genetics
🔒 Log in to trackDiscoveries about the cell
| Discovery | Scientist |
|---|---|
| Cell (in cork, 1665) | Robert Hooke |
| First living cells, bacteria (single-lens microscope) | Antonie van Leeuwenhoek |
| Nucleus (1831) | Robert Brown |
| Cell theory | Schleiden (plants) and Schwann (animals) |
| 'Omnis cellula e cellula' — new cells come from existing cells | Rudolf Virchow |
Cell organelles
| Organelle | Function / nickname |
|---|---|
| Mitochondria | Powerhouse of the cell — aerobic respiration, ATP; have their own DNA |
| Ribosomes | Protein synthesis (no membrane) |
| Lysosomes | 'Suicide bags' — digestive enzymes |
| Golgi apparatus | Packaging and secretion |
| Endoplasmic reticulum | Transport; rough ER (with ribosomes) makes proteins, smooth ER makes lipids |
| Chloroplast | Photosynthesis (plants only); has its own DNA |
| Nucleus | Control centre; holds chromosomes (DNA) |
| Vacuole | Storage; large in plant cells |
| Cell wall | Plants (cellulose), fungi (chitin) — absent in animal cells |
- Prokaryotes (bacteria, blue-green algae) have no nuclear membrane and no membrane-bound organelles. Eukaryotes have a true nucleus.
- Largest cell: ostrich egg. Longest cell: nerve cell (neuron). Smallest cell: Mycoplasma (PPLO). Mature mammalian RBCs have no nucleus.
- Mitosis — body growth and repair, gives 2 identical cells. Meiosis — forms gametes, halves the chromosome number (4 cells).
Genetics
- Gregor Mendel — father of genetics; experiments on garden pea (Pisum sativum); laws of dominance, segregation and independent assortment.
- DNA double helix — Watson and Crick (1953), using X-ray data of Rosalind Franklin and Maurice Wilkins. DNA bases: A, T, G, C (RNA has uracil instead of thymine).
- Humans have 46 chromosomes (23 pairs) in body cells: 22 pairs of autosomes + 1 pair of sex chromosomes. Gametes have 23.
- Sex of the child is decided by the father: XX = female, XY = male.
- Haemophilia and colour blindness are X-linked disorders — more common in males. Down syndrome — an extra chromosome 21 (trisomy 21).
- Hugo de Vries — mutation theory. Darwin — natural selection (On the Origin of Species, 1859). Lamarck — inheritance of acquired characters.
- Har Gobind Khorana — Nobel Prize 1968 for work on the genetic code.
Detailed notes
The cell — discovery and the cell theory
Robert Hooke saw cork cells under his microscope in 1665 (book: Micrographia) — he named 'cells' but saw only dead walls. Leeuwenhoek first saw living cells ('animalcules'). Robert Brown discovered the nucleus (1831). The cell theory: Schleiden (1838, plants) and Schwann (1839, animals) said all living things are made of cells; Virchow (1855) completed it — every cell arises from a pre-existing cell (Omnis cellula e cellula).
Organelles — who does what
| Organelle | Job / title |
|---|---|
| Mitochondria | Respiration, ATP — 'powerhouse of the cell'; has its own DNA |
| Ribosome | Protein synthesis — 'protein factory' |
| Golgi body | Packs and dispatches secretions — 'post office' |
| Lysosome | Digestive enzymes — 'suicide bags' |
| Chloroplast | Photosynthesis (plastid); other plastids store colour/starch |
| Nucleus | Controls the cell; stores DNA — the 'brain' |
| Vacuole | Storage; big and central in plant cells |
| Cell wall | Rigid cellulose coat — plant cells only |
| Plant vs animal cell: plant has a cell wall, chloroplasts and a large central vacuole; animal cells have centrosomes instead. |
Cell types and cell division
Prokaryotes (bacteria, blue-green algae) — no true nucleus, no membrane-bound organelles. Eukaryotes (plants, animals, fungi, protists) — true nucleus. Cell division: mitosis keeps the chromosome number (body growth, 46 → 46); meiosis halves it (gamete formation, 46 → 23) — the basis of sexual reproduction.
Genetics — the science of heredity
Gregor Mendel, the father of genetics, worked on garden pea (Pisum sativum) and gave the laws of inheritance — dominant vs recessive traits. Watson and Crick (1953) gave the DNA double-helix model; a gene is a stretch of DNA coding a trait. Humans carry 46 chromosomes = 23 pairs; the 23rd pair decides sex — XX female, XY male, so the father determines the child's sex. Blood groups too are inherited traits.
Cell records and blood cells
- Largest human cell — the ovum (egg); longest — the nerve cell (up to a metre).
- RBC (red blood cells): biconcave, no nucleus, carry oxygen on haemoglobin (iron); live about 120 days.
- WBC: the defence cells (immunity); platelets: clot the blood.
- Normal counts: RBC ≈ 5 million per mm³, WBC ≈ 6,000–8,000 per mm³.
Quick revision
- Hooke named cells (1665); Leeuwenhoek saw living ones; Brown found the nucleus; Schleiden + Schwann + Virchow = cell theory.
- Powerhouse–mitochondria; protein factory–ribosome; suicide bag–lysosome; post office–Golgi.
- Mitosis 46→46; meiosis 46→23; XX female, XY male; father decides sex.
- Mendel = father of genetics (pea plant); Watson & Crick = DNA double helix.
- Largest human cell–ovum; longest–nerve cell; RBC survives ~120 days.
Types of questions asked
Every way this subtopic shows up in exams — how to recognise it, the formula or logic to use, and a solved example.
Type 1: Cell theory and discoverersvery common3 practice Q
'Who discovered the cell / nucleus / living cells', 'the cell theory was given by…', 'omnis cellula e cellula' attribution, year-based questions (1665, 1831, 1855).
- Timeline: 1665 Hooke (cork, named cells) → Leeuwenhoek (living cells) → 1831 Robert Brown (nucleus) → 1838–39 Schleiden + Schwann (theory) → 1855 Virchow (cells from cells).
- Trap: Hooke saw dead cork walls; the LIVING cells credit goes to Leeuwenhoek.
- Latin tag omnis cellula e cellula belongs to Virchow. Why: each scientist has exactly one signature fact — match the year and the noun.
Example: The cell theory was proposed by —
Schleiden and Schwann (1838–39), completed by Virchow. Hooke only named the cell.
Type 2: Organelle ↔ function/nicknamevery common3 practice Q
'Powerhouse of the cell', 'suicide bags', 'which organelle makes protein / packages secretions', 'which organelle has its own DNA'.
- Nickname table: powerhouse–mitochondria; protein factory–ribosome; post office–Golgi; suicide bag–lysosome; kitchen–chloroplast; brain–nucleus.
- Special facts: mitochondria and chloroplasts have their OWN DNA; RBCs are the odd cells WITHOUT a nucleus.
- Photosynthesis needs chloroplast + chlorophyll; respiration is the mitochondrion's job. Why: the question is always one nickname or one job — recall the pair, don't reason.
Example: Which organelle is called the 'suicide bag' of the cell?
Lysosome — its enzymes digest the cell itself if the membrane bursts, hence the nickname.
Type 3: Prokaryote vs eukaryote, plant vs animal cellcommon2 practice Q
'Which is a prokaryote', 'bacteria lack a true nucleus', 'cell wall is made of cellulose in…', 'which structure is absent in animal cells'.
- Prokaryote = no true nucleus, no membrane-bound organelles (bacteria, blue-green algae); eukaryote = everything else.
- Plant-only structures: cell wall (cellulose), chloroplasts, large central vacuole. Animal-only: centrosome.
- 'Absent in animal cells' almost always answers cell wall or chloroplast.
Example: The cell wall of a plant cell is made of —
Cellulose — absent in animal cells. Chitin instead builds the fungal wall.
Type 4: Cell division and chromosome numberscommon2 practice Q
'The human body cell has how many chromosomes', 'chromosome number in a gamete', 'mitosis vs meiosis', 'which division keeps the number the same'.
- Human body cell = 46 chromosomes (23 pairs); gamete = 23 — meiosis halves it.
- Mitosis: growth and repair, number preserved (46→46). Meiosis: gamete formation, number halved (46→23).
- Sex chromosomes: 22 pairs are autosomes; the 23rd pair (XX/XY) decides sex — the father's sperm decides it.
Example: A human gamete contains how many chromosomes?
23 — meiosis halves the body-cell number of 46; fertilisation restores 46.
Type 5: Genetics: Mendel, DNA, inheritancecommon4 practice Q
'Father of genetics', 'the DNA double helix model was given by…', 'Mendel worked on which plant', 'who determines the sex of a child', 'a gene is a segment of…'.
- Mendel = father of genetics; plant = garden pea (Pisum sativum); laws of dominance and segregation.
- DNA double helix — Watson and Crick, 1953; gene = segment of DNA.
- Sex determination: mother always gives X; father gives X (girl) or Y (boy) — so the FATHER decides. Why: one name + one organism + one year is the whole question.
Example: Watson and Crick are famous for —
The double-helix model of DNA (1953). Mendel is the genetics-father; Crick later also worked on the genetic code.
Type 6: Blood cells and cell recordscommon2 practice Q
'Which blood cells lack a nucleus', 'lifespan of RBC', 'which cells fight infection', 'largest / longest cell in the human body', 'which cell carries oxygen'.
- RBC: no nucleus, biconcave, haemoglobin + iron, oxygen transport, ~120-day life.
- WBC: defence/immunity; platelets: clotting (the third, fragment-type cell).
- Records: ovum = largest human cell; nerve cell = longest.
Example: The average lifespan of a human red blood cell is about —
120 days; worn-out RBCs are destroyed mainly in the spleen (the 'graveyard of RBCs').
Shortcut tricks
⚡ Organelle nicknames: 'Power-Mito, Suicide-Lyso, Protein-Ribo, Packing-Golgi'
These four nicknames answer most organelle questions.
Example: Which organelle is called the 'suicide bag' of the cell?
Lysosome.
⚡ Cell discoverers in order
Hooke saw it (1665, dead cork) → Leeuwenhoek saw it alive → Brown saw the nucleus → Schleiden–Schwann made the theory → Virchow said cells come from cells.
Where students lose marks
Saying Leeuwenhoek discovered the cell — Hooke coined the term; Leeuwenhoek first saw living cells.
Saying the mother decides the child's sex — the father's X or Y sperm does.
Giving animal cells a cell wall — only plants, fungi and bacteria have one.
Practice sets — 18 questions
Sets of 10, mixed across the question types above. Each answer comes with a step-by-step explanation.
Topic test · 10 questions
Suggested time 3 min · wrong answers go to your mistake notebook automatically.