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.
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.
Question types you will see
Each type: how to recognise it, the method step by step, and one question to try.
Cell theory and discoverers
'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.
each scientist has exactly one signature fact — match the year and the noun.
The cell theory was proposed by —
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Schleiden and Schwann (1838–39), completed by Virchow. Hooke only named the cell.
Organelle ↔ function/nickname
'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.
the question is always one nickname or one job — recall the pair, don't reason.
Which organelle is called the 'suicide bag' of the cell?
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Lysosome — its enzymes digest the cell itself if the membrane bursts, hence the nickname.
Prokaryote vs eukaryote, plant vs animal cell
'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.
The cell wall of a plant cell is made of —
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Cellulose — absent in animal cells. Chitin instead builds the fungal wall.
Cell division and chromosome numbers
'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.
A human gamete contains how many chromosomes?
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23 — meiosis halves the body-cell number of 46; fertilisation restores 46.
Genetics: Mendel, DNA, inheritance
'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.
one name + one organism + one year is the whole question.
Watson and Crick are famous for —
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The double-helix model of DNA (1953). Mendel is the genetics-father; Crick later also worked on the genetic code.
Blood cells and cell records
'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.
The average lifespan of a human red blood cell is about —
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120 days; worn-out RBCs are destroyed mainly in the spleen (the 'graveyard of RBCs').
Shortcuts that save time
These four nicknames answer most organelle questions.
Which organelle is called the 'suicide bag' of the cell?
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Lysosome.
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.
Mistakes to avoid
Where most students lose marks on this subtopic.
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.
Quick revision
Read this the night before the exam.
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.
Practice: 18 questions
Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.
Topic test · 10 questions
Suggested time 3 min · wrong answers go to your mistake notebook automatically.