How cells were discovered and the cell theory
A cell is the smallest unit that is alive. Most cells are too small to see with the eye, so we need a microscope (an instrument that makes small things look big).
- 1665 – Robert Hooke saw tiny boxes in a slice of cork and called them cells (Latin cella = small room). These were dead cells.
- 1674 – Leeuwenhoek saw living cells, like bacteria, in pond water with a better lens.
- 1831 – Robert Brown found the nucleus.
- 1838–39 – Schleiden and Schwann said all plants and animals are made of cells.
- 1855 – Virchow added that new cells come only from cells that already exist.
Cell theory in three lines
- All living things are made of one or more cells.
- The cell is the basic unit of structure and function.
- All cells come from pre-existing cells.
Organisms with one cell (amoeba, bacteria) are unicellular. Those with many cells (you, a mango tree) are multicellular.
Try it at home
Peel the thin skin from the inside of an onion layer, put it on a glass slide with a drop of water and look with a school microscope or a strong phone macro lens. You will see neat brick-like cells.
Prokaryotic vs eukaryotic cells, plant vs animal cells
Prokaryotic and eukaryotic
| Prokaryotic | Eukaryotic |
|---|---|
| Very small (1–10 µm) | Bigger (5–100 µm) |
| No nuclear membrane; DNA lies in a region called the nucleoid | True nucleus with a nuclear membrane |
| No membrane-bound organelles (no mitochondria, no chloroplast) | Many membrane-bound organelles |
| Bacteria, blue-green algae | Plants, animals, fungi, amoeba |
Plant cell and animal cell
| Plant cell | Animal cell |
|---|---|
| Cell wall made of cellulose outside the membrane | No cell wall |
| Fixed box-like shape | Shape can change, often round |
| Chloroplasts present (in green parts) | No chloroplasts |
| One large central vacuole | Small or no vacuoles |
| Centrioles usually absent | Centrioles present |
Both have a cell membrane, cytoplasm, nucleus, mitochondria, ER, Golgi and ribosomes.
Cell organelles and their functions
Organelles are tiny parts inside a cell, each with one job – like rooms in a factory.
- Cell membrane (plasma membrane): thin outer skin. It is selectively permeable – it lets some things pass and stops others.
- Cell wall (plants, fungi): stiff layer of cellulose outside the membrane. Gives shape and strength and stops the cell from bursting in water.
- Nucleus: the control room. It holds chromosomes made of DNA, which carry instructions and pass traits to the next generation.
- Cytoplasm: the jelly that fills the cell; organelles sit in it.
- Mitochondria: the powerhouse. They break down food to release energy as ATP. They have their own DNA and ribosomes.
- Plastids: chloroplasts (green, with chlorophyll – make food by photosynthesis), leucoplasts (colourless, store starch, oil, protein). Plastids also have their own DNA.
- Endoplasmic reticulum (ER): a network of tubes. Rough ER has ribosomes and helps make proteins; smooth ER makes fats (lipids) and helps remove poisons in liver cells. Together they form membranes (membrane biogenesis).
- Ribosomes: make proteins.
- Golgi apparatus: stores, changes and packs materials and sends them out; also forms lysosomes.
- Lysosomes: bags of digestive enzymes that clean up waste and worn-out parts. If the cell is badly damaged they burst and digest the cell itself, so they are called suicide bags.
- Vacuoles: storage sacs. Very large in plant cells (can fill most of the cell) and keep the cell firm (turgid).
Diffusion and osmosis
Diffusion: particles spread on their own from where they are more crowded to where they are less crowded. Oxygen enters a cell and carbon dioxide leaves it by diffusion.
Osmosis: movement of water through a selectively permeable membrane from a region of more water (dilute solution) to a region of less water (concentrated solution).
Three kinds of outside solution
- Hypotonic (more dilute outside, e.g. pure water): water goes in, the cell swells. Animal cells may burst; plant cells stay firm because of the wall.
- Isotonic (same strength): no net movement, no change.
- Hypertonic (more concentrated outside, e.g. salty water): water comes out, the cell shrinks. In plant cells the contents pull away from the wall – this is plasmolysis.
Cells also take in food by endocytosis: amoeba wraps its membrane around food. This works because the membrane is flexible.
Try it at home
Take two bowls: one with plain water, one with very salty water. Put 5 raisins in each. After 2 hours compare. Predict first, then check.
Cell division: mitosis and meiosis
New cells are needed to grow, to replace old or injured cells and to make sex cells (gametes).
| Mitosis | Meiosis |
|---|---|
| 1 cell → 2 daughter cells | 1 cell → 4 daughter cells (two divisions) |
| Same number of chromosomes as the mother cell | Half the number of chromosomes |
| Daughter cells are identical | Daughter cells are different |
| For growth and repair (skin, root tip) | Only in reproductive organs to make sperm and eggs |
Why half? When a sperm and an egg join, the halves add up to the full number again. In humans: body cells 46 chromosomes, sex cells 23.
Key formulas and definitions
- Cell = basic structural and functional unit of life
- Prokaryote: no nuclear membrane (nucleoid); Eukaryote: true nucleus
- Plant cell only: cell wall, chloroplasts, large vacuole
- Mitochondria = powerhouse (ATP); Lysosome = suicide bag
- Osmosis: water moves from dilute to concentrated through a selectively permeable membrane
- Hypotonic → swells; Isotonic → no change; Hypertonic → shrinks (plasmolysis)
- Mitosis: 1 → 2 identical, same chromosome number; Meiosis: 1 → 4, half the number
Worked examples
1. A cell has no nuclear membrane and no mitochondria. Is it prokaryotic or eukaryotic? Name one example.
No nuclear membrane means the DNA is loose in a nucleoid, and no membrane-bound organelles. So it is prokaryotic, e.g. a bacterium.
2. A cell has a cell wall and a large vacuole but no chloroplasts. Can it still be a plant cell?
Yes. A cell wall and large central vacuole show a plant cell. Cells in roots are not green, so they have no chloroplasts (they may have leucoplasts to store starch).
3. Why do raisins swell in water but a salted mango piece becomes watery?
Raisins are more concentrated inside than water, so water enters by osmosis (hypotonic outside). Salt outside the mango makes the outside more concentrated (hypertonic), so water leaves the cells.
4. Human body cells have 46 chromosomes. How many will each cell have after mitosis? After meiosis?
Mitosis keeps the number: 46 in each of 2 cells. Meiosis halves it: 23 in each of 4 cells.
5. Why can a plant cell kept in pure water not burst, but a red blood cell can?
Water enters both by osmosis. The plant cell's stiff cellulose wall pushes back and stops bursting (the cell becomes turgid). A red blood cell has no wall, so it keeps swelling and can burst.
6. If the Golgi apparatus of a cell stops working, what will be affected?
Golgi packs and sends out materials made by the ER and forms lysosomes. So packaging and secretion (like enzymes and hormones) will stop and new lysosomes will not form.
Common mistakes
- Saying Robert Hooke saw living cells. The cork cells he saw were dead; Leeuwenhoek first saw living cells.
- Thinking bacteria have no DNA. They do; it is just not enclosed in a nuclear membrane.
- Mixing up diffusion and osmosis. Osmosis is only about water moving through a selectively permeable membrane.
- Writing that meiosis gives 2 cells. Meiosis gives 4 cells with half the chromosomes; mitosis gives 2.