The Fundamental Unit of Life
In one line
Every living thing is made of cells, and each part inside a cell has its own job, like rooms in a house.
Key points
- Robert Hooke first saw cells in 1665, in a thin slice of cork. Leeuwenhoek first saw living cells, like bacteria, in pond water.
- Cell theory (Schleiden, Schwann, and later Virchow): all living things are made of cells, the cell is the basic unit of life, and new cells come only from existing cells.
- Unicellular organisms are a single cell (Amoeba, bacteria). Multicellular organisms have many cells (plants, animals, humans).
- Prokaryotic cells have no proper nucleus or membrane-bound organelles (bacteria). Eukaryotic cells have a nucleus with a membrane (plants, animals, fungi).
Parts of a cell and their jobs
| Part | Job |
|---|---|
| Plasma membrane | Outer covering that lets only some substances in and out (selectively permeable) |
| Cell wall (plants only) | Tough outer layer made of cellulose; gives shape and support |
| Nucleus | Control centre; holds chromosomes made of DNA, which carry genetic information |
| Mitochondria | "Powerhouse of the cell"; releases energy as ATP |
| Endoplasmic reticulum (ER) | Network of tubes for transport. Rough ER has ribosomes and makes proteins; smooth ER makes fats |
| Ribosomes | Make proteins |
| Golgi apparatus | Packages and sends out materials made by the cell |
| Lysosomes | Break down waste and worn-out parts; called "suicide bags" |
| Plastids (plants) | Chloroplasts contain chlorophyll for photosynthesis |
| Vacuole | Stores water and substances; very large in plant cells |
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Plant cell vs animal cell
| Plant cell | Animal cell | |
|---|---|---|
| Cell wall | Present | Absent |
| Chloroplasts | Present | Absent |
| Vacuole | One large central vacuole | Small or none |
| Shape | Usually fixed, box-like | Usually round or irregular |
How things move in and out
- Diffusion
- Particles spread from where there are more of them to where there are fewer. Oxygen and carbon dioxide move this way.
- Osmosis
- Water moves through a selectively permeable membrane from a dilute solution to a concentrated one. A raisin swells in water because of osmosis.
- Plasmolysis
- A plant cell loses water in a strong salt or sugar solution, and its contents shrink away from the cell wall.
Common mistakes
- "Animal cells have a cell wall." Only plant, fungal and bacterial cells do.
- Mixing up plasma membrane and cell wall. All cells have a membrane; the wall is an extra layer outside it.
- "Bacteria have no DNA." They do, but it isn't enclosed in a nuclear membrane.
Quick check
Revise in 30 seconds
| Discovery | Hooke 1665 (cork) · Leeuwenhoek (living cells) |
|---|---|
| Prokaryote | No nuclear membrane (bacteria) |
| Eukaryote | True nucleus (plants, animals, fungi) |
| Powerhouse | Mitochondria |
| Protein makers | Ribosomes |
| Plant only | Cell wall, chloroplasts, large vacuole |
| Osmosis | Water moves from dilute to concentrated through a membrane |