Every living thing — a blade of grass, a whale, you — is built from microscopic units called cells. You are made of roughly 37 trillion of them, and each one is a tiny, self-contained machine doing a specific job. Because a cell is the smallest thing that counts as "alive", biologists call it the basic unit of life.
What each part does
A cell isn't just a bag of jelly — it's packed with tiny structures called organelles, each with its own job. It helps to picture the cell as a factory:
- Nucleus — the control centre. It holds the cell's DNA: the coded instructions that tell the cell what to do and how to build every protein. (The manager's office, holding the master plans.)
- Cytoplasm — the jelly-like fluid that fills the cell and where most chemical reactions take place. (The factory floor where the work gets done.)
- Cell membrane — the thin outer skin. It is partially permeable, carefully controlling what comes in (like oxygen and glucose) and what goes out (like waste). (The security gate.)
- Mitochondria — the cell's power stations, where aerobic respiration releases energy from glucose. Very active cells, such as muscle cells, are packed with them.
- Ribosomes — the tiny sites where proteins are made, following the instructions copied from the DNA.
Animal cells vs plant cells
All cells share the five parts above. Plant cells add three more, because a plant has jobs an animal doesn't: it has no skeleton (so it needs support) and it makes its own food.
| Part | Animal | Plant |
|---|---|---|
| Nucleus, cytoplasm, membrane | ✓ | ✓ |
| Mitochondria & ribosomes | ✓ | ✓ |
| Cell wall | ✗ | ✓ |
| Chloroplasts | ✗ | ✓ |
| Permanent vacuole | ✗ | ✓ |
- Cell wall — a tough outer layer of cellulose that gives the cell strength and shape (it's why plant stems can stand upright).
- Chloroplasts — green organelles full of chlorophyll; this is where photosynthesis makes food from sunlight.
- Permanent vacuole — a large sac of cell sap that keeps the cell firm and the plant standing up.
Cells with special jobs
As an organism grows, many cells specialise — their shape changes to suit a particular job. This is called differentiation:
- Red blood cells are flat discs with no nucleus, leaving more room to carry oxygen.
- Nerve cells are long and thin to carry electrical signals across the body.
- Root hair cells have a long "finger" that increases the surface area for absorbing water.
How do we actually see cells?
Cells are far too small for the naked eye, so we use a microscope. A school light microscope magnifies up to about ×1500; a powerful electron microscope reaches around ×500,000 and reveals even the smallest organelles. Because cells are so small we measure them in micrometres (µm) — and there are 1000 µm in a single millimetre. A typical animal cell is only about 20 µm across.
Memory hook: animal and plant cells share the basics — plants just add the "wall, chloroplast, vacuole" trio. And cells are tiny: a typical one is about 0.02 mm across, which is why you need a microscope to see it.