Cells: the units of life
The cell theory, developed in the 1800s, states that all living things are made of one or more cells, that the cell is the basic unit of life, and that new cells come only from existing cells. Animal cells areeukaryotic: their DNA is enclosed in a nucleus, and they contain membrane-bound organelles, each specialised for a particular job — like departments in a factory.
The main organelles
| Organelle | Job |
|---|---|
| Cell membrane | Controls what enters and leaves the cell |
| Nucleus (with nucleolus) | Stores DNA and controls the cell; the nucleolus makes ribosome parts |
| Mitochondria | Release energy by aerobic respiration |
| Ribosomes | Build proteins |
| Rough endoplasmic reticulum | Folds and transports proteins made by its ribosomes |
| Smooth endoplasmic reticulum | Makes lipids and breaks down toxins |
| Golgi apparatus | Modifies, sorts and packages proteins into vesicles |
| Lysosomes | Digest worn-out organelles and engulfed material |
| Centrioles | Organise the spindle during cell division |
All of these sit in the cytoplasm, a jelly-like fluid where many chemical reactions happen. A network of protein fibres, the cytoskeleton, gives the cell its shape and moves organelles around.
A protein’s journey
Organelles work together. Here is how a cell makes and exports a protein such as a digestive enzyme or hormone:
- The nucleus copies the gene into messenger RNA, which leaves through a nuclear pore.
- Ribosomes on the rough ER read the RNA and build the protein.
- The protein is folded inside the rough ER and carried in a vesicle to the Golgi apparatus.
- The Golgi modifies and packages it into a new vesicle.
- The vesicle fuses with the cell membrane and releases the protein outside the cell.
Mitochondria supply the energy (ATP) needed at every step.
How small is a cell?
A typical animal cell is about 10–30 micrometres (µm) across — roughly 1/50 of a millimetre. A mitochondrion is about 1 µm long, and a ribosome only about 25 nanometres. That is why cells were not discovered until microscopes were invented: Robert Hooke first described “cells” in cork in 1665. Organelles such as ribosomes can only be seen with an electron microscope.
Prokaryotic and eukaryotic cells
Bacteria are prokaryotic: they are much smaller, have no nucleus (their DNA floats in the cytoplasm) and lack membrane-bound organelles, though they do have ribosomes. Animals, plants, fungi and protists are made of eukaryotic cells like the one in this model.
Key terms
- Organelle
- A specialised structure within a cell that performs a particular job.
- Cytoplasm
- The jelly-like material inside the cell membrane, surrounding the nucleus.
- Eukaryotic cell
- A cell with a nucleus and membrane-bound organelles.
- ATP
- Adenosine triphosphate, the molecule that carries energy for cell processes.
- Vesicle
- A small membrane bubble that transports substances within the cell.
Why it matters
- Medicine: many diseases involve faulty organelles — for example, some inherited disorders affect mitochondria or lysosomes (educational context only).
- Antibiotics: some target bacterial ribosomes, which differ from ours, so they harm bacteria but not human cells.
- Specialised cells: muscle cells are packed with mitochondria; gland cells have extensive ER and Golgi to make secretions.
- Biotechnology: cells are used as factories to make insulin and vaccines.
Common misconceptions
- “Cells are flat, like textbook diagrams.” They are three-dimensional and crowded with organelles.
- “The nucleus is the brain of the cell and does everything.” Organelles divide the work; the nucleus provides instructions.
- “All cells look the same.” Cells vary hugely in shape and contents depending on their function.
Key takeaways
- Animal cells are eukaryotic, with a nucleus and membrane-bound organelles.
- Each organelle has a specific job; together they make, package and export products.
- Mitochondria release energy; ribosomes build proteins; the Golgi packages them.
- A cell’s structure reflects its function.
Model note: organelles are shown larger and fewer than in a real cell so that each can be seen and selected.