Cell theory and structures common to all cellsSpec A2.2.1, A2.2.4
In short
Cells are the basic structural unit of all living organisms: every organism consists of one or more cells. Typical cells share three structures: DNA as the genetic material, a cytoplasm composed mainly of water, and a plasma membrane composed of lipids that encloses the cytoplasm. Each is needed for the cell to function and reproduce.
Cell theory is one of the most important theories in biology. It states that:
- All living organisms are composed of one or more cells.
- Cells are the basic structural unit of all living organisms, and the smallest unit of life.
- Cells only arise from pre-existing cells.
Cell theory was built up from a very large number of observations made with microscopes. It can then be used to make predictions: a newly discovered organism can be predicted to consist of one or more cells.
Nature of science: deductive reasoning generates predictions from a theory. If an observation does not fit, such as cells with many nuclei or none, the theory may need refining.
Structures common to cells in all living organisms
| Structure | Reason it is needed |
|---|---|
| DNA as genetic material | Stores the instructions for making proteins, including enzymes, and can be replicated so that copies pass to daughter cells |
| Cytoplasm composed mainly of water | Water is the solvent in which most metabolic reactions happen; enzymes and substrates dissolve and move in it |
| Plasma membrane composed of lipids | A hydrophobic barrier that encloses the cytoplasm, keeps its contents in and controls what enters and leaves |
All cells also contain ribosomes for protein synthesis, although ribosomes in prokaryotes are smaller than those in the cytoplasm of eukaryotes.
Written and checked against the IB Biology SL specification · Updated October 2026