The first of the characteristics of life is that living things are made of cells. That idea took about two hundred years of looking through microscopes to settle, and today it is one of the foundations of biology: the cell theory.
Key idea
The cell theory has three parts. (1) All living things are made of one or more cells. (2) The cell is the basic unit of structure and function in living things. (3) All cells come from cells that already existed.
| Year | Scientist | What they found |
|---|---|---|
| 1665 | Robert Hooke | Looked at thin slices of cork and named the tiny boxes "cells". |
| 1670s | Antonie van Leeuwenhoek | Saw living single-celled organisms in pond water with his own lenses. |
| 1838 | Matthias Schleiden | Concluded that plants are made of cells. |
| 1839 | Theodor Schwann | Concluded that animals are made of cells too. |
| 1855 | Rudolf Virchow | Argued that every cell comes from another cell. |
Why it works
Every living thing scientists have examined, from bacteria to whales, turns out to be made of cells. And no one has ever seen a cell form from nonliving matter: new cells appear only when an existing cell divides. The theory holds because every test so far agrees with it.
Worked examples
Worked example: Which part?
A scientist watches one yeast cell split into two. Which part of the cell theory does this support?
- The observation is about where new cells come from.
- The two new cells came from one cell that already existed.
Answer: All cells come from existing cells.
Worked example: Is it made of cells?
Is a mushroom made of cells?
- A mushroom is a living thing (a fungus).
- Every living thing is made of cells.
Answer: Yes.
Common mistake
Thinking only animals, or only large living things, are made of cells, or that cells can form from mud or broth. Plants, fungi, protists and bacteria are all made of cells, and new cells come only from existing cells. Check: if it is alive, it is made of cells.