The cell theory

The smallest unit of life

What is the smallest thing that can be called alive? A single cell. An Amoeba is one cell, and it feeds, moves, grows and divides on its own. Nothing smaller than a cell can live independently.

• Unicellular organisms are a single cell. They can carry out every function of life alone. Bacteria, Amoeba and Chlamydomonas are examples.
• Multicellular organisms, like you, are made of many cells. The cells share out the work of the body.

So the cell is the fundamental structural and functional unit of all living organisms.

Seeing the first cells

Cells are far too small for the eye. Their story begins with the microscope.

• Robert Hooke, in 1665, looked at a thin slice of cork. He saw tiny empty boxes and called them cells. They were the walls of dead plant cells.
• Anton von Leeuwenhoek made fine lenses. He was the first to see and describe a live cell, in pond water.
• Robert Brown, in 1831, discovered the nucleus.

Building the cell theory

The microscope kept improving. Two Germans then looked at many tissues.

• Matthias Schleiden, a botanist, studied a large number of plants in 1838. He saw that all plants are made of different kinds of cells.
• Theodor Schwann, a zoologist, studied animal cells in 1839. He reported a thin outer layer, now called the plasma membrane. He also saw that a cell wall is a feature of plant cells only.

Schwann then proposed that the bodies of animals and plants are made of cells and their products. Schleiden and Schwann together gave the cell theory.

The first theory had a gap. It did not say how new cells form.

Virchow fills the gap

In 1855, Rudolf Virchow explained it. He said that cells divide, and new cells come from cells that already exist. He put it in Latin: *Omnis cellula-e cellula*, every cell from a cell.

He reshaped the theory into its final form. The cell theory today says:

1. All living organisms are made of cells and the products of cells.
2. All cells arise from pre-existing cells.

Making cells look bigger

A microscope shows a small cell as a larger image. The magnification tells how many times larger. The image width is the real width multiplied by the magnification.

Cells are measured in micrometres, µm. One micrometre is a thousandth of a millimetre: $1000\ \mu\text{m} = 1\ \text{mm}$. Smaller parts are measured in nanometres: $1000\ \text{nm} = 1\ \mu\text{m}$.

You should find that a $10\ \mu\text{m}$ cell at a magnification of $400$ looks $4\ \text{mm}$ wide. Halve the magnification and the image halves too.

Try it in Python

The program sorts the discoveries by year, prints them, and finds how long the cell theory took to reach its final form.

Practise with new numbers

Common mistakes

• Thinking Hooke saw living cells. Cork cells are dead; he saw their walls. Leeuwenhoek first saw live cells.
• Giving Schleiden the animal cells. Schleiden studied plants and Schwann studied animals.
• Leaving Virchow out of the cell theory. He added that every cell arises from a cell.
• Forgetting to change units. An image in millimetres comes from a real size in micrometres.

Remember these

• The cell is the structural and functional unit of life.
• Hooke named cells; Leeuwenhoek saw live cells; Brown found the nucleus.
• Schleiden (plants) and Schwann (animals) gave the cell theory.
• Virchow: Omnis cellula-e cellula. The modern theory: all organisms are made of cells, and all cells come from cells.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why is a single Amoeba called a complete organism?

2. Who first saw and described a live cell?

3. What did Schwann say is found only in plant cells?

4. What gap in the first cell theory did Virchow fill?

5. How many micrometres make one millimetre?