Course 1.1Lesson 1 of 3
Robert Hooke gave the cell its name. All living things are made of cells, and every cell comes from another cell. Cells are measured in micrometres, and a microscope makes them look bigger: size in the picture = actual size × magnification.
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Cork is the light, soft bark of a tree. In 1665, a scientist called Robert Hooke cut a very thin slice of cork. He looked at it with a microscope that he had built himself.
He saw many small boxes, joined side by side, like the rooms of a honeycomb. He called each box a cell. The word comes from a Latin word for a small room.
What Hooke saw were only the walls. Cork is dead tissue, and the living parts of its cells were gone. But the name stayed. Today a cell is the small unit that makes up every living thing.
Around 1674, Anton van Leeuwenhoek made very good lenses. He looked at a drop of pond water and saw tiny creatures that swam. These were the first living cells that anyone had seen. Later he also saw bacteria.
Many scientists looked at plants and animals during the next 200 years. Their ideas together became the cell theory.
We can write the cell theory in three lines.
Some living things have only one cell, such as an amoeba or a bacterium. The one cell does all the work: it eats, breathes, grows and makes new cells.
A human being has many trillions of cells, and different kinds do different jobs. A nerve cell carries messages. A red blood cell carries oxygen. A muscle cell makes the body move.
Most cells are too small to see. We measure them in micrometres, written µm.
| Cell | About how wide |
|---|---|
| Bacterium | 2 µm |
| Red blood cell | 7 µm |
| Plant leaf cell | 50 µm |
| Human egg cell | 100 µm |
The smallest thing that you can see without a microscope is about 100 µm wide. So you can only just see a human egg cell.
A microscope makes things look bigger. The number of times that it makes them bigger is the magnification.
A plant cell is 50 µm wide. Under a magnification of 400 times, it looks wide. That is 20 mm, about the width of your thumb. For the actual size, we divide: actual size = size in the picture ÷ magnification.
You should find that a bacterium looks less than 1 mm wide even at 400 times, while an egg cell nearly fills the picture. That is why bacteria need the strongest microscopes.
This program finds how many cells of each kind fit across 1 mm, and how wide each looks under a microscope.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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