Cells, Their Parts and Their Size

A brief note on Cells, Their Parts and Their Size. Try each question at the end before you open its answer.

The cell theory

• Hooke named the cell in 1665; Leeuwenhoek first saw living cells.
• All living things are made of cells; the cell is the basic unit of life; all cells come from other cells.

Size and magnification

• $1\ \text{mm} = 1000\ \mu\text{m}$. A bacterium is about $2\ \mu\text{m}$, a leaf cell about $50\ \mu\text{m}$.
• Size in the picture = actual size $\times$ magnification. Actual size = size in the picture $\div$ magnification.

Parts and their jobs

Plant cells also have one large vacuole. Bacteria are prokaryotes: no nucleus with a membrane. Plants, animals and fungi are eukaryotes.

Why cells are small

For a cube of side $L$, surface area $\div$ volume $= \dfrac{6}{L}$. A bigger cell has less surface for each part of its volume, so cells divide instead of growing large.

A worked example

A guard cell is $40\ \mu\text{m}$ wide.

1. At 400 times. $40\ \mu\text{m} \times 400 = 16\,000\ \mu\text{m} = 16\ \text{mm}$.
2. As a cube. Its ratio is $\dfrac{6}{40} = 0.15$ per µm, far less than a bacterium's $\dfrac{6}{2} = 3$ per µm.

Practice with new numbers

A program to try

The program works out the worked example.

Mistakes to avoid

• Mixing up micrometres and millimetres. 1 mm is 1000 µm.
• Saying a plant cell has a wall instead of a membrane. It has both.
• Mixing up mitochondria and chloroplasts. One releases energy, the other makes food.
• Thinking all plant cells have chloroplasts. Root cells do not.

Check yourself

1. A cell is $15\ \mu\text{m}$ wide. How wide does it look at 200 times?

2. Which part would you find in a bacterium: a nucleus with a membrane, mitochondria, or a cell wall?

3. Which organelle would a cell that makes a lot of protein have many of?

4. A cube cell's side doubles. What happens to its ratio of surface area to volume?