Cell Theory and Microscopes

In short: Cell theory says all living things are made of cells, the cell is the basic unit of life, and all cells come from existing cells. Light microscopes show living cells in colour, electron microscopes show far finer detail, and magnification is image size divided by actual size.

Starting point: what you already know

• Living things are made of cells, from Year 7 and 8 science.
• Plant cells have a cell wall and chloroplasts. Animal cells do not.
• Units of length: $1\text{ mm}=1000\mu \text{m}$ (micrometres).

Cell theory

Over about 200 years, many scientists added evidence until three ideas were accepted. Together they are called cell theory.

The third idea came from Rudolf Virchow in 1855. It replaced the old belief in spontaneous generation, the idea that life could appear from non-living things like mud or rotting meat. Louis Pasteur's swan neck flask experiment (1859) showed that broth stayed clear unless microbes from the air could reach it.

Microscopes

Resolution is the ability to see two close points as separate. A blurry picture made bigger is still blurry. That is why electron microscopes, with much better resolution, show organelles like ribosomes that light microscopes cannot.

There are two kinds of electron microscope. A transmission electron microscope (TEM) sends electrons through a thin slice and shows the inside of a cell. A scanning electron microscope (SEM) bounces electrons off the surface and gives 3D images of the outside.

Magnification and size

$$\text{magnification}=\frac{\text{image size}}{\text{actual size}}$$

Both sizes must be in the same units.

On a light microscope, total magnification = eyepiece × objective. A $\times 10$ eyepiece with a $\times 40$ objective gives $\times 400$.

Two kinds of cells

Quick summary

• Cell theory: all living things are made of cells, the cell is the basic unit of life, and all cells come from cells.
• Light microscopes view living cells in colour up to about $\times 1500$. Electron microscopes have far better resolution but need dead specimens.
• $\text{magnification}=\frac{\text{image size}}{\text{actual size}}$, with both sizes in the same units.
• Prokaryotes have no nucleus and no membrane-bound organelles. Eukaryotes have both.

Quiz