# Cell Theory and Microscopes

> What cell theory says, how light and electron microscopes work, magnification and cell size, and prokaryotic versus eukaryotic cells.

From HSC Biology on WhizzNotes, free study notes for NSW students: https://whizznotes.com/biology/cell-theory-and-microscopes/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The tiny boxes in a cork
In 1665 Robert Hooke looked at a thin slice of cork through a homemade microscope. He saw rows of tiny empty boxes. They reminded him of the small rooms monks slept in, called cells, so that is what he named them.

A few years later, Antonie van Leeuwenhoek, a cloth seller in the Netherlands, ground his own lenses and looked at pond water and the gunk on his teeth. He saw tiny things swimming about. He called them "animalcules". He was the first person to see bacteria.

Neither man knew it yet, but they had found the building blocks of every living thing.

**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**.

### Key idea: Cell theory
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.

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

| | Light microscope | Electron microscope |
|---|---|---|
| Uses | Light and glass lenses | A beam of electrons and magnets |
| Best magnification | about 1500 times | over 1 000 000 times |
| Resolution (smallest detail seen) | about $0.2 \ \mu\text{m}$ | about $0.0002 \ \mu\text{m}$ |
| Living specimens? | Yes | No, specimens are dead and in a vacuum |
| Colour | Real colour or stains | Black and white (colour is added later) |
| Cost and size | Cheap, small | Very expensive, fills a room |

**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.

### Worked example: How big is the cell really?
A cell in a photo is 30 mm long. The magnification is $\times 600$. What is its actual length in micrometres?

$$\text{actual size} = \frac{\text{image size}}{\text{magnification}} = \frac{30 \text{ mm}}{600} = 0.05 \text{ mm}$$

$$0.05 \text{ mm} \times 1000 = 50 \ \mu\text{m}$$

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

## Two kinds of cells

*Diagram: A prokaryotic cell with loose circular DNA and no nucleus next to a larger eukaryotic cell with DNA inside a nucleus and other organelles*
*Prokaryotes keep their DNA (red) loose in the cytoplasm. Eukaryotes keep it inside a nucleus and also have membrane-bound organelles such as mitochondria (orange). Grey dots are ribosomes.*

| | Prokaryotic | Eukaryotic |
|---|---|---|
| Examples | Bacteria, archaea | Animals, plants, fungi, protists |
| Nucleus | No. DNA is in a loop in the cytoplasm | Yes. DNA is inside a nuclear membrane |
| Membrane-bound organelles | None | Many (mitochondria, chloroplasts, and more) |
| Ribosomes | Small (70S) | Larger (80S) |
| Size | Small, about 1 to 10 µm | Larger, about 10 to 100 µm |
| Extras | Often plasmids, a cell wall, sometimes a flagellum | Plants and fungi have cell walls |

### Exam tip: Memory hook
**Pro** sounds like "no": **pro**karyotes have **no** nucleus. **Eu** sounds like "do": **eu**karyotes **do**.

**Mind map**

```text
Cell theory and microscopes
  Cell theory
    All living things are cells
    Cell is the basic unit
    Cells come from cells
  Microscopes
    Light: living, colour, ×1500
    Electron: dead, high resolution
    TEM inside, SEM surface
  Size
    magnification = image ÷ actual
    1 mm = 1000 µm
  Cell types
    Prokaryotic: no nucleus
    Eukaryotic: nucleus and organelles
```

### Year 11: Where did eukaryotic cells come from?
The **endosymbiotic theory** says mitochondria and chloroplasts were once free-living prokaryotes. Long ago they were taken in by a larger cell and lived on inside it, helping each other. The evidence:

- Mitochondria and chloroplasts have their **own circular DNA**, like bacteria.
- They have **70S ribosomes**, like bacteria.
- They have a **double membrane** (their own plus the one from being engulfed).
- They divide by splitting in two, like bacteria.

Biologists also use modern tools beyond the simple microscope: **fluorescence microscopy** (glowing labels mark one kind of molecule) and **confocal microscopy** (sharp slices through thick samples that a computer stacks into 3D).

### Year 12: Why the model of the cell keeps changing
Cell theory is a good example of how science works. Each new tool, from Hooke's lens to the electron microscope to cryo-electron microscopy (which won the 2017 Nobel Prize in Chemistry), let scientists see more and update the model. In the HSC you may be asked how technology changed our understanding. Name the technology, say what new detail it revealed, and link it to the change in the model.

## 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.
- $\text{magnification} = \dfrac{\text{image size}}{\text{actual size}}$, with both sizes in the same units.
- Prokaryotes have no nucleus and no membrane-bound organelles. Eukaryotes have both.

## Quiz

Try these questions from the chapter quiz.

1. Who first used the word "cell" for the tiny boxes he saw in cork?
   - A. Robert Hooke
   - B. Louis Pasteur
   - C. Charles Darwin
   - D. Rudolf Virchow
   - Answer: A. Robert Hooke. Hooke named them in 1665.

2. Which is part of cell theory?
   - A. All cells have a nucleus
   - B. All cells come from cells that already existed
   - C. Cells can form from non-living matter
   - D. Only animals are made of cells
   - Answer: B. All cells come from cells that already existed. New cells come only from existing cells.

3. Which idea did Pasteur's swan neck flask experiment disprove?
   - A. Cell theory
   - B. Spontaneous generation
   - C. Natural selection
   - D. Endosymbiosis
   - Answer: B. Spontaneous generation. Broth stayed clear unless microbes could reach it.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/biology/cell-theory-and-microscopes/#quiz