Cell Structure and Function · Grade 10

What can a microscope show you, and what can it not?

A light microscope can magnify almost without limit, but its resolution stops at about two hundred nanometres — roughly the wavelength of light. Past that, magnifying only blurs. Small organelles were therefore only discovered with the electron microscope, and cell theory was built exactly at the edge of what could be seen.

Learning objectives

Cell theory, and what stood behind it

Cell theory makes three claims: all living things are made of cells, the cell is the basic unit of life, and every cell comes from an existing cell. Each rests on a particular observation somebody managed to make.

The first two became possible once microscopes were good enough to show cells in cork, in plants and in animal tissue. The third came later, when it turned out cells do not simply appear in a sterilised broth.

That is why the microscope is not a footnote to biology. What could be seen determined what could be claimed.

Magnification is not resolution

Magnification is how large something appears. Resolution is the smallest separation at which two points are still seen as two rather than as one blur.

A light microscope is limited to a resolution of roughly two hundred nanometres, because light itself is a wave of about that length. You can magnify a thousand or two thousand times — the image only becomes larger and blurrier.

An electron microscope uses a beam whose wavelength is far shorter, so it resolves detail a thousand times finer. Every image you have seen of a mitochondrion or a ribosome comes from one.

Why cells stay small

As a cell grows, its volume grows faster than its surface area. The volume is what needs supplying, and the surface is what supplies it.

So a cell that gets too large reaches a point where the membrane can no longer feed its interior. The answer is to divide, and it is one reason a large organism is built from many small cells rather than one big one.

Worked examples

  1. A cell appears 40 mm long in a photograph at 400× magnification. What is its real size?

    1. Real size equals image size divided by magnification
    2. 40 mm divided by 400
    3. That gives 0.1 mm

    Answer: 0.1 mm, which is 100 micrometres

  2. Can a ribosome be seen with a light microscope at very high magnification?

    1. A ribosome is under twenty nanometres
    2. A light microscope resolves about two hundred nanometres
    3. Extra magnification does not improve resolution

    Answer: No. Magnifying enlarges the blur, not the detail

  3. A cube of side 1 and a cube of side 2 — which has the larger surface-to-volume ratio?

    1. Side 1: surface 6, volume 1, ratio 6
    2. Side 2: surface 24, volume 8, ratio 3
    3. The larger cube ended with the smaller ratio

    Answer: The small one. And that is exactly why cells stay small

Common mistakes

Assuming stronger magnification shows more
Beyond the resolution limit, magnifying only enlarges the blur. Two points that have merged will not separate because you made them bigger.
Dividing by the magnification the wrong way
The real size is always smaller than what appears in the image. A result larger than the image means you multiplied where you should have divided.
Thinking a large cell simply produces more
Volume outgrows surface area, so supply cannot keep up with demand. Past a certain size the cell has to divide.

What to remember

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