This means that it can be used to study cells in much finer detail.

The idea

Because electrons have a much shorter wavelength than visible light, an electron microscope resolves points that are only about 0.2 nm apart, whereas a light microscope cannot separate points closer than about 200 nm. This thousand-fold improvement in resolving power means structures that blur together under a light microscope become sharp and separate in an electron micrograph. For instance, a light microscope shows a mitochondrion as a faint oval, but an electron micrograph reveals its double membrane and folded cristae. Similarly, the internal membranes of a chloroplast or the detail of a synapse become visible. Finer detail is therefore a direct consequence of better resolution, not merely of greater magnification, and it allows biologists to relate structure to function more precisely.

What earns the marks

• Electrons have a much shorter wavelength than visible light, so the resolution limit is far smaller.
• Resolution of about 0.2 nm allows points that a light microscope merges to be seen as separate.
• Finer detail includes internal membranes and organelles such as cristae in mitochondria and thylakoids in chloroplasts.
• The improvement follows from resolving power, not from magnification alone, because enlarging a blurred image adds no new information.
• Finer structural detail helps biologists link sub-cellular structure to its function.
• Specimens for electron microscopy are usually dead and dehydrated, which is a limitation when studying living cells.

Common mistakes and how to put them right

• Claiming that an electron microscope simply magnifies more.

  Put it right: Magnification without improved resolution would only produce a larger blur, so the key gain is resolving power.

• Assuming all cell structures are visible with a light microscope.

  Put it right: Organelles such as ribosomes and the internal details of mitochondria are below its resolution limit.

• Forgetting that electron microscopy usually requires dead, dehydrated specimens, so dynamic processes in living cells cannot be observed directly.

Exam tips

• Link the finer detail explicitly to the shorter wavelength of electrons rather than stating it as an isolated fact.
• Give a named example of detail revealed, such as cristae or thylakoid membranes, to show understanding.
• When interpreting an electron micrograph, use scale bars and known organelle shapes to identify structures rather than guessing from size alone.