Microscopy: explain how electron microscopy has increased understanding of sub-cellular structures.

The idea

Light microscopes use visible light with a relatively long wavelength, so their resolution is limited and organelles smaller than about 200 nm cannot be distinguished. Electron microscopes use a beam of electrons with a much shorter wavelength, giving both higher magnification and higher resolution. This lets biologists see organelles inside cells, such as mitochondria, ribosomes, the internal membranes of chloroplasts and the details of the nucleus, and also study viruses and the surfaces of cells. For example, a transmission electron microscope reveals the folded inner membrane (cristae) of a mitochondrion, which a light microscope shows only as a faint granule. Seeing these structures allows scientists to link each organelle's structure to its function, so understanding of sub-cellular structures has increased.

What earns the marks

• States that light microscopes use visible light, whereas electron microscopes use a beam of electrons.
• Explains that electrons have a much shorter wavelength than visible light, so the resolution of an electron microscope is much higher.
• Links higher resolution to seeing smaller sub-cellular structures, such as ribosomes, mitochondria and internal membranes, that a light microscope cannot resolve.
• Explains that higher magnification allows images of organelles to be enlarged more without becoming blurred, so fine detail can be studied.
• Uses a named example, such as cristae in mitochondria or thylakoid membranes in chloroplasts, to show how electron microscopy revealed structure linked to function.
• Recognises that electron microscopes can also show viruses and cell surface detail, extending understanding beyond structures visible with light.
• Compares the two microscopes explicitly, rather than describing electron microscopy alone.

Common mistakes and how to put them right

• Saying electron microscopes simply 'magnify more' without mentioning resolution.

  Put it right: Magnification enlarges an image, but resolution is the ability to distinguish two close points, and the short electron wavelength is what improves resolution.

• Confusing the roles of magnification and resolution.

  Put it right: State that increased magnification is useful only if resolution is also sufficient, otherwise the image is enlarged but blurred.

• Claiming electron microscopes can view living cells.

  Put it right: Samples are usually dead and prepared in a vacuum, so electron microscopy shows fixed, non-living material.

• Writing that electrons are 'smaller than light' rather than having a shorter wavelength.

  Put it right: It is the much shorter wavelength of electrons that gives the higher resolution.

Exam tips

• Plan a comparison: one sentence on light microscopy limits, one on electron microscopy, then a named organelle example.
• Use the words 'resolution' and 'magnification' correctly and separately; examiners look for the distinction.
• Include a specific sub-cellular structure, such as ribosomes or cristae, to move from a general answer to a developed explanation.
• Check that every claim about seeing smaller structures is tied back to the shorter wavelength of electrons.