Explain why the resolution of a transmission electron microscope (TEM) is far greater than that of a light microscope, and explain how this improved resolution allows biologists to study cell ultrastructure in detail.

OCR A-Level Biology A (H420) — 2.1 Cell structure · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A researcher wishes to investigate the internal membrane systems of a pancreatic exocrine cell, including structures such as the rough endoplasmic reticulum and Golgi apparatus. They must choose between a light microscope and a transmission electron microscope for their study.

Model answer (4 marks)

A transmission electron microscope (TEM) uses a beam of electrons instead of visible light. Electrons have a de‑Broglie wavelength of about 0.005 nm at 200 kV, which is roughly 10 000 times shorter than the 400–700 nm wavelength of visible light.

Because the resolving power of a microscope is limited by the wavelength of the radiation used, the much shorter electron wavelength gives a far higher resolution – the minimum distance that can be distinguished is only a few tenths of a nanometre.

With this higher resolution, structures that are far below the light‑microscope limit (≈200 nm) can be seen. Ribosomes, the unit membranes of the rough endoplasmic reticulum, the cis‑ and trans‑cisternae of the Golgi apparatus, and the cristae of mitochondria become distinguishable.

Thus biologists can examine the precise arrangement and fine detail of internal membrane systems, resolving closely spaced membranes as separate structures and revealing the true ultrastructure of the cell.

Examiner tips

  • Use the key phrase ‘electrons have a much shorter wavelength than visible light’ to show understanding of the physics.
  • Explain that resolution is limited by wavelength – shorter wavelength = higher resolution – to link to the first point.
  • Give specific examples of sub‑micron structures that become visible (ribosomes, unit membranes, cristae).
  • Mention that this allows the study of arrangement and fine detail of organelles.

Common mistakes

  • Confusing the type of microscope (light vs. electron) or stating that light microscopes have higher resolution.
  • Failing to mention the wavelength or the de‑Broglie wavelength of electrons.
  • Giving examples that are larger than the light‑microscope limit (e.g. whole cells) instead of sub‑micron structures.

Mark scheme (4 marks)

  1. The TEM uses electrons rather than light, and electrons have a much shorter wavelength than visible light.
  2. Resolution is limited by the wavelength of the radiation used; a shorter wavelength gives a higher resolution / smaller minimum resolvable distance.
  3. The higher resolution of the TEM means that structures smaller than the resolution limit of a light microscope can be distinguished, such as unit membranes / ribosomes / cristae.
  4. Biologists can therefore study the arrangement and fine detail of internal membrane systems (e.g. the rough ER or Golgi apparatus) because closely adjacent membranes can be resolved as distinct structures.

Key terms in this question

resolution · transmission electron microscope · ultrastructure

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