Explain why the resolution of a transmission electron microscope is greater than that of a light microscope, and explain how this difference in resolution has contributed to our understanding of cell ultrastructure.

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).

Scientists studying cell organelles must choose appropriate microscopy techniques depending on the level of detail required. A transmission electron microscope (TEM) can resolve structures separated by as little as 0.1 nm, whereas a light microscope has a resolution limit of approximately 200 nm.

Model answer (4 marks)

Resolution is limited by the wavelength of the radiation used. Electrons have a wavelength of about 0.005 nm at 200 kV, far shorter than visible light (≈400–700 nm). A shorter wavelength gives a smaller diffraction limit, so objects only 0.1 nm apart can be distinguished, whereas light microscopy is limited to ≈200 nm.

Because TEM can resolve such small distances, sub‑cellular structures that are too small for light microscopy – for example the cristae of mitochondria, ribosomes, the detailed arrangement of membrane proteins – can be visualised. This has allowed a detailed description of organelle structure, the fluid‑mosaic model of membranes and the relationship between structure and function at the ultrastructural level.

Examiner tips

  • Mention the wavelength difference first, then the diffraction limit, then give a concrete example of a structure only visible by TEM.
  • Link the higher resolution to a specific advance in cell biology (e.g. mitochondria, ribosomes, membrane model).

Common mistakes

  • Confusing resolution with magnification; stating that TEM is simply more powerful without explaining wavelength.
  • Failing to give a specific example of an ultrastructure that TEM reveals, or not linking it to functional understanding.

Mark scheme (4 marks)

  1. Resolution is limited by the wavelength of the radiation used, and electrons have a much shorter wavelength than visible light.
  2. A shorter wavelength means closer objects can be distinguished as separate, giving greater resolving power.
  3. The greater resolution of the TEM has allowed subcellular structures / organelles that are too small to be seen with a light microscope to be visualised.
  4. This has led to a more detailed understanding of organelle function / the fluid-mosaic model of membranes / the relationship between structure and function at the ultrastructural level.

Key terms in this question

resolution · transmission electron microscope · ultrastructure

Related

More Cell structure questions

▶ Try answering this question with AI marking (free) →