Students should be able to: • understand how microscopy techniques have…

The idea

Microscopy has developed from simple single-lens instruments to powerful electron microscopes. Early light microscopes, refined through the 17th century, allowed cells and some nuclei to be seen, but their resolution is limited by the wavelength of visible light to roughly 200 nm, so many sub-cellular structures remain indistinct. Electron microscopes use a beam of electrons with a much shorter wavelength, giving far greater resolution and useful magnification. Transmission electron microscopes produce high-resolution 2D images of thin sections, revealing internal structures such as mitochondria, chloroplasts and ribosomes. Scanning electron microscopes produce detailed 3D surface images. This increased understanding lets biologists relate structure to function, for example the folded inner membrane of a mitochondrion to aerobic respiration, and ribosomes to protein synthesis.

What earns the marks

• State that light microscopes use visible light and lenses, and can magnify but have limited resolution, so only larger structures such as nuclei and some organelles are seen clearly.
• Explain that resolution is the ability to distinguish two close points as separate, and that the wavelength of light limits light microscope resolution to about 200 nm.
• Describe electron microscopes as using a beam of electrons, which has a much shorter wavelength than light, giving higher resolution and allowing greater useful magnification.
• Distinguish transmission electron microscopy, which gives high-resolution 2D images of internal structure in thin specimens, from scanning electron microscopy, which gives detailed 3D surface images.
• Link improved resolution to increased understanding of sub-cellular structures, for example mitochondria, chloroplasts, ribosomes and the internal membranes of cells.
• Relate a named sub-cellular structure to its function, such as mitochondria for aerobic respiration or ribosomes for protein synthesis, using evidence from electron micrographs.

Common mistakes and how to put them right

• Confusing magnification with resolution.

  Put it right: Magnification makes an image larger, whereas resolution is the ability to distinguish separate points; a blurred image can be magnified but not resolved.

• Believing that electron microscopes can be used to view living cells.

  Put it right: Specimens must be dead and prepared in a vacuum, so dynamic processes cannot be observed directly.

• Thinking that light microscopes cannot show any organelles.

  Put it right: They can show larger structures such as nuclei and sometimes chloroplasts, but not ribosomes or internal mitochondrial detail.

Exam tips

• Use the terms resolution and magnification precisely; define resolution when explaining why electron microscopes reveal more detail.
• When comparing microscope types, structure your answer around wavelength, resolution, image type and whether the specimen is living.
• Support explanations with a named sub-cellular structure and its function to show how better resolution increased understanding.