Electron Microscopes (OCR Gateway GCSE Biology: Combined Science)

• Light microscopes use light and lenses to form a magnified image (make the specimen look larger)
• With a light microscope it is possible to see images of cells and large sub-cellular structures (like nuclei and chloroplasts), although stains are often required to highlight certain parts of cells
• Light microscopes have limited magnification and resolution (how well two points that are close together can be distinguished)
  • Electron microscopes helped address these limitations
• The first electron microscopes were developed in the 1930s.
  • Electron microscopes use beams of electrons, rather than light, to visualise specimens
  • The wavelength of an electron beam is much smaller than that of visible light, which gives electron microscopes a much higher resolution and magnification
  • Electron microscopes allow smaller structures can be viewed in more detail
  • This has improved our understanding of sub-cellular structures such as the internal structure of mitochondria

• Transmission Electron Microscopes (TEMs) are the most powerful type of electron microscopes
• They have a higher magnification and resolution than other microscopes with a maximum magnification of approximately 50 000 000 X (50 million times)
  • This has helped biologists develop a better understanding of the structure of the nucleus and cell membrane
• You will not find an electron microscope in school. They are very expensive, not portable and it is a complicated process to prepare samples

Images from a light microscope

• The image below represents an image of a cross-section of a leaf taken with a light microscope
• The sample can be stained to help visualise distinct structures 
• The overall structure of the leaf can seen and different types of individual cells identified
  • Nuclei (pink dots) in some cells can be observed
  • under a higher magnification, chloroplasts would be seen

Light microscope photomicrograph of the cross-section of a leaf

Images from an electron microscope

• The image below represents a Transmission Electron Micrograph of a palisade mesophyll cell
• A much higher magnification can be attained with the TEM
• Sub-structures like the cell wall, chloroplasts and nucleus can be observed in more detail
  • Distinct layers can be seen inside the chloroplasts
  • Granular material can be seen in the nucleus - this is DNA 
• Electron micrographs are black and white although false colour can be added afterwards 

Electron micrograph and a drawing of a palisade mesophyll cell

Table to compare electron microscopes and light microscopes