Electron Microscopes (OCR GCSE Combined Science A (Gateway))

Increased Resolution

• 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 Microscopy

• 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

Comparing Light Microscopes & Electron Microscopes

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