Electron Microscopy of Animal Cells (Edexcel International A Level Biology): Revision Note

Electron Microscopy of Animal Cells

Organelles under the electron microscope

• There are two types of electron microscope

  • Transmission electron microscopes (TEMs)

  • Scanning electron microscopes (SEMs)

• Transmission Electron Microscopes 

  • TEMs use electromagnets to focus a beam of electrons

  • This beam of electrons is transmitted through a thin specimen

  • Denser parts of the specimen absorb more electrons; these denser parts appear darker on the final image, producing contrast between different parts of the object being observed

  • The internal structures within cells, or even within organelles can be seen as a 2D image

  • The resolution of these images is very high

• Scanning Electron Microscopes

  • SEMs scan a beam of electrons across a specimen

  • This beam bounces off the surface of the specimen and the electrons are detected, forming an image

  • This means SEMs can produce 3D images that show the surface of specimens

  • Since they scan the outside surface it means that the specimen viewed does not have to be thin

  • The images they form are of a lower resolution than TEMs

A stained TEM microscope of the nucleus. It is clear this is a TEM micrograph as the image is 2D and in high resolution; the inside of the nucleus would not be clear to view at low resolution.

A stained TEM micrograph of a mitochondrion. It is clear this is a TEM micrograph as the image is 2D and in high resolution; the inside of a mitochondrion would not be clear to view at low resolution.

A SEM of a spiracle (part of an insect). You can tell this is a SEM micrograph as the image is 3D.

Cell organelles can be identified in a micrograph produced using a TEM