Edexcel International A Level Biology

Revision Notes

3.6 Electron Microscopy of Animal Cells

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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.

Hodder Edexcel micrograph of mitochondria

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.

Spiracle Electronmicrograph

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

Electron micrograph and drawing of an exocrine gland cell of the pancreas

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

Exam Tip

You need to be able to recognise organelles from electron microscope images; cells in real life are not always as easy to observe as cells in diagrams, so be sure to get practice at looking at electron micrographs of cells

Generally, if you can see internal structures the image would have been taken with a TEM and if the image appears 3D then an SEM would have been used.

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Author: Alistair

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.