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
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.
