Electron Microscope
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of the limitations of optical microscopes and the need for higher resolution in cell imaging. You can test your knowledge on these below.
What is meant by “resolution” in microscopy?
The ability to distinguish between two points that are close together.
Why are some organelles, like ribosomes, not visible under a light microscope?
Because they are smaller than the resolving power (200 nm) of light microscopes.
Why is a stain necessary when observing cells using a microscope?
The stain binds to specific structure and provides contrast making them easier to see.
Topic Explainer Video
Check out this @LauraDoesBiology video that explains electron microscope or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Electron Microscopes Overview
Electron microscopes use electrons instead of light to create highly detailed images of biological specimens. They focus the electrons using magnets. Due to the shorter wavelength of electrons, they have a much higher resolution than light microscopes.
Types of Electron Microscopes
Transmission Electron Microscope (TEM)
- A beam of electrons passes through a very thin specimen.
- Denser areas absorb more electrons and appear darker, while less dense areas appear lighter.
- Produces a 2D, black-and-white image with extremely high resolution.
Uses of TEM:
- Studying the ultrastructure of cells (e.g., smaller organelles like ribosomes, mitochondria, and endoplasmic reticulum).
- Examining internal details of viruses and bacteria.
Scanning Electron Microscope (SEM)
- A beam of electrons is directed onto the surface of a specimen.
- Electrons are reflected off the surface and detected to produce a 3D image.
- Provides detailed surface structure rather than internal details.
Uses of SEM:
- Studying surface structures of cells, viruses, and tissues.
- Examining insects, pollen grains, and other surfaces in high detail.
Advantages and Disadvantages of Electron Microscopes
Advantages:
- Much higher magnification (up to ×1,000,000) than light microscopes.
- The magnification of a TEM is higher than an SEM but both are higher than an optical microscope.
- Higher resolution than light microscopes (TEM: 0.1 nm, SEM: 20 nm), allowing for finer details to be seen.
- Can reveal details of smaller organelles (TEM) and 3D surface structures (SEM).
Disadvantages:
- Specimens must be dead: as electron microscopes require a vacuum.
- Complex sample preparation: requires very thin sections for TEM and coating with metals for SEM.
- Expensive: requires specialist training and maintenance.
- Images are black and white: although false colouring is often added later.
Key Terms
- Magnification: How many times larger the image is compared to the actual object.
Formula: Magnification = Image Size / Actual Size. - Resolution: The ability to distinguish between two close objects as separate.
- Transmission Electron Microscope (TEM): Uses electron beams passing through a specimen to produce high-resolution, 2D images of internal structures.
- Scanning Electron Microscope (SEM): Uses electron beams bouncing off the surface to produce detailed, 3D images.
- Vacuum Chamber: A sealed space with no air to prevent electrons from scattering.
No answer provided.
Exam Tips
You are often asked which type of microscope has been used to produce an image.
If an image shows the ultrastructure of mitochondria, or any organelle, a TEM has been used.
If the image shows the shape and surface of cells, an SEM has been used.
If the image has low resolution and smaller organelles cannot be seen, an optical microscope has been used.
Make sure you can compare and contrast the advantages and disadvantages of all three types of microscope.
No answer provided.
Contrast how an optical microscope and a transmission electron microscope work and contrast the limitations of their use when studying cells. (6 marks)
- TEM use electrons and optical use light.
- TEM focuses using magnets and optical uses glass lenses.
- TEM allows a greater resolution.
- With TEM, smaller organelles / named example can be observed.
- TEM view only dead specimens and optical can view live specimens.
- TEM does not show colour and optical can show colour.
- TEM requires thinner specimens.
- TEM requires a more complex/time consuming preparation process.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!