1. Electron Microscope

2. Learning Objectives
Know the difference between a transmission and a scanning electron microscope
Explain the benefits of electron microscopes
Compare and contrast EM & LM

3. Starter – light microscopes
Magnification X1500
Resolution 200nm
Light passes through so need thin specimens
Can be alive or dead
Stains can be used to enhance contrast

4. Electron Microscope EM generates beam of electrons
Focused using magnets
Electron beam passes through very thin prepared sample (TEM) or bounces off sample (SEM)
Stain with heavy metals
Vacuum – sample is dead
Resolution is 0.2nm
(1000x more than light microscope)

5. Light vs EM
Which is which? What are the main differences?

6. Preparing Slide of EM Fix specimen in gluteraldehyde
Dehydrate with ethanol
Embed in resin
Slice thinly
Stain (salts of heavy metals – eg lead, uranium)
Mount on copper grid
Place in vacuum (allows electrons to travel towards specimen)

7. SEM vs TEM
A seed viewed with a scanning EM

8. Transmission Electron Microscope
Electrons pass through denser part of sample less easily so create contrast 2D
Magnification up to x

9. Scanning Electron Microscope
Electrons ‘bounce’ off metal-salt-stained sample 3D
Magnification x

10. Task
Construct a table/diagram/revision resource which compares and contrast Light and Electron microscopes.
Include: resolution, magnification, stains, types of samples viewed, level of detail seen.
Biofactsheet 75 will help.

11. False colour can be added by computers
Light
Electron
Resolution
200nm
0.2nm
Magnification
X1500
TEM x
SEM x100000
Stains
Dyes
Heavy metals
Types of sample
Living and dead
Only dead samples
Level of detail
2D colour
Black and white
TEM 2D
SEM 3D
False colour can be added by computers

12. Advantages and disadvantages of EM
Resolution is x1000 more than LM
Samples have to be placed in a vacuum
Produces detailed images
Very expensive
SEM produces 3D images
Need to be highly skilled to create samples

13. Essay Questions
Using your textbook and your notes answer the essay question:
Compare and contrast the advantages and limitations of EM and LM [12]

14. Mark Scheme Resolution (marks for actual figures) [2]
Detail in image: SEM, TEM, LM [3]
Preparing samples [2]
How they work: SEM, TEM, LM [3]
EM expensive [2]
QWC [1]