3.1.6Unit 3: Practical Skills in Biology I
Electron Microscopy of Animal Cells — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 8 of 27 Unit quiz
Part 1 of 5Two Types of Electron Microscope
Electron microscopes use beams of electrons instead of light.
Transmission (TEM)
- Electromagnets focus a beam of electrons that passes through a very thin specimen to form a two-dimensional image.
TEM contrast
- Denser parts of the specimen absorb more electrons and appear darker, so stains containing heavy metals can be used.
TEM use
- It shows the internal structure of cells and organelles at very high resolution, such as mitochondria and nuclei.
Scanning (SEM)
- A beam of electrons is scanned over the surface of a specimen, and the electrons that bounce off are detected.
SEM use
- It produces three-dimensional images of surfaces, and the specimen does not have to be thin.
Resolution
- The SEM image has lower resolution than a TEM image, but both have much higher resolution than a light microscope.
Part 2 of 5Why Electrons Give Better Resolution
1
Resolution is the ability to distinguish two points that are close together as separate points in the image.
2
The resolution of any microscope is limited by the wavelength of the radiation it uses.
3
Visible light has a long wavelength, so a light microscope cannot resolve objects closer than about 200 nm.
4
Electrons have a much shorter wavelength, so electron microscopes can resolve far smaller structures and give much greater useful magnification.
5
A disadvantage is that specimens must be placed in a vacuum, so living cells cannot be observed.
| Feature | Light microscope | TEM | SEM |
|---|---|---|---|
| Radiation | Visible light | Electron beam | Electron beam |
| Image | Colour, 2D | Black and white, 2D | Black and white, 3D |
| Resolution | About 200 nm | Very high | High, lower than TEM |
| Living specimens | Possible | No | No |
Part 3 of 5Identifying Organelles in a Micrograph
Identifying Organelles in a Micrograph
Nucleus
- A large round structure with a double membrane containing dark-staining chromatin, and sometimes a darker nucleolus.
Mitochondria
- Oval or rod-shaped with a double membrane, and the inner membrane folded into cristae.
Rough ER
- Flattened sacs arranged in stacks, with small dark dots of ribosomes on the outer surface.
Golgi apparatus
- A stack of flattened curved sacs with small vesicles budding off near the edges.
Part 4 of 5Worked Example
Question
- A TEM micrograph of a nucleus has a scale bar 30 mm long that represents 2 μm.
- Calculate the magnification of the image.
- Convert the scale bar to micrometres30 mm = 30 × 1000 = 30 000 μm.
- Magnification = image size ÷ real size = 30 000 ÷ 2.
- 30 000 ÷ 2 = 15 000.
Answer The magnification is ×15 000.
Part 5 of 5Exam Checklist & Tips
3.1.6 – Key Points for the Exam
- Transmission electron microscope A microscope in which electrons pass through a thin specimen to give a 2D image of internal structure.
- Scanning electron microscope A microscope that gives a 3D image of a surface using reflected electrons.
- Resolution The smallest distance at which two points can be distinguished as separate.
- Micrograph A photograph taken with a microscope.
Common Mistakes
- Saying that electron microscopes have higher magnification because of better lenses, rather than because of the shorter wavelength.
- Stating that a SEM shows internal organelles, when it shows the surface.
- Saying living cells can be viewed with an electron microscope, as specimens are in a vacuum.
- Believing that the colours in electron micrographs are real, when they are usually added later.
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Cambridge 9700 AS Biology
Core Formula
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.