Methods of studying cells (AQA A Level Biology): Revision Note
Exam code: 7402
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Microscopes
Microscopes can be used to analyse cell components and observe organelles
Key terms when discussing microscopy include:
Magnification: how many times larger the image is than the actual object
Resolution: the ability to distinguish two close objects as separate
Optical (light) microscopes
Use light to form images
Light microscopes are limited by low resolution and magnification
Maximum resolution is around 0.2 µm (200 nm)
It is possible to view the nucleus, mitochondria and chloroplasts
It is not possible to view ribosomes, ER or lysosomes
Maximum magnification for light microscopes is around ×1500
It is possible to observe live specimens and produce colour images with a light microscope
Electron microscopes
Use a beam of electrons to form the image
This means a shorter wavelength and higher resolution
Maximum resolution is around 0.0002 µm (0.2 nm)
It is possible to view smaller organelles (e.g. ribosomes, ER)
Maximum magnification is around ×1,500,000
Electron microscopes produce black and white images and specimens must be dead
There are two types of electron microscopes:
Transmission electron microscopes (TEMs)
Scanning electron microscopes (SEMs)
Transmission electron microscopes (TEMs)
Electrons pass through specimen
Gives high-resolution, 2D images of internal structures
Limitations include:
Specimens must be thin
Cannot view live cells
May introduce artefacts
Scanning electron microscopes (SEMs)
Electrons scan the specimen surface
Produces 3D images of external surfaces
Limitations include:
Lower resolution than TEM
Cannot view live specimens
Comparing microscopes
Feature | Light microscope (optical) | Transmission EM (TEM) | Scanning EM (SEM) |
|---|---|---|---|
Radiation used | light | electrons | electrons |
Resolution | ~0.2 µm (200 nm) | ~0.0002 µm (0.2 nm) | ~0.002 µm (2 nm) |
Magnification | up to ×1500 | up to ×1,500,000 | up to ×500,000 |
Image type | 2d, colour | 2d, black and white | 3d, black and white |
Specimen state | living or dead | dead only (due to vacuum) | dead only (due to vacuum) |
Preparation | simple | complex, may introduce artefacts | complex, may introduce artefacts |
Sample thickness | thick acceptable | must be very thin | can be thick or 3d |
Structures visible | nucleus, mitochondria, chloroplasts | internal structures, organelles | surface details, external structures |
Cost and availability | inexpensive, common in schools | expensive, specialised | expensive, specialised |
Examiner Tips and Tricks
Focus on how each microscope works and be ready to justify which is most suitable in a given scenario.
Early scientists using electron microscopes struggled to tell apart real cell structures and artefacts (e.g. dust, air bubbles or fingerprints)
They had to prepare samples in different ways to see if a structure was real or an artefact
Over time, improved techniques helped reduce artefacts and increase confidence in identifying organelles
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