2-2-1-methods-of-studying-cells

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Exam code:7401

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

Biology AS AQA