A level, cell structure microscopy topic

1. We will cover
• Microscopy and Types of microscopes
• Some important terms in microscopy
• Units conversions and magnification calculations

2. Cell structure
With a few exceptions, cells are only visible under a microscope.
A microscope produces a magnified image of an object.

3. Cell structure
With a few exceptions, cells are only visible under a microscope. A microscope produces a magnified image of an object.
Technical terms used in microscopy
• Magnification and the measurement units
magnification = size of image / size of real object
NOTE: Greater the magnification greater the size of an image. The bigger image shows more details,
but after a certain limit It might only produced a more blurred image.
Table: Units of length
Example: An object that measures 100nm in length appears 10mm long in a photograph.
What is the magnification of the object?
Units Symbols Equivalent in meters
kilometre km 1000
metre m 1
millimetre mm 10-3
micrometre µm 10-6
nanometre nm 10-9

4. Resolution/Resolving power
‘The minimum distance apart two objects can be in order for them to appear separate’
The resolution of a microscope depends on the
• Wavelength of the radiation used
• Type of the radiations used
High resolution means greater clarity, which means more clear and precise image (precision ?).
Type of a microscope Resolution
Compound light-microscope 0.2µm
Electron microscope 0.1nm

5. Types of microscopes
1. Simple convex lenses
2. The compound light-microscope
3. The electron microscope
a. The transmission electron microscope
b. The scanning electron microscope

6. Types of microscopes
1. Simple convex lenses
2. The compound light-microscope
3. The electron microscope
a. The transmission electron microscope
b. The scanning electron microscope
Light Microscope Electron Microscope
Low purchase and operation cost High purchase and operation cost
Small and portable Large and requires special operation room
Simple and easy sample preparation Lengthy and complex sample preparation
The material under study is rarely damaged The material under study is damaged during sample
preparation
Vacuum is not required Requires vacuum
Natural colors of sample are maintained All images are in black-and-white
Magnification is up to 2000 times Magnification over 500, 000
Both living and dead specimens can be studied Only dead specimens are observed as they need to be fixed in
plastic and viewed under vacuum
Specimen stained to improve visibility Specimen stained with electron dense material (heavy
metals like gold.

7. Types of microscopes
1. The electron microscope
In an electron microscope an electron beam is used instead of photons (light), which increases the resolution
of the microscope 20 (in SEM) to 2000 (in TEM) times.
a. The transmission electron microscope
b. The scanning electron microscope

8. Comparison of a transmission electron microscope (TEM) and scanning electron microscope (SEM)
Transmission Electron Microscope (TEM) Scanning Electron Microscope (SEM)
A beam of electrons passes through the specimen, which on
reflection is directed on a fluorescence screen to get a
photomicrograph.
The beam of electrons scans the surface of the specimen rather
than penetrating into the specimen. The scattered electrons
generates an image of the surface contours.
A flat 2D images 3D images
Resolution 2000 time better than the light microscope Resolution 20 time better than the light microscope
Requires a very thin specimen which is hard to prepare and
introduces artifacts in the end image
Thick sample can be tested
Image not in natural colours Coloured image
Limitation of the TEM
• The upper limit of the resolution is hard to achieve due to difficulties in preparing an adequately thin sample.
• The sample gets damaged due to vacuum and electrons energy/living sample cannot be observed.
• No coloured image