l2 electron microscope

8/11/2019 L2 Electron Microscope

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COPY LO:

To understand how the electronmicroscope works.

Starter

Write down whatis meant byresolution


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BATS

Describe how electron microscopes work

Explain the differences between transmissionand scanning electron microscopes

Describe the limitations of the transmissionand the scanning electron microscopes


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Light microscopes have poor resolutionbecause of the relatively long wavelength

of light.

B y u s i n g m o r e l e n s e smicroscopes can magnify by alarger amount, but this doesn't

always mean that more detailcan be seen.

The amount of detail dependson the resolving power of amicroscope, which is the

smallest separation at whichtwo separate objects can bedistinguished (or resolved).


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The resolving power of amicroscope is ultimately

limited by thewavelength of light

(400-600nm for visiblelight).

To improve the resolvingpower a shorter

wavelength of light isneeded, and sometimesmicroscopes have bluefilters for this purpose

(because blue has theshortest wavelength of

visible light).


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The Electron Microscope

Developed in 1930s

The electron beam has a veryshort wavelength - thereforeit has a high resolving power

The electrons are

negatively charged so the

beam can be focussed usingelectromagnets


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Modern Electron Microscopes - canresolve ob ects that are ust 0.1 nm a art

2000 timesbetter than

light

microscope

Electrons are

absorbed by

molecules in the air so

a near vacuum has to

be created in the

chamber of the

electron microscope


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2 Main types of Electron Microscopes

1. The transmissionelectron microscope(TEM)

2. The scanning electronmicroscope (SEM)


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Transmission Electron Microscope

Using books writedown as many facts

as you can about

the electron

microscope.

What are the main

differences

between the two?


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Transmission Electron Microscope

Here the beam passesthrougha thin section

of the specimen

Part of the specimen

absorb electrons and

appear dark

An image is produced on ascreen which is photographed

to give a photomicrograph


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Resolving power of TEM = 0.1 nm

Limitations:

- The whole system must be in avacuum

- A complex staining processisrequired

- Specimen must be extremely thin

- Specimen may contain artefacts


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Limitations:

- The whole system must be in a

vacuum - no living specimens

- A complex staining process isrequired - even then only black andwhite

- Specimen must be extremely thin

- Specimen may contain artefacts -things that result from the way thespecimen is prepared. We cannot

always be sure that what we see on aphotomicrograph really exists in thatform


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How might we build

up a 3 D image?

Take a series of

sections through a

specimen


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Rat Liver Cells


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Scanning Electron Microscope

Same as forTransmission Electron

Microscope except

specimen does not have

to be so thin

Beam of Electrons are

aimed at the surface

of the specimen andthe specimen scatters

them


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A computer analyses it and builds upa 3D image

Basic SEM has lower

resolving power than

a TEM - about 20nm

(still 10 times betterthan a light

microscope)

Ragweed pollen


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Summary Questions -

Page 45

Exam Questions


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Homework

References

Glenn Toole, Susan Toole (2008).A2 Biology. Cheltenham: Nelson

You Tube clips to watch

1. Revise for Test next week

2.

l2 electron microscope

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