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AS Biology Core Principles
The Electron Microscope
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Aims
Resolving powerThe resolving power of light &electron microscopesThe difference between the light &electron microscopeTransmission & scanning electronmicroscopy
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IntroductionMicroscopes magnify & resolve imagesMicroscopy began in 1665 when RobertHooke coined the word ‘cells’ to
describe the structure of corkYou need to know about 2 types ofmicroscope - light & electronYou need to know how they work and
the differences between them ‘Its not how much they magnify that iskey - but how well they resolve…’
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Resolving Power
The limit of resolution of a microscopeis the smallest distance between 2points that can be seen using a
microscopeThis is a measure of the clarity of theimageA microscope with a high resolving
power will allow 2 small objects whichare close together to be seen as 2distinct objects
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Resolving Power
Resolving power is inverselyproportional to the wavelength ofthe radiation it uses
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The Light MicroscopeSeries of lenses throughwhich ordinary white lightcan be focusedOptical microscopes cannot resolve 2 points closertogether than about half(0.45) the wavelength ofthe light used (450-600nm)How close is this?
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The Light MicroscopeThe total magnification isthe eyepiece magnificationmultiplied by the objectivemagnificationThe maximummagnification of a lightmicroscope is x1500What can it be used for?What can it not be usedfor?
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The Electron MicroscopeElectrons (negatively charged, very smallparticles) can behave as wavesThe wavelength of electrons is about 0.005nmWhat will this mean for the limit of resolution?Electrons are ‘fired’ from an electron gun atthe specimen and onto a fluorescent screen orphotographic plateWhere is this technique commonly used?There are 2 types of electron microscopy -transmission and scanningBoth focus an electron beam onto thespecimen using electromagnets
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Transmission Electron
Microscope (TEM)In transmission EM theelectrons pass through thespecimen
Specimen needs to beextremely thin - 10nm to100nmTEM can magnify objects up to500 000 timesTEM has made it possible tosee the details of and discovernew organelles - see page 9 inCollins
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Transmission Electron
Microscope (TEM)Cells or tissues are killed andchemically ‘fixed’ in acomplicated and harsh
treatment (in full detail in table3.1 pg 52 Rowland)How does this differ to lightmicroscopy?This treatment can result inalterations to the cell - knownas artefactsWhat will this mean for theimages produced?
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Transmission Electron
Microscope (TEM)
Transmission electronmicrograph ofepithelial cells from arat small intestine.Scale bar = 5 mm.
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Scanning Electron
Microscope (SEM)In Scanning EMmicroscopes theelectrons bounce offthe surface of thespecimenProduce images witha three-dimensionalappearanceAllow detailed studyof surfaces
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Scanning Electron
Microscope (SEM)Now watch the following clip explaining SEM
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Scanning Electron
Microscope (SEM)
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Links
www.learn.co.uk/
www.microscopy-uk.org.uk/intro/index.html
www.mwrn.com/feature/education.asp
http://www.feic.com/support/tem/transmis.htm
http://anka.livstek.lth.se:2080/microscopy/foodmicr.htm
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Light & Electron Microscopes
Feature LightMicroscope
ElectronMicroscope
Radiation used
Radiation source
Nature of lenses
Lenses used
Image seen
Radiation medium
Magnification
Limit of resolution
What it can show
Copy & complete the following table
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