Light microscopes - low resolution. If
magnification is above x1500 - unclear
image. Visible light wavelength = 400 -
750nm so strucutres closer than
200nm appear as 1 object.
Achieve higher resolution
with electron microscopes.
Electron microscopes (EM)
generate electron beam with
wavelength of 0.004nm - 100 000x
shorter than light wavelength.
EM can distinguish between
objects 0.2nm apart. Use
electromagnets to focus electron
beam onto prepared specimen.
Electrons not visible to human eye -
image produced from electron beam
projected onto photgraphic paper to
make greyscale image = micrograph.
Electron micrographs sometimes in
colour. Final image produced from EM
always black, white + grey; colours
added after using specialised software.
Such images labelled 'false
colour' electron micrographs.
Resolution of EM is 500 000x
greater than the human eye.
2 types of EM:
Transmission Electron
Microscope (TEM)
1. Electron beam
passes through v. thin
prepared sample.
2. Electrons pass through
denser parts of sample less
easily - giving more contrast.
3. Final image
produced is 2D.
Magnification
possible - x500 000.
Scanning Electron
Microscope (SEM)
1. Electron beam directed
onto sample. Electrons don't
pass through specimen.
2. They're bounced
off the sample.
3. Final image
produced is 3D view of
surface of the sample.
Magnification
possible - x100 000.
Advantages and
DIsadvantages of EM
Advantages
Resolution - 0.1nm (2000x more
than light microscope) - means EM
can be used to prooduce detailed
images of organelles inside cells.
SEM produces 3D images - can
reveal detail of contours +
cellular/tissue arrangements - not
possible with light microscopes.
Disadvantages
Electron beams deflected by
molecules in air, so samples
have to be placed in vacuum.
EM are extremely expensive.
Preparing samples +
using EM both require high
degree of skill + training.
Use of EM
When EM first developed, many scientists
thought complicated preparation of
specimens produced artefacts (structures
that result from preparation process).
Not true representations of
specimen's original structure.
1980s, scientists arguing about true
value of EM but tech + quality has
improved in recent years. Scientists
also used X-ray crystallography to
investigate cell structure.
Confirmed that EM do give true
insight into internal workings of cells.