3.1 AS Unit F211: Cells, Exchange and
Transport
• The cell is the basic unit of all living things.
• How to use a light microscope
• Why electron microscopes are so important in
biology.
• Details of cell structure and ultra-structure
• The roles of cells and organelles.
3.1 AS Unit F211: Cells, Exchange and
Transport
• Candidates should be able to:
• state the resolution and magnification that
can be achieved by:
1. a light microscope
2. a transmission electron microscope
3. a scanning electron microscope
MAGNIFICATION AND RESOLUTION
• Cells are too small to be seen with the naked eye.
• The light microscope was developed to produce enlarged
and more detailed images of cells.
• The magnification of an image is how much bigger it
appears under the microscope than it is in real life, and is
worked out using the following formula:
magnification = image size ÷ actual size
MAGNIFICATION AND RESOLUTION
• Magnification on its own does not increase
the level of detail seen, it just increases the
size.
• Resolution refers to the ability to see two
distinct points separately.
•
MAGNIFICATION AND RESOLUTION
If the resolution of a light microscope is 200nm
(0.2μm), this means it can see any two
different points as separate objects if they are
200nm apart or more.
If they are any closer than this amount, they
appear as one object
THE LIGHT MICROSCOPE
• Light microscopes use a number of lenses to
produce an image that can be viewed directly
at the eyepiece.
• Light passes from a bulb under the stage,
through a condenser lens and then through
the specimen.
• This beam of light is passed through an
objective lens and then the eyepiece lens.
THE LIGHT MICROSCOPE
THE LIGHT MICROSCOPE
• The light microscope usually has a number of objective
lenses which can be rotated into position; a x10 lens
will magnify the image 10 times.
• The eyepiece lens then magnifies the image again by
x10. So using these two lenses the final magnification
the microscope is capable of producing is: (which)
• x40
• x100
• x400
• x1000
THE LIGHT MICROSCOPE
• Using the light microscope some specimens
may be seen directly, live specimens may be
seen.
• Some tissues may need to be stained to add
colour to aid seeing detail.
• The more preparation a specimen needs the
more it may be distorted
THE ELECTRON MICROSCOPE
• Electron microscopes can achieve higher
resolutions .
• Electron microscopes generate a beam of
electrons, which have a wavelength of
0.004nm.
• They can distinguish objects 0.2nm apart
THE ELECTRON MICROSCOPE
Transmission Electron Microscope (TEM)
• The electron beam passes through a very thin sample
to which heavy metals have been added (e.g. Osmium
tetroxide).
• The electrons pass through dense, metal, parts less
easily than to the sections on which the metal is
absent.
• This gives a ‘shadowing effect’ and produces contrast
in the final 2D image produced.
• Maximum possible magnification of x500,000
THE ELECTRON MICROSCOPE
THE ELECTRON MICROSCOPE
Scanning Electron Microscope (SEM)
• The electron beam is directed onto a sample
which has been covered with heavy metal.
• The electrons don’t pass through the specimen,
they bounce off.
• This produces a final 3D image view of the
surface of the sample.
• Maximum possible magnification of x100,000
THE ELECTRON MICROSCOPE