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Monday, 10 September 2012
F211 Module 1 Cells and the Microscope
Weekly Learning Outcomes
Students should be able to:
State the resolution and magnification that can be achieved by a light microscope, a transmission
electron microscope (TEM) and a scanning electron microscope (SEM).
Explain the difference between magnification and resolution.
Explain the need for staining samples for use in light microscopy and electron microscopy.
Calculate the linear magnification of an image.
The Light Microscope
Light microscopes produce images
using a number of lenses that can be viewed
directly from the eyepiece.
Light passes from a bulb under the
stage, through a condenser lens, then through
This beam of light is focused through
the objective lens, then through the eyepiece
If you want to view a specimen at a
different magnification you can rotate the lens
Common lenses are x4, x10 and x40
but some will also have x100.
The eyepiece lens then magnifies the
image usually at x10.
To find the total magnification of a specimen you should multiply the objective magnification by the
Advantages and Disadvantages of the Light Microscope
Magnification Most light microscopes are capable of magnification up to a maximum of x1500.
Resolution The maximum resolving power using light is 200nm. This means objects closer together than
200nm will be seen together as one object. This limit is caused by the magnitude of the wavelength of
light. Two objects can only be distinguished if light waves can pass between them.
Specimens You can use light microscopes to view specimens including living organisms. Thin sections of
plants, animals and smear preparations of blood and cheek cells can also be viewed.
Preparation of Specimens for the Light Microscope
Some biological samples need staining, as you cannot otherwise see the detail. Other material distorts
when you cut it into thin sections.
To overcome the issues you need to follow these steps:
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Staining Coloured stains are chemicals that bind to chemicals on or in the specimen. This allows
the specimen to be seen. Some stains bind to specific cell structures. One example is iodine, which
stains starch a dark blue/ black colour.
2. Sectioning Specimens are embedded in wax. These sections are then cut without distorting the
structure of the specimen. This is particularly useful for making sections of soft tissue, such as the
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Electrons are not visible to the human eye. The image produced from the electron beam is
projected onto a screen or photographic paper to make a black-and-white image.
Such images are called electron micrographs.
The resolution of an electron microscope is about 500000 times greater than that of the human
Transmission Electron Microscope (TEM)
The electron beam passes through a very thin
Electrons pass through the denser parts of the
sample less easily, so giving some contrast.…read more
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Magnification The number of times greater an image is than the object.
Resolution The ability to distinguish two separate points as distinct from each other.
Cell surface membrane The membrane that surrounds every cell, forming the selectively permeable
boundary between the cell and the environment.
Ribosome The organelle, made of two subunits, on which proteins are synthesised inside the cell.
Organelle Structure inside a cell. Each structure has a specific function.…read more