Biology F211 Module 1 Cells

  • Created by: Bethany62
  • Created on: 18-04-15 09:38


Magnification - The degree to which the size of an image is larger than the object itself. Numerically, it is the image size divided by the actual size of the object, measured using the same units. Expressed as x10 x40 ect.

Resolution - The degree to which it is possible to distinguish between two objects that are very close together. The higher the resolution, the greater detail one can see.

The Light Microscope

  • Light passes through a condenser lens then through the specimen. This beam is focused through the objective lens then the eyepiece lens.
  • max x1500 magnification, and 200nm resolution. Specimen can be living but must be thin.
  • Staining- coloured chemicals bind to certain chemicals on or in the cell, allows the cell to be seen and some even highlight specific cell structures. eg. Acetic orcein stains DNA dark red.
  • Sectioning- Specimens are embedded in wax, then thin sections are cut.

um- micrometre, 0.000001 metres; nm- nanometre, 0.000000001 metres

actual size = image size/magnification (I AM written in a formula triangle)

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Electron Microscopes

A beam of electrons has a shorter wavelength than light so replacing the light beam in a microscope with an electron beam creates higher resolution (0.2nm). Magnets are used in place of lenses to focus and the reulting image is projected onto a screen or photographic paper forming a greyscale image called an electron micrograph.

Transmission Electron Microscope(TEM) - x500000 magnification, 2D image, electron beam travels through a thin prepared sample.

Scanning Electron Microscope(SEM)- x100000 magnification, 3D image, Electrons are bounced off the sample.

Advantages - resolution is 0.1nm, can produce images of organelles and cell structures, SEM can reveal contours or cellular/ tissue arrangements.

Limitations -Must be a vacuum, expensive, preparing samples require advanced training.

Specimens- fix the specimen, then dehydrate it, embed in a solid resin, cut thin slices with a diamond knife, stain with heavy metals to give contrast, mount on a copper grid.

Though electron micrographs are blank, white, and grey, computer software can give false-colour.

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