KEGS Biology Blog

Friday 15th February Section 4
February 15, 2013, 11:09 am
Filed under: Uncategorized

Cell sizes and magnification


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F211 – Microscopes

Animal cells are usually 20 – 40 micro metres
Cell membranes are usually 10 nano metres
Viruses are normally 40 – 100 nano metres

1000nm = 1 micro metre

Magnification – the ratio between the actual size of an object and the size of an image.

M= Image size/actual size

Resolution – the ability to see detail and distinguish between objects. A light microscope has a resolution of 0.0002mm. This means that two objects this far apart can be viewed as two separate objects.

SEM (Scanning Electron Microscope) have higher resolution: 0.20nm
SEMs work by firing a beam of electrons using magnets to focus the beam, to produce an electron micrograph a black and white image. The 0.004nm wavelength of electron beam much shorter than light gives a much higher resolution.

TEM (Transmission Electron Microscope) also exist which give an even higher magnification than SEM.

Resolution of light microscope: 200nm/0.0002mm
This means if the magnification is above x1500, the image isn’t clear. Light has a wavelength if 400-750nm so structures closer than 200nm will appear as one object

Resolution of human eye: 100 micro metres/0.1mm

Using a light microscope on a eukaryotic cell for example would mean features such as ribosomes, mitochondria would not be visible with a 200nm resolution

Actual size = size of image/magnification

Total magnification =
Magnification of eye piece x Magnification of objective lens

Comment by helenquah

These two websites are useful for the homework about pro/eukaryotic cells.The second is for IB Standard Level revision but the syllabus looks fairly similar and it has loads of useful resources.

Comment by hannahgooch

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