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Microscopy: 5. The scale of things in biology - orders of magnitude - comparison of cells with other objects with approximate size dimensions

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Sub-index of biology notes: investigations using microscopes


(5) The scale of things in biology and orders of magnitude

Comparison of cells with other objects with approximate dimensions

The practical limit of magnification for an optical light microscope is x1000 to resolve features e.g. structural parts of cells.

In terms of dimensions we are talking about ~0.5 m, the size of a larger virus.

Electron microscopes can resolve images down to ~0.2 nm, the size of a small atom.

This is a resolving power 25 000 times greater than that of an optical light microscope.

Object Size   Object Size
a common ladybird ~7 mm, 7000 m   HIV virus ~0.1 m, ~100 nm
diameter of a human hair ~0.1 mm, ~100 m   thickness of cell membrane ~0.007 m, ~7 nm
typical plant leaf cell ~0.07 mm, ~70 m   diameter of DNA strand ~2.5 nm
diameter of red blood cell ~0.007 mm, ~7 m, 7000 nm   diameter of a carbon atom ~0.34 nm

 When comparing the size of small objects like cells, scientists refer to differences in sizes as an order of magnitude.

This means that an object is described as being greater of smaller by a factor of 10.

For example, the leaf cell size is one order (10 x) in size than a red blood cell.

The common ladybird is two orders greater (100 x or 10 x 10) in size than a plant leaf cell.

The thickness of the cell membrane is three orders smaller {1/1000 or 1/(10 x 10 x10)} than the diameter of a red blood cell.


Keywords, phrases and learning objectives for this part on relative size and orders of magnitude

Appreciate from investigative techniques, including microscopy, you can measure the scale of objects in biology.

This can be expressed as orders of magnitude in powers of 10.

Appreciate the comparison of size of cells with other objects and sub-structures including insects, human hair, leaf cell, blood cell, virus, cell membrane, DNA molecules and carbon atom in terms of their approximate size and dimensions.


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