This document compares the relative sizes of various biological structures using the SI unit of length. It states that molecules are about 1 nanometer in size, cell membranes are 10 nanometers thick, viruses are around 100 nanometers, bacteria are 1 micrometer, organelles are generally less than 10 micrometers, plant and animal cells are less than 100 micrometers, and plant cells tend to be larger than animal cells. It also provides a formula for calculating actual size based on known image size and magnification.

1. Cell Size and microscopy
Carissa Fletcher
Inter Community School
http://tysontrepidations.wordpress.com/2011/07/02/say-cheese-for-scanning-electron-microscopy/

11. 2.1.4 Compare the relative sizes of molecules cell membrane
thickness, viruses, bacteria, organelles and cells, using the
appropriate SI unit (L3 Give an account of similarities and differences between two (or more) items, referring
to both (all) of them throughout.)
Relative sizes:
1. molecules (1nm).
2. cell membrane thickness (10nm).
3. virus (100nm).
4. bacteria (1um).
5. organelles (less 10um).
6. cells (<100 um). http://www.cellsalive.com/howbig.htm
7. generally plant cells are larger than
animal cells. http://click4biology.info/c4b/2/cell2.1.htm#size

12. 2.1.5 Calculate the linear magnification of drawings and the actual size
of specimens in images of known magnification. (L2 Calculate means find a
numerical answer showing the relevant stages in the working )
1m = 103 mm (millimetres)
I = Image 1m = 106 µm (micrometres)
M=Magnification 1m = 109 nm (nanometres)
A = Actual Size
I
M A

13. Actual Size = Image/Magnification
Magnification X 100000