Showing SCALE in biology


Working out scales (of drawings or photomicrographs) is a common exam question in IB papers – usually in the Paper 1 Multiple Choice. It is most simply done with a scale bar.


I think the most useful tip is to make sure that you have some idea of what real size actually should be:

  1. Molecule size – a few nanometres
  2. Membrane width – about 8 nanometres
  3. Virus diameter – 10 – 100 nanometres
  4. Bacteria length or width – a few micrometres
  5. Animal cell – up to about 30 micrometres
  6. Plant cell – up to 120 micrometres


Another tip is ensure that you are clear about linear units in biology

  1. Start with 1 METRE (m)
  2. 1 m x 10(3) = 1 KILOMETRE (km)
  3. 1m x 10(-3) = 1 MILLIMETRE (mm)
  4. 1m x 10(-6) = 1 MICROMETRE (µm)
  5. 1m x 10(-9) = 1 NANOMETRE (nm)

Here is a very clear and useful link to help:

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Surface area to volume ratio – why cells cannot be huge


I hope that this is a fairly straightforward way to show the calculations for working out surface area and volume of a cuboid cell, and then putting the two together to show the all-important surface area to volume ratio. Volume increases faster than surface area, thus the ratio of the two (SA : Vol) becomes smaller and smaller.



  • Become multicellular
  • Create a convoluted or highly irregular surface area


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Elizabeth Week 2


Let’s sprint through:

  • 1.2 Ultrastructure of cells
  • 1.3 Membrane structure

Both these units now have dedicated pages under the Cell Biology Topic 1 main page:

I will post up some past paper and revision questions later in the week.

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Elizabeth Week 1


Topic 1 Cells

Unit 1.1 Introduction to Cells


Reading, learning and resources are posted on a Page in this Blog:


Some questions to try: 

I have tried to create the typical boxes which IB uses, inside which you should answer the questions. The necessary skill here is PLANNING and MORE PLANNING, so that what you eventually write is not coming straight and immediately out of your head but is prepared. Work on it!

If you have trouble downloading the Scribd file, it might work better through Google:

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