A good fiend of mine, who enjoys mountaineering, loves stepping from boulder to boulder especially when he can link a few brave leaps in a chain. This autumn I think I achieved that in the start of an IB Biology class using some simple experiments to navigate the perculiar statistics topic arriving at a classic investigation of the size of human cells vs moss cells.

There is no time to waste in IB bio but there are some new ideas about experimental uncertainties, precision of equipment,sample sizes, measn and standard deviation which are essential for Internal Assessment.

I asked students to draw a biological diagram of one cell, using a microscope and some algae rich water from the lab aquarium. Summer holidays inevitably mean that the aquarium has been a little neglected and the wow factor of finding, long threads full of chloroplasts, and a whole range of single celled organisms movingin through. It's not quite a hay infusion, but there is plenty to see, and we even found a nematode.

The second experiment was to measure the volume of water in a small crucible using measuring cylinders, syringes, or graduated pipettes and to work out the precision of the equipment and the uncertainties in the method. Asking students to put the measuring methods into order, the most precise first was a great way to tempt students into asking how to calculate uncertainties for different pieces of equipment. Better than telling them, while they wonder what the devil you are on about. The fact that the surface tension of the water allows about 4ml extra to be added to a crucible after it looks full illustrates the idea that bio expts have to deal with varied specimens, and so variation in results may be an accurate representation of the material, and it also introduces one property of water which we will revisit in topic 3 and the HL plants topic.

The third experiment was to sample beans from an envelope. The aim was to deduce the ratio of red to white. By taking 10 samples and calculating a running average students learnt that 3 repeats is not really enough, and that the more repeats there are the less the averages vary. Handy ideas in themself but also gives an opportunity to introduce standard deviation as an easier way to measure this variation in data. It's much easier to calculate a standard deviation using Excel than it is to calculate a running average of 10 repeats.

Rounding this off with an investigation to find out if human cheek cells are bigger than moss leaf cells using eyepiece graticules gives the students chance to use these new ideas in a piece of assessed experimental work and the first week of IB Biology has not yet had a theory lesson.

This is a great example of planning a scheme of work backwards like the model used by

Tony Sherborne called CRACKING Science.
If you'd like copies of the worksheets I used please leave a comment with your email address.