Agar Cell Races

Planning lessons on the hoof can be a bit nerve shredding and means I often have very little idea how something will work out. Indeed, “planning” probably isn’t the right word, but I can’t think of a better one. I find myself lurching from class to class, relying far too much on bravado, improvisation and the teaching equivalent of muscle memory to survive.

Continuing with the Year 9 diffusion work, I had had this vague idea of how to approach the question of Surface Area to Volume ratio. It’s a difficult concept, especially for Year 9 sized brains, and in the past I’ve kind of rushed past it. But this year, having let them design their own experiments to investigate factors affecting diffusion rate, I came up with a follow up thought which combines some of the most motivating features of any lesson:

  • autonomy
  • competition
  • cutting up pink agar blocks

So I divided the students into teams of two with a large cube of agar + indicator, a tile and a knife each. I told them to cut a cell with a volume of 8000mm3. They had to bring their cells to me with my large tray of 1M HCl, and then they would race to see which cell went clear first….


Simple idea. Luckily, I had somehow remembered to tell the technician.

I thought it might be quite fun. But I had not remotely imagined how much they would go for it. They desperately wanted to WIN, so there was frantic calculator action as they went through all the combinations of dimensions that would give them the specified volume. It differentiated! Weaker students took a while to see that it didn’t have to be a 20x20x20 cube…. There was creativity (see picture of second race below!), there was dawning realisation, but above all, there was genuine excitement.


OK, so the volume was too big – next time I’ll ask for 1000mm3 – but it worked. I got them to fill in the details of their dimensions in an Excel spreadsheet and filled in the times for the different cells. That took us to the end of the single period – I know so many teachers who at this point would have been in a blind panic as the students had not written anything down (and so couldn’t possibly have learned anything…) – but it’s OK. Tomorrow we will revisit the data.

How can there be different times when we controlled the Volume of the cells?

Ah, they will say, but the Surface Areas are different!

and I will get them to calculate the Surface Areas of the cells.They will see that this also isn’t enough to explain the pattern. At which point I will introduce the idea of SA/Vol ratio and get them to calculate this. It works beautifully! I will also bet a large bar of Green and Blacks orange flavoured chocolate that they will remember it.


Half term is here. Not a moment too soon.


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