Resources from the Computers in Science Book
PDF of this chapter
Worksheet
Growth spreadsheet (for PCs)
Growth spreadsheet (for Macs)

You can find some superb science simulations on the UCLES website. Most are for Key Stage 4, but the water weed simulation is suitable for Key Stage 3. It counts oxygen bubbles and you can vary the light intensity, the CO2 concentration and the colour of light. You will need the Shockwave plugin to run the simulation.
http://ital.ucles-red.cam.ac.uk/htdocs/enigma/simulations/elodea/elodea.html