Cabomba is no longer available in the UK, due to the recent invasive plant directive. We are currently trialling alternatives, and currently recommend Myriophyllum scabratum or Egeria densa
Watching gas bubble up from a pondweed as it photosynthesises can be a great demonstration or student practical. When placed closer to a light source, the rate of bubbling will speed up, and as the pondweed is taken further away, the bubbles will slow down again - an instant and visual indicator of the importance of light intensity in photosynthesis. Our video demonstrates how best to use this protocol with your students in the lab.
The bubbles can be counted and the rate of bubbling can serve as an indication of the rate of photosynthesis, or the gas can be collected in a pipette or microsyringe and the amount measured. Students can investigate the effects of either light intensity or the wavelength (colour) of light on photosynthesis.
This resource includes student sheets with 4 different investigations, technical notes and full teachers' notes.
There are a few tricks to getting this practical to work at its best, and we outline them in detail in the technical notes given here. Your choice of pondweed is key - although a pondweed named Elodea is often used to demonstrate this, it is very unreliable. We recommend Cabomba, a tropical pondweed easily sourced from aquatic shops and school science suppliers. You will also need to ensure that your light source is sufficiently intense, and we recommend placing the pondweed in a solution of sodium hydrogencarbonate rather than water. By following these simple tips, you'll have bubbling pondweed to show to your students all year round.
Always dispose of non-native plants like Cabomba appropriately - be plant wise.
Download the resources from the link on the right.