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.
Cabomba is no longer available in the UK, due to the recent invasive plant directive.
We have trialled various alternatives. These do not bubble as freely as Cabomba, but enable you to continue this practical. These species are likely to give off more bubbles if placed in lukewarm water, rather than cold, and given some time to come up to temperature. However, you may wish to consider alternative practicals to look at photosynthesis, such as the ‘algal balls’ practical (see www.saps.org.uk).
Ceratophyllum demersum (Hornwort) A British native species, suitable for growing in a school pond, or in an indoor tank. This is a robust, fast-growing, easily-propagated plant.
Myriophyllum scabratum (note – do not use other Myriophyllum species, as these can be invasive). This is a fast growing aquarium plant, which appreciates bright light and an aquarium tank at room temperature. It is widely available from aquarium suppliers online, and from Blades Biological.
Egeria densa (formerly Elodea densa). This is a non-native species that can tolerate relatively cold temperatures and low light levels. However, we would not recommend growing this in school ponds, as it is an invasive species that can cause harm to invertebrates and fish.
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. 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.
Download the resources from the link on the right.