Demonstrating Oxygen Evolution during Photosynthesis using Pondweed

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, which species of pondweed can be used, how to care for pondweeds and a short explanation on the physiology that allows pondweed to bubble.

The bubbles produced by pondweed 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. We have also provided a PowerPoint presentation and notes on pondweed physiology.

Previously, Cabomba caroliniana was recommended as a species for this protocol but this is no longer available in the UK, due to the invasive plant directive. You may find that some exam specifications and text books still mention this species but you can use any of those mentioned in this video or any other species that you find work for you.

In addition, you may wish to consider alternative practicals to look at photosynthesis, such as the ‘algal balls‘ practical.

Note: When buying from pet stores in particular, the staff may not be able to identify the type of pondweed they are selling but we hope that you will be able to use the video and images in the guide to pondweeds to help.