Video clip – Production of oxygen in plants

This video clip from the BBC Four Botany: A Blooming History demonstrates a classic experiment to investigate the production of oxygen by plants through photosynthesis.

In the late 18th century, pioneering scientist Jan Ingenhousz placed plants under water and watched. When placed in the dark, plants produced no gases. When placed in direct sunlight the leaves released tiny bubbles of gas. The gas that accumulated in the container was able to re-light a glowing splint, indicating that it was oxygen. Ingenhousz repeated his experiment many times, with many different species. They all produced oxygen gas. To show that it is the light energy and not the heat of sunlight which is necessary he repeated the experiment with leaves placed near a fire. He established that the production of oxygen was a universal process for which plants need light.

The design of Ingenhousz’ experiments can be discussed. The control of variables such as light and heat etc. can be considered, and the way that only one variable can be changed while all others are controlled if firm conclusions are to be drawn. The repeating of the experiments in order to consolidate the data and get more reliable results upon which to base a conclusion can be discussed. The process of collecting the oxygen from plants can also be developed as practical work. Pondweed can be exposed to a variety of light intensities and the relationship between light intensity and rate of photosynthesis can be investigated. This can be done with a variety of ages and levels of ability of student. Older students can use this process to investigate the light dependent stage of photosynthesis and then develop the concept to look at the effect of different wavelengths light.