Watching Photosynthesis in Action: Elodea’s Colour Change
We thank our Biology technician, Mrs Wansbury, who has made this wonderful time-lapse video of the pondweed Elodea. The spectacular colour change which occurs when the plant is exposed to bright light for just a few hours allows us to “see” photosynthesis taking place, in a demonstration which is based on some interesting biochemistry.
The water in which the Elodea is submerged contains a bicarbonate indicator, a chemical cocktail that responds to pH by changing colour. In low pH or acidic conditions, the indicator will be yellow. At neutral pH, around 7, the indicator begins to turn red. In high pH or alkaline conditions, the indicator darkens to purple.
At the start of the clip, the water is yellow. This is because it contains an artificially high concentration of carbon dioxide molecules, which react with water to form carbonic acid, which readily dissociates into an acidic hydrogen ion and the bicarbonate ion.
When the lamp is directed at the Elodea and it begins to use up the dissolved carbon dioxide in photosynthesis, this sequence of chemical events is pulled in the opposite direction. As the acidic hydrogen ions disappear from the solution, the pH creeps up and the indicator turns from yellow to red to purple.
Our thanks again to Mrs Wansbury for this fascinating demonstration.
