More about carbon dioxide

Chloroplasts contain a powerful enzyme called carbonic anhydrase that turns hydrogen carbonate back into carbon dioxide. This is then used in photosynthesis to make glucose.

Carbon dioxide dissolves easily in water to form hydrogen carbonate ions (sometimes called bicarbonate). Elodea takes hydrogen carbonate ions into its cells and uses it as a source of carbon dioxide.

Carbon dioxide is often a limiting factor in photosynthesis and we can increase the amount of hydrogen carbonate in the water in a number of different ways.

Soda water contains dissolved carbon dioxide as well as bubbles of carbon dioxide gas.

We can make our own soda water by adding carbon dioxide (from a gas cylinder) to water. Gas cylinders can be dangerous, and this should only be tried if your teacher is supervising you.

Blackman states that water cannot be saturated with carbon dioxide by 'merely bubbling gas through it, even in a very long time; prolonged shaking of the water with a large volume of pure CO₂ will, however, achieve this.' The CO₂ saturated water can be diluted with water to get the appropriate dilution. Boil tap water and allow it cool to get CO₂-free water.

Potassium hydrogen carbonate can also be added to water to enrich the supply of carbon dioxide. About 10 grams in each litre of water should be enough. Sodium hydrogen carbonate is not recommended for this purpose by McLean and Ivimey-Cook's Textbook of Practical Botany (1952).

Remember to allow time for the Elodea to take up the hydrogen carbonate before measuring its effects on the rate of photosynthesis.