Carbon dioxide from the air

So far: sunlight, water and chlorophyll. Is anything else needed? Bhaskar's sister says air plays a part. Which gas in the air?

Activity 10.4 is a teacher demonstration. First, a potted green plant is kept in the dark for two to three days to destarch it — to use up any stored starch — so that any starch found later must be newly made.

A wide-mouthed bottle holds some caustic soda (sodium hydroxide), which soaks up carbon dioxide from the air. Caustic soda is a strong chemical that can burn the skin, so only teachers should handle it.

Half of one destarched leaf is pushed into the bottle through a split cork; the other half stays outside. The set-up stands in sunlight for a few hours, then the leaf is tested with iodine.

The half outside the bottle turns blue-black: starch was made there. The half inside does not: no food was made there. Why? The caustic soda had taken away the carbon dioxide from the air around that half.

So carbon dioxide from the air is essential for plants to make starch.

Now the list is complete. Sunlight, water, chlorophyll and carbon dioxide are all needed for plants to make food. The process by which plants make food using sunlight and chlorophyll is called photosynthesis ('photo' means light, 'synthesis' means putting together). The leaf is the main place for it, but any green part of a plant can do it.