Proving What Photosynthesis Needs
Four experiments, and the control that makes each one work.
To prove that carbon dioxide is necessary for photosynthesis.
Before any experiment on photosynthesis, the plant is destarched — kept in the dark for 24 to 48 hours so its leaves are free from starch. Now set this one up.
Apparatus
How do you deprive one part of a leaf of carbon dioxide while it is still attached to the plant?
The starch test itself: dip the leaf in boiling water to kill the cells, boil it in methylated spirit over a water bath to remove the chlorophyll, soften it in hot water, then add iodine. Blue-black means starch is present, brown means it is not.
Free play — set the experiment up, choose its control, and commit to a prediction before anything is revealed. Switch to when you're ready to test yourself.
What this lesson covers
Predict first
After a few hours in sunlight you test the whole leaf with iodine. Predict what you will see.
Challenges to clear
- Set the experiment up and identify its control.
- Predict the result before it is revealed.
- Draw the right conclusion.
The task
Before any experiment on photosynthesis, the plant is destarched — kept in the dark for 24 to 48 hours so its leaves are free from starch. Now set this one up.
The experiment
To prove that carbon dioxide is necessary for photosynthesis.
How do you deprive one part of a leaf of carbon dioxide while it is still attached to the plant?
Hint
The starch test itself: dip the leaf in boiling water to kill the cells, boil it in methylated spirit over a water bath to remove the chlorophyll, soften it in hot water, then add iodine. Blue-black means starch is present, brown means it is not.