Sara placed a healthy potted plant in a dark room for 48 hours to perform an experiment on photosynthesis. She plucked one of the leaves and tested it for starch. The leaf did not turn blue-black on adding Iodine solution.
Why was the plant placed in the dark for 48 hours?
Topic: Destarching a plant
Answer
To DESTARCH the plant — in darkness photosynthesis stops but respiration continues, so the starch already stored in the leaves is used up. This ensures that any starch found afterwards must have been made DURING the experiment.
The word to use is DESTARCHING, and the reason matters as much as the term.
THE LOGIC OF A CONTROLLED EXPERIMENT: if the leaf already contained starch at the start, a positive iodine test at the end would prove nothing — the starch might be left over. Emptying the leaf first means any starch present at the end must have been made during the test, so the result is valid.
This step appears in EVERY photosynthesis experiment — testing whether light, chlorophyll or carbon dioxide is necessary — and examiners ask about it repeatedly. Note also that the question tells you the leaf did NOT turn blue-black, which confirms the destarching worked.