In a class experiment, a potted plant is kept in a dark cupboard for two days
In a class experiment, a potted plant is kept in a dark cupboard for two days. Then a strip of black paper is clipped over part of one leaf, and the plant is placed in sunlight for a day. Finally the leaf is tested with iodine (the teacher first boils it in alcohol to remove the green colour). Prithvi predicts that only the uncovered part will turn blue-black. Saloni says the whole leaf will turn blue-black, because the covered part is still green and still has chlorophyll. Whose prediction is correct?
Who do you think is right? Pick one.
The answer: Prithvi is right, the covered part got no light, so no starch was made there
Why
Saloni is correct that chlorophyll is needed for photosynthesis — a leaf without it can't make food. But needed is not the same as enough. Photosynthesis needs chlorophyll AND sunlight AND carbon dioxide AND water, all together. The black paper blocked the sunlight, so the covered strip had chlorophyll but no light, and made no food. The two days in the dark cupboard matter too: they let the plant use up the starch already stored in its leaves, so any starch found afterwards must have been made during the sunny day. Iodine turns blue-black only where starch is present — so the uncovered part turns blue-black and the covered strip stays pale, in the exact shape of the paper. Test: repeat the experiment on a second plant without the black paper, and the whole leaf turns blue-black. Prithvi is right: only the part that received sunlight made starch, because chlorophyll alone can't make food without light.
On Socho, you explain your thinking out loud and a coach asks about YOUR reasoning — one question a day, free, in Hindi, Hinglish or English.
Try a question on Socho →More disagreements in Life Processes in Plants