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Question
Describe an experiment to show that “sunlight is essential for photosynthesis”.
Solution
Sunlight is essential for photosynthesis
Place a healthy, green, potted plant in a dark room for 1−2 days. This is done to ensure that the plant consumes all its reserve food and the leaves do not contain any starch. Then, cover a portion on both sides of a green leaf with two uniform pieces of black paper. Fix the cover in position with two paper clips.
Now, expose this plant to bright light. After a few hours, remove the leaf and decolourise it with alcohol. Now, test the presence of food (starch) by putting iodine solution on the leaf.
It can be observed that the covered portion of the leaf does not show any presence of starch (food).
This is because plants store the food prepared by through the process of photosynthesis as starch. Starch reacts with iodine solution to give blue−black colour.
In the performed experiment, only those portions of the leaf that were exposed to sunlight could photosynthesise. Hence, the uncovered portion of the leaf gives blue−black colour when tested with iodine.
The covered portion of the leaf did not receive any sunlight. Hence, starch was not produced in this portion. So, it does not change its colour when treated with iodine solution.
Thus, it can be concluded that the sunlight is essential for photosynthesis.
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