Light is one of the requirements for photosynthesis. When a part of the leaf is covered from both upper and lower sides with a black paper strip, the light will not reach to covered part. Hence, photosynthesis will not take place in the covered part of the leaf. On the other hand, as light will be available to uncovered part of the leaf, cells of this region will carry out photosynthesis actively and hence, glucose will be synthesised in the uncovered part of the leaf. In the leaf, a part of this glucose is utilised to carry out various metabolic activities and rest of the amount converts into starch. Presence of starch in the leaf, which can be detected by performing the iodine test, indicates the occurrence of photosynthesis.
6CO2 + 12H2O + Chlorophyll / Sunlight →C6H12O6 + 6H2O + 6O2
This procedure should be followed:
1. Take the potted plant and keep it in the dark place for 2-3 days so that the leaves get destarched.
2. Cover a part of one of its leaves with the strip of black paper. Make sure that you cover both the sides of the leaf.
3. Now place this plant in sunlight for 3—4 hours.
4. Pluck the selected covered leaf and remove the black paper covering it.
5. Place this leaf in the beaker containing water and boil it for about 10 minutes.
6. Take out the leaf and now boil it in alcohol, using the water bath, for 10 minutes. This removes the chlorophyll.
7. Take out the leaf and wash it under running water.
8. Place this leaf in the Petri dish and put a few drops of iodine solution on it. Now observe the change in colour.
The leaf turns blue-black except in the covered region. As this covered region did not receive light, photosynthesis did not occur. Hence no starch was formed there. The uncovered region received light and starch was formed there due to photosynthesis.
This tells us that light is essential for photosynthesis.
Hence Student D made the right suggestion because destarching of the leaf by keeping the plant in darkness before starting the experiment is essential. Because then only photosynthesis the process of making food will occur efficiently.
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