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Question

Experiment to show that sunlight is essential for photosynthesis.

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Solution

Experiment to prove that sunlight is essential for photosynthesis:

PROCEDURE:

Step 1: Keeping the plant in the darkroom:

  1. At first, we need to place a healthy green potted plant in a dark room for 1-2 days.
  2. This is done to ensure that the plant consumes all its reserve food and the leaves do not contain any starch. ​

Step 2: Covering a specific part of a leaf:

  1. A part of the leaf is covered on both sides with two uniform pieces of black paper, fixed in position with two paper clips on both sides so that the leaf region below the black paper is not exposed to sunlight. ​

Step 3: Exposing the plant to sunlight:

  1. Now the plant is exposed to bright sunlight with the specific part of the leaf staying covered. ​

Step 4: Decolorizing the leaf:

  1. After a few hours the specific leaf with the covering is removed.
  2. The black papers are removed and then the leaf is subjected to decolorization using alcohol in boiling water.
  3. The decolorization occurs as the chlorophyll is removed from the leaf and hence the leaf loses its pigmentation. ​

Step 5: Adding iodine solution:

  1. To the decolorized leaf, iodine solution is added. ​


OBSERVATION:

After adding the iodine solution the part of the leaf that was covered by black paper remains colorless while the region that was exposed to sunlight will have blue-black color.


INFERENCE:

  1. The food prepared by plants (carbohydrates) through the process of photosynthesis is stored as starch.
  2. This starch reacts with the iodine solution to change to a blue-black color.
  3. Only those portions of the leaf that were exposed to sunlight could photosynthesize and hence, change to a blue-black color when tested with iodine. ​
  4. The region that was closed by black papers on both sides were not able to photosynthesize and hence the color did not appear.
  5. This proves the importance of sunlight in the process of photosynthesis as it does not occur in the parts of leaves that are not exposed to sunlight.

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