Question

A destarched leaf on a potted plant was covered with black (A) transparent (B) strips of paper as shown in figure. After 6 hours of exposure to sunlight the leaf was removed from the plant and tested for starch. Which are the following will be the correct observation?

A. Whole leaf turned blue black
B. Only B portion turned black
C. Only A & B portions remained colorless and the rest of the leaf turned blue black
D. Only A portion remained colorless and the rest of the leaf remained blue black

Answer 1

Hint: Sunlight is an essential factor in the process of photosynthesis. Plants capture photons and use them to synthesize chemical energy in the form of ATP and NADPH that is used for the synthesis of carbohydrates.

Complete answer: The portion A was covered with a black strip and did not receive sunlight to initiate the process of photosynthesis. Sunlight is an essential factor that is required to initiate food production in plants. So, when the destarched leaf is tested for starch then the ‘portion A’ remains colorless because it did not receive sunlight. Whereas, the portion B is covered with a transparent strip and receives sunlight and therefore, it initiated the process of starch synthesis. When the destarched leaf is tested for starch, then the portion B turns blue-black due to the presence of the starch. Therefore, portion A is colorless and portion B produces a blue-black color. This experiment proves that sunlight is an essential factor that initiates the process of photosynthesis in leaves. Photosynthesis involves two phases; light and dark phases. The actual fixation of carbon dioxide occurs in the light independent phase of photosynthesis.
Hence, the correct answer is option D.

Note: Starch synthesis is initiated in the presence of sunlight. In sunlight, chemical energy is synthesized in the light phase of photosynthesis. Whereas, in the dark phase carbohydrates are synthesized by using this chemical energy received from the light dependent reactions.