A candidate studied the importance of certain factors in photosynthesis. He took a potted plant and kept it in the dark for over 24 hours. In the early hours of the next morning, he covered one of the leaves with dark paper in the centre only. Then he placed the plant in sunlight for a few hours and tested the leaf which was covered with black paper for starch.

(a) What aspect of photosynthesis was being investigated?

(b) Is there any control in this experiment? If so, state it.

(c) Why was the plant kept in the dark before the experiment?

(d) Describe step by step, how the candidate proceeded to test the leaf for the presence of starch.

(a) The candidate was studying the effect/importance of sunlight on the rate of photosynthesis.

Explanation: The candidate had covered one part of the leaf to study the rate of photosynthesis in the absence of sunlight. The candidate would compare it with the rate of photosynthesis on the uncovered parts of the leaf and thus, determine the role of sunlight during photosynthesis.

(b) Yes, the uncovered portion of the leaf of the potted plant is used as the control here.

Explanation: The control is used to compare the results which will be obtained after the experiment with the covered portion of the leaf to obtain conclusions.

(c) Plant was kept in the dark before the experiment, to remove the starch present in the leaf, to the storage organs.

Explanation: Destarching is done by placing the plant for 24-48 hours in darkness before starting the experiment. This is done to make sure that all starch has been formed under experimental conditions only.

(d) 1) First, the candidate would have dipped the prepared leaf in boiling water to kill the leaf cells.

2) Then he would have boiled the leaf in methylated spirit to remove the chlorophyll.

3) The leaf would have then been placed in hot water to soften it.

4) he would have placed it in a dish and poured iodine solution. The presence of starch is confirmed in regions that turn bluish-black in colour.

The uncovered portion of the leaf would have turned bluish black and the cover region would have turned brown in colour.

Explanation: The uncovered portion of the leaf would have turned bluish black as starch would have formed in those regions. Sunlight activates the chlorophyll, which in turn catalyzes the photosynthesis reaction. In the covered portion, on which sunlight could not have penetrated, the region turns brown due to the absence of chlorophyll. This implies that photosynthesis would not have taken place on the covered portion of the leaf.