. A candidate studied the importance of certain factors in photosynthesis

 A candidate studied the importance of certain factors in photosynthesis. He took a potted plant and kept in the dark for over 24 hours. In the early hours of the next morning, he covered one of the leaves with black 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?


Best Answer

(a) To study the effect of sunlight on the rate of photosynthesis.

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

(c) A plant used for an experiment on photosynthesis should initially be placed in the dark for 24 to 48 hours to distarch the leaves. During this period, all the starch will be removed from the leaves and stored in the storage organs. The leaves will not show the presence of starch.

(d) (a) The candidate dipped the leaf in boiling water for a minute to kill the cells.

(b) Then boiled the leaf in methylated spirit over a water bath till it becomes pale-white due to the removal of chlorophyll. The leaf now becomes hard and brittle.


(c) Then place it again in hot water to soften it.

(d) Spread the leaf in a dish and pour iodine solution on it. The presence of starch will be indicated by a blue-black colour. A leaf without starch will show brown colouration.



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