Question

Given alongside is a sketch of a leaf partially covered with black paper and which is to be used in the experiment to show that light is compulsory for the process of photosynthesis. At the end of the experiment, which one of the leaf parts labelled I, II and III will become black when dipped in iodine solution?

A. I only
B. II only
C. I and III
D. II and III

Answer 1

Hint: Photosynthesis is the process by which green plants prepare their own food in the presence of sunlight and chlorophyll by using carbon dioxide and water from the atmosphere and soil respectively. Chlorophyll is the green pigment present on the leaves. The food prepared is in the form of glucose and is stored in the leaf in the form of starch.

Complete answer:
- The plants prepare their food in the presence of sunlight. The sunlight is the limiting factor for photosynthesis. In the absence of sunlight, the plants are not able to synthesise glucose or starch.
- The presence of starch in any food or substance can be tested by the help of iodine solution. The iodine turns black in the presence of starch and remains unchanged if starch is absent.
- In the experiment given in the question, part II is covered with a black strip. So that region will not receive any sunlight. In the absence of sunlight, it will not be able to synthesise starch. As iodine is an indicator used to test the presence of starch, it will not turn black.
- The part I and II are exposed to sunlight and will do photosynthesis and prepare starch. The iodine solution will turn black indicating the presence of starch.

Hence the correct answer is C.

Note: In photosynthesis, the exchange of gases takes place through the fine openings at the base of the leaf called stomata. The chloroplast is the organelle in a plant cell that contains chlorophyll which is an essential pigment for photosynthesis. The carbon dioxide is reduced to sugars and starch during this process.