Question

A potted plant is placed in a dark room for about 2 days to de-starch its leaves. One of its leaves is covered with the thin strip of black paper as shown in the picture, make sure that the leaf is covered on both sides.

The potted plant is kept in bright sunlight for 4 to 6 hours. The selected covered leaf is plucked and the black paper is removed.

The leaf is immersed in boiling water for a few minutes and then in alcohol to remove chlorophyll. The leaf is now tested with iodine solution for the presence of starch. The covered part of the leaf does not turn blue-black whereas the uncovered part of the leaf turns blue-black colour. Why are the changes in colour noted in the covered and uncovered part of the leaf?

Answer 1

Experiment to that sunlight is essential for photosynthesis:

Aim: To show that sunlight is necessary’ for photosynthesis.

Material required: potted plant, black paper, boiling water, alcohol and iodine solution.

Procedure: A potted plant is placed in a darkroom for about 2 days to de-starch its leaves. One of its leaves is covered with the thin strip of black paper as shown in the picture, make sure that the leaf is covered on both sides.

The potted plant is kept in bright sunlight for 4 to 6 hours. The selected covered leaf is plucked and the black paper is removed. The leaf is immersed in boiling water for a few minutes and then in alcohol to remove chlorophyll. The leaf is now tested with iodine solution for the presence of starch. The covered part of the leaf does not turn blue-black whereas the uncovered part of the leaf turns blue-black colour. The covered part of the leaf which did not receive the sunlight was unable to synthesize starch. Hence it does not turn blue black colour. But the uncovered part of the leaf which received sunlight was able to synthesise starch and so it turns blue-black in colour.