Question

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.

Describe step by step, how the candidate proceeded to test the leaf for the presence of starch?

Answer 1

• The student dipped the leaf in boiling water for a minute to kill the cells.
• Then he boiled the leaf in alcohol/methylated spirit over a water bath to remove chlorophyll. The leaf becomes hard and brittle.
• He then places the leaf in hot water to soften it.
• Next the student spreads the leaf in a dish and pours iodine solution on it. The presence of starch is indicated by a blue-black colour.
• The uncovered portion (exposed to sunlight) turned blue-black colour and the covered portion showed brown colour. The difference in the colours of covered and uncovered part of leaves indicates the importance of sunlight in photosynthesis.