Question

Sara placed a healthy potted plant in a dark room for 48 hours to perform an experiment on photosynthesis. She plucked one of the leaves and tested it for starch. The leaf did not turn blue-black on adding Iodine solution.

1. Why was the plant placed in the dark for 48 hours?
2. What is the significance of boiling the leaf in alcohol during the starch test?

Answer 1

1. The plant was kept in the dark to destarch it, meaning all the previously stored starch in the leaves is used up by respiration. This ensures that any starch detected later during the test is freshly produced during the experiment and is a result of photosynthesis, not from earlier storage.
2. Boiling the leaf in alcohol removes chlorophyll, which gives the leaf its green colour. This decolourisation helps in clearly observing the colour change when iodine solution is added. As a result, the blue-black colour indicating the presence of starch can be seen more distinctly.