Explain why the potted plant is placed in the dark for 24 hours. Explain the role of leaf Y in the experiment.

To accurately test for starch production during the experiment, any pre-existing starch must be removed.

During this period, the plant uses up its stored starch reserves through respiration.

• The potted plant is placed in the dark for 24 hours to destarch it. This means all the starch previously stored in the leaves is used up for respiration, ensuring that any starch detected at the end of the experiment was newly produced during the experiment.

4. (a) (ii)

It is exposed to the atmosphere (providing carbon dioxide) and light, but a strip of black paper covers part of it.

It blocks light from reaching the covered part of the leaf, while the rest of the leaf is exposed to light.

• Leaf Y serves as a control to demonstrate that light is necessary for photosynthesis. The part of leaf Y covered by the black paper will not photosynthesize and thus will not produce starch, while the uncovered part will. This allows for a direct comparison to show the effect of light.