(iii) Describe how to safely test the leaves for starch. (b) Explain how the results of this experiment would show that a plant needs light for photosynthesis.

4. (a) (iii)
To safely test the leaves for starch:

1. Boil the leaf in water for a few minutes. This kills the cells and breaks down the cell walls and membranes, making it easier for iodine to penetrate.
2. Decolorize the leaf: Place the boiled leaf in a test tube containing ethanol (alcohol). Place this test tube in a beaker of hot water (a water bath). Do not heat ethanol directly over a flame as it is flammable. The ethanol will remove the chlorophyll, making any colour change from the iodine test visible.
3. Rinse the leaf: Remove the leaf from the ethanol and rinse it thoroughly with cold water to rehydrate it and remove any remaining ethanol.
4. Test for starch: Spread the leaf on a white tile and add a few drops of iodine solution. Observe any colour change.

4. (b)
The results of this experiment would show that a plant needs light for photosynthesis by comparing the starch test results of leaf Y:

• The part of leaf Y that was exposed to light would turn blue-black when tested with iodine solution, indicating the presence of starch.
• The part of leaf Y that was covered by black paper (and therefore deprived of light) would remain brownish-yellow (the colour of iodine solution), indicating the absence of starch.
• This difference in starch presence demonstrates that light is essential for the plant to carry out photosynthesis and produce starch.