Activity to prove that water is necessary for photosynthesis
Answers
Explanation:
1. De-starch a potted plant by putting it in complete darkness for two days.
2. Fill partly a wide-mouthed bottle with strong solution of caustic potash and fit a split cork on its mouth.
3. Insert about half of the portion of a leaf of the de-starched plant into the bottle through the split cork (Fig. 36).
Demonstration of Moll's half-leaf experiment
4. Place the whole apparatus in light after applying grease on the upper portion of split cork, and test the leaf for stach after about 10 hours.
Observations:
Portions of the leaf inside the bottle as well as in between the split cork show negative test for starch indicating the absence of photosynthesis while the portions outside the split cork show positive test for starch indicating the presence of process of photosynthesis in this region.
Results:
Negative starch test by the leaf portion present inside the bottle indicates that process of photosynthesis is absent in this region. This portion of leaf is getting all the essential requirements, i.e., light, chlorophyll and water except the CO2 because the latter is absorbed by the caustic potash. Thus, it can be concluded that CO2 is necessary for this process.
Negative test of starch, which is also shown by the portion of the leaf present in between the split of the split cork, can be explained that it is due to the lack of CO2 and light, thus indicating that both of them are essential requirements.
Positive test of starch shown by the portions of the leaf present outside the bottle indicates that photosynthesis process is continuously going on there because all the essential requirements, i.e., light, chlorophyll, water and CO2 are readily available to this portion.
That the chlorophyll is also an essential requirement for photosynthesis can be shown by testing starch in a variegated leaf. Only green portions of the leaf show positive starch test.