Question

Write an experiment to show that water is necessary for photosynthesis.

Answer 1

It is a part of the process of how a plant produces glucose andother sugars in order to live. In order to photosynthesize, a plantneeds water from what is in the air (measured by humidity) or fromwhat it takes from the ground in its roots, or on its leaves whenit rains, and carbon dioxide from the atmosphere. The plant usesthese two elements to produce by products oxygen, which is releasedas a waste product to the atmosphere, and glucose, which is usesfor energy. Specifically, water is needed for photosynthesis because it'selectrons are used to replenish the lost electrons from thechlorophyll in photosystem II. The hydrogens from water are alsoused to make a proton gradient that powers the ATP synthase whichis used to make ATP for the Calvin cycle. The oxygen is a wasteproduct of the process.
Stable isotopes of carbon and hydrogen are useful indicators of the photosynthetic pathway.

Use a hydrogen isotope labelled water to see that the water is split in the photolysis reaction. Then track the hydrogen ions to see where they end up in the plants.

2 (H2O) --- photolysis reaction --> O2 + ATP + NADPH (carrier molecule will transfer H isotope to calvin cycle if water provides the H to build glucose)

NADPH transfers to the Calvin cycle step of photosynthesis and provides the hydrogen to build the CO2 into an organic molecule.

Photosynthesis
CO2 + H2O → → O2 + CH2O

Answer 2

1. Take a potted plant and destarch it by keeping it in dark room for 2-3 days.

2. Now place it in sunlight but donot water it for 1-2 days.

3. After 3-4 days , you will observe that plant starts wilting.

4. Now take its one of the leaves and boil it in alcohol for 20-25 minutes.Now cool it and put few drops of iodine solution on it.

5.You will find no color change .  

Explanation:

The above experiment shows that:-

1. After destarching, it was left with no starch.
2. When we did not water it for long time, it became impossible for it to synthesize glucose.
3. No change in color shows the absence of starch.

This happens because in the process of  photosynthesis ( light reaction) the step of photo oxidation of water did not take place and as a result the dark reaction also did not take place.

This shows that water is essential for the process of photosynthesis.