Experiment to demonstrate that light is necessary for photosynthesis.

Apparatus and materials required: A healthy potted plant, a Petri, a beaker containing water, forceps, a water bath, a piece of wire gauzes, a burner, a box of matches, alcohol a strip of black paper, iodine solution, and clips.

Theory: Photosynthesis is a biochemical process by which green plants synthesized simple sugar in the presence of sunlight using carbon dioxide from the atmosphere and water from the soil. This simple sugar (glucose) is later converted to starch.

6Co₂ +12H₂O  — Chlorophyll/sunlight →  C₆H₁₂O₆ + 6H₂o + 6O₂

The most important factor for photosynthesis is light. The rate of photosynthesis depends on the quantity and quality of light. The chlorophyll molecule in green leaves absorb light, get excited, and emit electrons. The emitted electrons are used in the production of adenosine triphosphate (ATP) finally the solar energy is converted into chemical energy are stored in the glucose produced. The rate of photosynthesis is the maximum in the presence of red and blue lights, while in green light the rate is minimum because green light is reflected by the chlorophyll molecules.

Procedure:

1. Take the potted plant and keep it in a dark place for 2-3 days so that the leaves get detached.
2. Cover a part of one of its leaves with a strip of black paper. Make sure that you cover both sides of the leaf.
3. Now place this plant in sunlight for 3-4 hours.
4. Pluck the selected covered leaf and remove the black paper covering it.
5. Place this leaf in a beaker containing, water and boil it for about 10 minutes.
6. Take out the leaf and now boil it in alcohol, using the water bath, for 10 minutes. This removes the chlorophyll.
7. Take out the leaf and wash it under running water.
8. Place this leaf in the petri dish and put a few drops of iodine solution on it. Now observe the change in color.

Observations:

The leaf turns blue-black except in the covered region. As the covered region did not receive light photosynthesis did not occur. Hence no starch was formed there. The uncovered region receives light and starch was formed there due to photosynthesis.

Result: Light is essential for photosynthesis.

Precautions:

1. Before starting the experiment, the leaf must be distracted.
2. The leaf must be covered with black paper properly to prevent the entry of light.
3. Boling the leaf in alcohol should be done in a water bath.