Question

(a) Define 'Photosynthesis'. Apart from sunlight, what other conditions are necessary for the process of photosynthesis.
(b) Describe an experiment to show that "sunlight is essential for photosynthesis".

Answer 1

(a) Photosynthesis is the process by which plant cells prepare food (carbohydrates) from inorganic raw materials such as carbon dioxide and water in presence of sunlight and chlorophyll.

The factors other than sunlight affecting the rate of photosynthesis are:

• Carbon dioxide − Carbon dioxide acts as the substrate for photosynthesis. During this process, carbon dioxide and water are converted into carbohydrates.
• Water − During photosynthesis, water is photolysed into oxygen and hydrogen and it provides the energy for photosynthesis.
• Chlorophyll − Chlorophyll is the green pigment present in the leaves which absorbs sunlight.

(b) Experiment to demonstrate that light is necessary for photosynthesis:

1. Take two potted plants. Place one plant in a dark room and the other in a room receiving sufficient sunlight.
2. After 3-4 days, take one leaf each from both the plants. Bathe the two leaves first in hot water and then in alcohol.
3. Add some drops of iodine solution on the leaves.

Observation:

It can be observed that the leaf taken from the plant kept in sunlight develops blue-black patches whereas the leaf taken from the plant kept in the dark room does not show any patches.

Reason for the observation:

Plants which receive sunlight prepare their own food through the process of photosynthesis. It is the presence of starch that turns the colour of iodine solution blue-black. Therefore, blue-black patches are observed on the leaf taken from the plant kept in sunlight. In contrast, the leaf of the plant kept in the dark room does not produce any starch. Therefore, no patches are observed on it.