It has been found by experiments that the presence of sunlight, chlorophyll, carbon dioxide and water is necessary for the process of photosynthesis. So, we can say that: The conditions necessary for photosynthesis to take place are:
3. Carbon dioxide, and
Please note that the conditions necessary for photosynthesis are also the conditions necessary for autotrophic nutrition. We will now describe some experiments to show that sunlight, chlorophyll and carbon dioxide are necessary for photosynthesis by green plants. These experiments will also show that leaves finally make ‘starch’ as food by photosynthesis.
The experiments on photosynthesis depend on the fact that green leaves make starch as food. And that starch gives a blue-black colour with iodine solution. Now, ordinarily all the plants have starch in their green leaves, so before we can use a plant in a photosynthesis experiment, the initial starch present in its leaves must be removed. In other words, we should destarch the leaves of a plant before using it in a photosynthesis experiment. The green leaves of a plant are destarched by keeping this plant in a completely dark place in a room for at least three days.
When the plant is kept in a dark place, it cannot make more starch (food) by photosynthesis because there is no sunlight. So, the plant kept in dark place uses the starch already stored in its leaves during respiration. The plant will use up all the starch stored in its leaves in about three days time. So, after about three days, the plant leaves will not have any starch left in them.
And we say that the leaves have been destarched. This plant with destarched leaves can now be used in the photosynthesis experiments. Please note that we will be using a plant growing in a pot in these experiments. The ‘plant growing in a pot’ is called ‘potted plant’. Let us describe the experiments now.