No animal can utilize the solar energy directly for its life activities. The green plants can utilize the solar energy to prepare food because they contain chlorophyll which can trap solar energy. During the process of photosynthesis the plants utilize carbon dioxide, water andsunlight, in the presence of chlorophyll to build up complex carbohydrate molecules. In these carbohydrate molecules the solar energyis trapped as potential chemical energy. Thus, solar energy is introduced into the biosphere through photosynthesis. Photosynthesis is one of the most important natural processes, which converts the radian energy into chemical energy and links the non-living world with living world.

“Photosynthesis is the phenomenon of synthesis of carbohydrates (mostly glucose) by photoautotrophs (producers – green plants in terrestrial ecosystem and phytoplanktons in aquatic ecosystem), from carbon dioxide (CO₂) and water (H₂O) using enzymes in presence of sunlight and chlorophyll, and releasing oxygen (O₂) as a by-product”. This O₂ comes from water and not from CO₂.