Photosynthesis is the process of formation of organic compounds from Carbon dioxide and hydrogen donor like water using radiations or light energy trapped by chlorophyll. Oxygen is evolved where water is used as hydrogen donor. During photosynthesis, light energy is changed into chemical or bond energy present inside the organic compounds.
The overall equation for photosynthesis is
6 CO2 + 12 H2O + photons → C6H12O6 + 6 O2 + 6 H2O
Water is the hydrogen donor. A molecule of water can donate only one hydrogen. As photosynthesis converts inorganic carbon into organic state, it is also called carbon assimilation.
Carbon dioxide as raw material: It is obtained from air. Concentration of Carbon dioxide in the atmosphere is only 0.03%.However the total amount is quite large, sufficient to last for several years even without replenishment. For passing into photosynthetic cells,carbondioxide dissolves in water .Aquatic plants also depend upon dissolved carbon dioxide present in water .If we increase the concentration of carbon dioxide in atmosphere by hundred times the rate of photosynthesis goes on increasing. But in practice it is extremely difficult to increase carbon dioxide concentration in the atmosphere to the optimum level required for photosynthesis.
Water as raw material: Water is a raw material for all photosynthetic organisms except bacteria. Bacterial photosynthesis is anoxygenic.Photosynthetic bacteria used either organic acids or reduced sulphur as reluctant for carbon assimilation. It is amazing that of the total water absorbed by a plant body only 1% is used in photosynthesis. Water may become a limiting factor during drought condition.
Photosynthesis is a redox process in which hydrogen is obtained from various sources. In oxygenic photosynthesis, water is used as reductant and oxygen is evolved from water.