Photosynthesis

Under standard conditions, the complete oxidation of glucose liberates 686 kcal/mol; the synthesis of a mole of glucose from carbon dioxide and water thus minimally requires the input of an equivalent amount of energy. In photosynthesis, visible light provides this energy. When it is considered that the only source of carbon for the tens of thousands of organic compounds synthesized in green plants is from the assimilation of carbon dioxide by means of photosynthesis, the inadequacy of Eq. (5) to describe photosynthesis, despite its usefulness, is readily apparent.

Inspection of Eq. (5) reveals that photosynthesis is an oxidation–reduction process. Simply put, photosynthesis is the light-driven reduction of carbon dioxide to the oxidation–reduction level of a carbohydrate by using water as the electron and hydrogen donor. In the process, water is oxidized to molecular oxygen. As stated previously, water is a very poor reducing agent. However, water at an effective concentration of 55 M is readily available in the biosphere. Although organic compounds and inorganic molecules such as hydrogen sulfide are more powerful reducing agents than water is, their use in photosynthesis as the source of electrons for photosynthesis is restricted to certain species of bacteria. The thermodynamically very unfavorable reduction of carbon dioxide bywater is driven by light.