Sequential transduction of light into redox and acid—base energy in photosynthesis

Photosynthesis consists essentially in the conversion, by the plant kingdom, of sunlight energy into chemical energy (cell material and molecular oxygen). From a physicochemical point of view, photosynthesis is intrinsically a light-driven oxidation—reduction/acid—base process. In the course of photosynthesis, water is oxidized to molecular oxygen, whereas carbon dioxide, nitrate or dinitrogen, and sulfate are respectively reduced to carbohydrate, ammonia and sulfide; water is moreover ionized, in similar amounts, into protons and hydroxide anions. Phosphorus does not change its oxidation state, but orthophosphate becomes energized to metaphosphate, at the expense of the ionization products of water, in a peculiar acid—base process. Metaphosphate is mostly used as an energy shuttle in many cell processes, among them in the reduction and assimilation of the primordial bioelements, as well as in the polymerization of the resulting monomers — sugars, lipid components, amino acids, nucleotides — and deenergizes itself back to orthophosphate, again in acid—base reactions. The role of the oxide anion, that is, an oxygen atom with two additional electrons, O 2− , in these bioenergetic processes is especially discussed.