equation for photosynthesis

Before 1930, it was considered by physiologists that one molecule each of CO2 and H2O form a molecule of formaldehyde (HCHO), of which 6 mols are polymerized to one molecule of glucose (a hexose sugar).LightCO2 + H2O -> HCHO + 02Chlorophyll (Formaldehyde) Polymerization6CH20 (or 6HCHO)-> C6Hi206(Formaldehyde) (Hexose sugar)However formaldehyde is a toxic substance which may kill the plants. Hence, formaldehyde hypothesis could not be accepted.On the basis of discovery of Nicolas de Saussure (1804) that "The amount of O2 released fromplants is equal to the amount of CO2 absorbed by plants", it was considered that O2 released inphotosynthesis comes from CO2, but later on it was proved wrong.In 1930, C.B. van Niel proved that, sulphur bacteria use H2S (in place of water) and CO2 tosynthesize carbohydrates as follows :6CO2 + 12H2S-> C6H1206 + 6H20 + 12SThis led van Niei to the postulation that in green plants, water (H2O) is utilized in place of H2Sand O2 is evolved in place of sulphur (S).6C02 + 12H20-> C6H1206 + 6H20 + 602This was confirmed by Ruben and Kamen in 1941 using chlorella a green alga. He used water(H20), having heavy isotope of oxygen (O18), oxygen of 18 atomic weight (normal oxygen is of 1618atomic weight) and found that oxygen released in the process of photosynthesis was of O 0 type (i.e., heavy oxygen). On the contrary when CO28 was used, the released oxygen was of normal type, i.e., O26. From this it can be concluded that, oxygen released by green plants comes from splitting of water (oxidation of water).Furthermore because 6 molecules of oxygen can be produced by 12 molecules of water only, the summary equation of photosynthesis is written as follows :Oxygenic and Anoxygenic Photosynthesis(i) Oxygenic photosynthesis (with evolution of O2) is found in eukaryotes and cyanobacteria. Water acts as H-donar to produce NADP.2H thus photolysis of water is essentialLightC02 + H2O-> Sugar + OxygenChlorophyll(ii) Anoxygenic photosynthesis (without the evolution of O2) is found in photosynthetic bacteria. H2S acts as H-donar and no photolysis of water is needed.LightCO2 + H2S-> Sugar + Sulphur or other oxidised compounds orChlorophyll some other inorganic compoundsAnoxygenic photosynthesizing forms are strictly anaerobic and thus O2 acts as a poison for them.