8o 



PHOTOSYNTHESIS 



the green plant, i.e. CO2+2H2O -- CHgO+HgO+Oa, water 

 might be considered to act only as a hydrogen donor. Con- 

 sequently it would be expected that the oxygen liberated 

 should come entirely from the water molecules. By the use 

 of the isotope ^^O which can be introduced into either the 

 water or the carbon dioxide molecules (as bicarbonate or 

 carbonate) the composition of the oxygen evolved may be 

 compared with that of the reactants. The results are consis- 

 tent with the interpretation that the whole of the oxygen 

 originates from the water molecules (Table 5.3) and that 

 the equation for green plant photosynthesis should be 

 written: — 



CO2+2H2O* - (CHoO)+H20+02* 



TABLE 5.3 



THE COMPOSITION OF THE OXYGEN EVOLVED IN PHOTOSYNTHESIS 



IN Chlorella 

 {Ruben, Randall, Kamen, and Hyde (1941)) 



A complication arises in the interpretation of these experi- 

 ments since it is possible that exchange of oxygen isotope 

 may occur between carbon dioxide and water molecules and 

 that the oxygen liberated during photosynthesis may equili- 

 brate with the water present in the system either outside or 

 within the cell. In vitro these reactions are slow compared 

 with the rate of photosynthesis but the possibility arises that 

 the exchange reactions may be catalysed by living cells. Sub- 

 sequent attempts to repeat this experiment modifying the 

 procedure in such a way as to minimize the importance of 

 exchange reactions have not produced indisputable evidence 

 either for or against the theory that all the oxygen arises from 

 water. Further evidence which makes this interpretation of 

 green plant photosynthesis still more probable has come 



