PHOTOSYNTHESIS AS A SENSITIZED OXIDATION-REDUCTION 



55 



(1941) introduced heavy oxygen (0^^) into the carbon dioxide and water 

 used for photosynthesis of Chlorella, and determined the concentration 

 of the heavy isotope in the liberated oxygen. The results are given in 

 table 3. VI. It shows that the proportion of 0^^ in oxygen is under all 



Table 3.VI 



IsoTOPic Ratio in Oxygen Evolved in Photosynthesis by Chlorella "» 



(after Ruben, Randall, Kamen and Hyde) 



■■ The volume of evolved oxygen was large compared with the amount of atmospheric oxygen present 

 at the beginning of the experiment. 

 ^ Calculated values. 



circumstances equal to its proportion in water, and independent of its 

 concentration in the carbonate — thus disproving the hypotheses (a) and 

 (c). As each experiment progresses, the isotopic exchange between 

 carbon dioxide and water, brought about by reaction (3.15), tends to 

 equalize the isotope distributions in the two reaction components; but 

 since the concentration of HCOa" is high and that of CO2 low {cf. page 

 178), the equalization proceeds slowly and several collections of oxygen 

 can be made before its completion. In chapter 26 (Volume II), we will 

 encounter an additional quantitative argument against Willstatter and 

 StoU's scheme (c)— the inability of the hydration reaction (3.15) to keep 

 pace with photosynthesis in very intense light. 



The fact that most if not all oxygen molecules liberated by photo- 

 synthesis, originate from water, was confirmed by Vinogradov and Teis 

 (1941) who determined the density of water synthesized from this oxy- 

 gen, and proved that the isotopic composition of the latter is similar to 

 that of oxygen in natural water (rather than to that of oxygen in carbon 

 dioxide). 



As to the two schemes, (h) and (d), which predict that all oxygen 

 should come from water, the difference between them is that the older 



