Photosynthesis 397 



CO ■» are not separated in time, whereas 2 development and C0 2 -binding are 

 separated. The unraveling of these relations has cost many experiments. 



3) The unit of 2500 Chlorophyll molecules. In 1932 Emerson and Arnold 13 at- 

 tempted to apply our methods of intermittent illumination 14 to photosynthesis. 

 For example, with very short, very bright light flashes and relatively long dark 

 periods, they determined the maximum quantity of 2 that appeared to be devel- 

 oped in one light flash. Comparison of this quantity of 2 with the Chlorophyll 

 content of the cells showed that 2500 molecules of Chlorophyll could develop one 

 molecule of 2 . In contrast, we find, without intermittent light, and with direct 

 measurement of the light reaction during inhibition of the dark reaction, that one 

 molecule of Chlorophyll can develop one molecule of 2 . There is thus a discrepany 

 of three powers of 10, depending upon whether the ratio ch!orophyll/oxygen is 

 measured with intermittent light or directly. As Dean Burk 15 , 16 has shown, how- 

 ever, the intensity of the light flash in the experiments of Emerson and Arnold 

 was several Orders of magnitude too low— that is, entirely insufficient— to decom- 

 pose all the photolyte in the very short time of the light flash (~ 10~ 5 second). 



4) Burst of carbon dioxide. According to Emerson and Lewis 17 , photosynthesis 

 begins with a burst of C0 2 evolution. This phenomenon was discovered mano- 

 metrically with the twovessel method, without, however, maintaining the essential 

 requirement of the method. Instead of both vessels being illuminated simultane- 

 ously, an interval of 8 to 24 hours took place between illumination of one vessel 

 and the other. This procedure removed the essential condition of the two-vessel 

 method, namely, that in the two unequal vessels the same chemical change must 

 occur. If one properly measures, as nowadays prescribed, the 2 deve!opment in 

 both vessels simultaneously with a divided light beam, one never finds at the be- 

 ginning of the illumination a burst of CO- 2 out of the living cells, but always and 

 only a burst of 2 corresponding to true photosynthesis. 



5) The experiments of Rüben and Kamen. When Chlorella were illuminated in a 

 bicarbonate Solution in which the oxygen of the water, but not that of the carbon 

 dioxide, was isotopically marked, the 2 developed was marked. Rüben and 

 Kamen 18 concluded that the light decomposed primarily the H 2 but not the CO- 2 . 

 Obviously this conclusion would have been correct only if one could have brought 

 forth the improbable argument that in light not the hydrate but only the anhy- 

 dride of carbon dioxide reacted. 



6) "Hill" reactions. If one suspends Chlorella cells in nitrate-nitric acid mixture, 

 the cells develop 2 for many hours when illuminated in the absence of CO- 2 , 

 according to the equation 



HN0 3 + H 2 = NH 3 + 202, 

 a reaction that was discovered in 1920 19 . The mechanism of this reaction has been 

 clarified as follows : the nitric acid oxidizes in a dark reaction the carbon of cell 

 organic matter to carbon dioxide, and then, in light, Splitting of the carbon dioxide 

 into C and 2 occurs, as in ordinary photosynthesis : 



(Dark) HNO ;3 2C + HoO = NH 3 - 2C0 2 

 (Light) 2CO2 = 2C + 2O2 



(Balance) HNO3 H 2 = NH 3 i 20 2 . 



