THE MECHANISM OF PHOTOSYNTHESIS 323 



chloroplasts produce oxygen without simultaneous reduction of carbon 

 dioxide. From these facts tliey concluded that carbon dioxide fixation 

 and oxygen liberation are only "loosely connected"; this was the reason 

 for their attempt to obtain oxygen from illuminated Chlorella cells by 

 supplying reducible materials other than carbon dioxide. They suc- 

 ceeded in obtaining oxygen evolution with a variety of reducible sub- 

 stances, including ferric salts, acetaldehyde, benzaldehyde, and nitrourea. 

 The following were among those which did not yield oxygen: potassium 

 nitrate, p-dimethylaminobenzaldehyde, formaldehyde, butyl aldehyde, 

 cystine, methylene blue, urea, methyl urea, succinate, citrate, fumarate, 

 acetate, lactate, malate, glucose, and hexose mono- and diphosphate. 

 Their conclusion (p. 17) is of interest: "It is also evident that the 

 reactions with which we are dealing are not the results of a simple reduc- 

 ing action of illuminated tissue since only a relatively small number of 

 materials are capable of causing oxygen production and a slight change 

 in their structure alters this reaction." The reaction with benzaldehyde 

 was studied in greater detail. No intermediate formation of carbon 

 dioxide was involved. The chief conversion appeared to be: 



2C6H5CHO + 2H2O -^ 2C6H5CH2OH + Oo. 



The experiments were complicated by reactions causing disappearance of 

 benzaldehyde in the dark. 



Clendenning and Ehrmantraut (1950) compared photosynthesis with 

 Hill reactions — the photochemical oxygen evolution from oxidizers other 

 than carbon dioxide — in entire Chlorella cells in continuous and flashing 

 light. An interesting observation is that the ability to photosynthesize 

 in Warburg's carbonate-bicarbonate buffer No. 9 is irreversibly lost when 

 the cells are exposed to a quinone solution in either light or dark. The 

 maximum rate of oxygen production at light saturation was about the 

 same with quinone as with carbon dioxide. Light saturation in the qui- 

 none reaction required, however, higher intensities. In flashing light it 

 was found that the time required for completion of the limiting dark 

 reaction was 0.03-0.04 sec at 10°C for both processes. The authors con- 

 cluded that in both processes the same dark reaction, which thus can- 

 not be concerned directly with carbon dioxide assimilation, enters as the 

 rate-limiting process. In carbon dioxide assimilation proper, much 

 slower reactions seem to be involved, as is evident from the work of Calvin 

 et al. (see Calvin, 1949), showing that, after admission of C^^02 for a few 

 seconds during stationary photosynthesis, the C^^ still is present chiefly 

 in a "first product." Clendenning and Ehrmantraut (1950) also refer to 

 work by Rieke and Gaffron (1943), who found that the time required to 

 complete the limiting dark reaction in flashing-light experiments is the 

 same for photosynthesis and photoreduction of hydrogen-adapted algea. 

 This would rule out a reaction directly involved in oxygen hberation. 



