THE MECHANISM OF PHOTOSYNTHESIS 



325 



in the first and last and is very high in the second case. When the light 

 is shut off, the potential quickly falls almost to the initial value. Obvi- 

 ously no relation exists between the over-all gas exchange and the poten- 

 tial shift. This means that the potential shift is not due to final products 

 or to waste products from the over-all process. The conclusion seems 

 justified that the potential change is related to a shift in the state of 

 oxidation-reduction of an essential intermediate catalyst; this shift is due 

 to the interference of the light. 





< - 

 a. — 



3 LU 

 I- Q 



< O 



(n a. 



-100 



-200 - 



< 



O 

 a 



-300 



-400 



- 300 



- 200 



- 100 



c 



E 

 o 



E 

 E 



< 

 o 



li. 

 o 



UJ 



< 

 a 



D 



-40 

 2 3 4 



TIME, hr 



Fig. 5-15. Redox potential and gas exchange in suspensions of Chromatium, strain D. 

 Solid lines: potentials; broken lines: uptake of gas. Curves No. 1: No + 30% 

 H, + 5% COo; No. 2: No + 15% Ho; No. 3: No + 5% CO2. jlight on; jlight off. 

 Reference electrode: saturated KCl-calomel electrode; suspension medium: phos- 

 phate buffer, pH 6.6, 29°C. {From Wassink, 1947.) 



The chain of photosynthesis is essentially a hydrogen-transfer chain. 

 In the absence of hydrogen and with excess carbon dioxide, the inter- 

 mediate reversible redox systems will be readily dehydrogenated, causing 

 a shift of their stationary potential to the oxidized side. It seems logical 

 to assume that, in the beginning, intermediate products and reduced sub- 

 stances will be able to supply some hydrogen but that this source will 

 quickly become exhausted. This is in perfect agreement with the obser- 

 vation that in N2 + COo the potential, after a quick, steep rise immedi- 

 ately after the start of the illumination, subsequently shows a slower, 

 gradual rise for a long time (Fig. 5-15). On the contrary, with nitrogen -f 

 hydrogen the potential does not rise much above the dark value and, 

 moreover, tends to decrease during illumination, so that, when the illumi- 

 nation is discontinued, hardly any change is observed. This indicates 

 that initially some hydrogen acceptors still are available, but they gradu- 

 ally become exhausted, so that the intermediate redox system^ shift more 

 to the reduced side. Perhaps the initial rise is due chiefly to carbon diox- 



