16 PROBLEMS IN PHOTOSYNTHESIS 



changed. We shall see later that in photosynthesis also the value of the 

 quantum requirement of the over-all reaction depends upon the experimental 

 conditions. 



§ 7 The Energy Turnover of the Photochemical Reaction 



Let us consider the endothermic reaction 



E cal _ 



A - > B + C - U cal 



which proceeds to the right when one mole of the substance A absorbs the 

 radiation energy E. The reaction thus needs E cal/mole but, according to 

 thermochemical measurements, only U cal/mole should be necessary because 

 the reaction would proceed to the left evolving this amount of energy. The 

 ratio -q oi U and E is called the efficiency of the photochemical reaction 



As an example we may take the photochemical dissociation of hydro- 

 bromic acid 



2 HBr ''" , H. + Br. - 24000 cal 



As in the photochemical dissociation of hydriodic acid, the quantum yield 

 has the value 2. E. Warburg (58), who was the first to measure photochemical 

 yields, irradiated HBr with ultraviolet light of wave-length 2090 A. Accord- 

 ing to equation 2, the photochemical equivalent is 137000 cal so that 2 mole 

 HBr use this amount of radiation energy, although only 24000 cal per 2 mole 

 HBr are necessary for the dissociation of HBr. Thus, we find for the effi- 

 ciency 



'^4000 



In the same way, E. Warburg found an efficiency of only 2.1% for the photo- 

 chemical dissociation of hydriodic acid. 



The ozonisation reaction of oxygen has the highest efficiency. The pri- 

 mary photochemical reaction is the formation of an excited molecule Oo*. 

 This is followed by two secondary dark reactions 



O., + hv -^ O2* 



O2* + Oo -> O.; -t- O 



O + O2 -^ 0.3 



The over-all reaction is represented by 



30o + hv -^ 2O3 



The quantum yield has the value 3. E. Warburg used light of wave-length 

 2070 A, corresponding to the quantum energy of 137000 cal/mole. As the 



