THERMOCHEMICAL CONSIDERATIONS 1969 



hydrogen atoms (or four electrons) moved. The standard free energy 

 change is, in the reduction of carbon dioxide by H2, only about 2 kcal/mole 

 CO2; in the reduction by H2S0O3, about 29 kcal/mole, and in that by H2- 

 S4O6, about 30 kcal/mole — as contrasted to about 120 kcal/mole in ordi- 

 nary photosynthesis. The identical results oV)tained with the three reduct- 

 ants clearly show that the maximum quantum requirements are deter- 

 mined by the (probably identical) mechanism, and not by the (vastly dif- 

 ferent) energy requirement of the photochemical process. No utilization of 

 the abundant excess quantum energy, enabling the over-all process to pro- 

 ceed at the cost of a smaller number of quanta, is attempted by nature in 

 these cells. (Such a utilization has been suggested for higher plants in 

 various "energy dismutation" hypotheses of photosynthesis.) 



The results of Larsen et al. are in reasonable agreement with the earlier 

 data for purple bacteria and hydrogen-adapted green algae (chapter 29, 

 section 3). 



5. Thermochemical Considerations 



Franck (1953) reviewed once again the question of the probable yrac- 

 tical energy requirement of photosynthesis-^', e., energy requirement 

 that includes the inevitable (or highly probalile) losses in the presently rec- 

 ognizable individual steps of the process. This analysis is similar to the 

 earlier one, reported on p. 1089, but makes use of new and more specific ideas 

 concerning the photochemical and enzymatic stages of photosynthesis. 



When Warburg and Negelein's finding of a quantum requirement of 4 

 stood unchallenged, Franck had tried to devise a mechanism of photo- 

 synthesis incorporating this feature, but noted that this offered great ther- 

 mochemical di culties (noted also by Wohl) . The experimental criticism of 

 Warburg and Negelein's conclusions by Daniels et al. (p. 1121) and Emer- 

 son and Lewis (p. 1091) provided a welcome way out of these difficulties 

 and the search for the mechanism of photosynthesis was switched to models 

 involving more than four elementary photochemical steps — with eight a 

 favorite number, both for experimental reasons, and because of its easy in- 

 terpretation in terms of a two-stage transfer of four hydrogen atoms (c/. 

 chapter 7, "Eight Quanta Mechanisms"). Alternative hypotheses, en- 

 visaging 4 -j- a; photochemical reactions, storing the energy of x quanta 

 in high energy phosphate esters, also have been discussed (c/. for example, 

 the hypotheses of Ruben, and of van der Veen, p. 1116). 



This sequence of events, already told in chapter 29, is recalled here be- 

 cause of Warburg's repeated assertions that Franck had concluded on theo- 

 retical grounds that the quantum requirement of photosynthesis must be 

 11 (or, in another of Warburg's papers, 13) before Daniels and Emerson (and 

 others) began finding quantum requirements supporting his theory. The 



