Chapter 8 



NONPHOTOCHEMICAL PARTIAL PROCESS 

 IN PHOTOSYNTHESIS 



I. FIXATION OF CARBON DIOXIDE 



In the reaction schemes developed in chapter 7, the primary photo- 

 chemical process was coupled with several nonphotochemical reactions. 

 Since these reactions proceed at low temperatures, they probably require 

 catalysts. Some of these undoubtedly are true enzymes, while others 

 may be comparatively simple organic, or even inorganic, compounds. 



The realization that photosynthesis includes nonphotochemical re- 

 action steps first came from kinetic studies. About 1905, Blackman had 

 established that, under certain conditions, photosynthesis cannot be ac- 

 celerated by further increase in light intensity, or carbon dioxide supply, 

 but only by a raise in temperature; Willstatter and Stoll (1918) and War- 

 burg (1919) interpreted this as evidence that photosynthesis includes a 

 non-photochemical process (which they called "Blackman reaction") 

 whose maximum rate is limited by the available quantity of an enzyme 

 (Vol. II, Chapter 28). Willstatter and Stoll suggested, more specifically, 

 that the Blackman reaction may be the liberation of oxygen from perox- 

 ides. Warburg thought at first that the Blackman reaction consists 

 in a transformation of carbon dioxide preliminary to its participation in 

 the photochemical reaction, but later agreed with the Willstatter-Stoll 

 hypothesis because of the similarity which he found between the effects 

 of poisons on photosynthesis and on catalase activity (c/. page 284), 

 The assumption, which appeared natural at that time, of a single "Black- 

 man reaction" later led to various difficulties. Suggestions that there 

 may be several "Blackman reactions" were made repeatedly, but without 

 much conviction, until Franck postulated, on the basis of an analysis of 

 various kinetic data, that photosynthesis must include (at least) three 

 different catalytic reactions. In addition to the preliminary transfor- 

 mation of carbon dioxide (first postulated by Warburg), and the peroxide 

 decomposition (first suggested by Willstatter and Stoll), Franck assumed 

 a third catalytic reaction, the stabilization of the primary photochemical 

 products, which prevents their destruction by back reactions. He made 

 no suggestion as to the chemical nature of this reaction, but our discus- 

 sions in chapter 7, would indicate that it may possibly be a dismutation, 



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