PHOTOSYNTHESIS, PHOTAUTOXIDATION, PHOTORESPIRATION 527 



strates to suffer photoxidation in plants whose photosynthesis has been 

 inhibited ; rather, they remain intact as long as other oxidizable materials 

 are available to the cells. 



Van der Paauw (1932) had noticed that, subsequent to a period of 

 photoxidation in C02-starved cells, oxygen consumption in the dark also 

 was stronger than before the exposure. To explain this, he suggested 

 that photoxidation of cellular reserve materials is simply light-stimulated 

 respiration, and that stimulation persists for some time after the illumi- 

 nation has ceased. 



However, in all probability, photautoxidation and light-stimulated 

 respiration are two independent phenomena. Photautoxidation is a 

 chlorophyll-sensitized photochemical process; it takes place only in the 

 chloroplasts, and its mechanism may bear a close relationship to photo- 

 synthesis. "Light-stimulated respiration," on the other hand, often is 

 merely ordinary respiration enhanced by the accumulation of "photo- 

 synthates" (e. g., sugars), that is, a nonphotochemical process, which may 

 occur everywhere in the cell. True, some evidence has been found also 

 of a direct stimulation of respiration by light ("photorespiration"); but 

 the active rays seemed in this case to be those absorbed by the carotenoids 

 rather than by chlorophyll (cf. page 569). 



" Photorespiration " phenomena will be discussed later (Chapter 20). 

 That the phenomena which we will describe now are different from 

 photorespiration is shown: first, by their occurrence in red light (which 

 indicates sensitization by chlorophyll); second, by the much higher 

 partial pressure of oxygen required for their "saturation" (cf. Figs. 58 

 and 59) ; and third, by their occurrence in leaves killed by boiling (which 

 proves their independence from the heat-sensitive enzymatic apparatus 

 of respiration). 



One could argue that the last observation indicates that photoxidation 

 in vivo bears no relation to photosynthesis either, since the latter process 

 also depends on heat-sensitive enzymes. Gaffron (1939^-2) and Franck 

 and French (1941) suggested, in fact, that chlorophyll-sensitized photoxi- 

 dations in vivo are analogous to similar reactions in chlorophyll solutions, 

 rather than to any enzymatic life processes. However, the different 

 influence of oxygen concentration on sensitized photoxidations in vivo 

 and in vitro (cf. page 531) shows that the mechanisms of these processes 

 are different. We suggest, as a working hypothesis, that the primary 

 photochemical process of photautoxidation in vivo is identical with the 

 primary photochemical process of photosynthesis, but that it is coupled 

 with secondary reactions catalyzed by heat-resistant catalysts (e. g., 

 complex iron compounds, as contemplated by Noack in 1925) ; while, in 

 photosynthesis, the same primary process is associated with secondary 

 reactions catalyzed by true, heat-sensitive enzymes. This relationship 



