ROLE OF RESPIRATION IN INDUCTION PHENOMENA 1425 



Figure 33.51 shows the hypothetical changes in the amounts of the two 

 inhibitors at the beginning of iUumination, and the resulting time course of 

 fluorescence (the latter in agreement with the experiments). 



5. Role of Respiration in Induction Phenomena 



The above analysis of the possible mechanisms of induction was based 

 on the concept of photosynthesis as a separate system of chemical and 

 photochemical reactions, fundamentally independent of other metabolic 

 mechanisms in the cell. This has been a legitimate and useful concept, 

 since it permitted attention to be concentrated on a minimum number of 

 factors, and to ask questions conducive to meaningful experimentation. 

 It is likely that, in the main, the chemical schemes and kinetic relationships 

 based on this approach will prove vahd in the end. However, complete 

 isolation of photosynthesis from other cellular processes undoubtedly was 

 an oversimplification. More recently, evidence has begun to pile up con- 

 cerning the mutual interplay of photosynthesis and other biochemical 

 processes, above all, respiration. Instead of a separate, chemical struc- 

 ture with one inlet through which carbon dioxide and water molecules 

 enter, and one outlet through which sugar and oxygen escape — we now 

 look on photosynthesis as a more open structure, with several connections 

 to the ambient medium and to the other metabolic reaction sj^stems, on 

 different reduction levels. Of course, even in the explanation of induction 

 phenomena suggested in the preceding sections, Franck has assumed an 

 interference of metabohtes with the smooth working of the photosynthetic 

 apparatus, most prominently in his concept of "internal narcotics" (pro- 

 duced, e. g., by fermentation, or by the action of excess photoperoxides on 

 sugars), which were credited with inactivating the photochemical appara- 

 tus by settling on chlorophyll, or on catalysts in permanent association 

 with this pigment. 



The influence of such "internal poisons" appeared, however, as an inci- 

 dental interference, which could only slow down or temporarily stop alto- 

 gether the wheels of photosynthesis (although Franck also suggested that 

 this interference may be providential when the danger exists that the photo- 

 chemical apparatus, for lack of proper substrate, may begin to chew up it- 

 self). 



The evidence for placing greater emphasis on the intrinsic character of 

 cross-connections between photosynthesis and other metabolic processes 

 came from two areas. Biochemical studies with isotopic carbon tracers 

 have indicated that photosynthesis may have several intermediates which 

 also occur in catabolic oxidative processes. It was found that several 

 intermediates of respiration can participate re\'ersibly in the carbon dioxide 



