Chapter 20 



PHOTOSYNTHESIS AND RESPIRATION * 



We have discussed, in the preceding chapters, the chemical mecha- 

 nisms of photosynthesis and other photochemical processes sensitized by 

 I chlorophyll in the living cell, in the assumption that these processes occur 

 in a separate catalytic apparatus and are essentially independent of the 

 other metabolic activities of the organism. This assumption forms the 

 basis of the quantitative study of photosynthesis, and it will be the 

 subject of a critical discussion in volume II, chapter 26, which will form 

 an introduction to the treatment of the kinetics of photosynthesis. 



However, the same cells which engage in photosynthesis, also partici- 

 pate in many other metabolic activities, e. g., fat synthesis and protein 

 synthesis, as well as various degradation processes ; and all these processes 

 interfere, directly or indirectly, with the working of the photosynthetic 

 mechanism. Even inside the chloroplasts, photosynthesis is only one of 

 several simultaneous reactions which include, for instance, the poly- 

 merization and depolymerization of sugars, as well as the ubiquitous 

 respiration, which takes place, with varying intensity, in all cells, tissues, 

 and organs of a living organism. 



Usually, respiration is mentioned in the investigations of photo- 

 synthesis only as a bothersome source of uncertainty. Since the net 

 result of respiration is the reversal of photosynthesis, all measurements 

 of the later process must be corrected for respiration. The difficulty 

 of determining this correction was first mentioned in chapter 3 (page 32), 

 and found particularly irksome in chapter 10 (pages 264 et seq.) when we 

 tried to clarify the function of hydroxy acids in the photosynthesis of 

 succulents, and in chapter 19 (Section A) when the interplay of photo- 

 synthesis and respiration was discovered to be further complicated by 

 the superposition of a third process — photautoxidation. 



Despite the difficulty of a simultaneous determination of respiration 

 and photosynthesis, the relationship between these two processes could 

 be considered as merely incidental if it were not for some observations 

 which appear to indicate the existence of a more intimate connection 

 between the two catalytic mechanisms: we refer to the cyanide-resistant 

 residual photosynthesis, described in chapter 12 (page 302) and the 

 carotenoid-stimulated respiration, which will be discussed further below. 



* Bibliography, page 570. 



561 



