PHOTOSYNTHESIS AND RESPIRATION 



1933 



Experiments such as those in fig. 37D.19 showed that, in the case of cells 

 whose respiration is found increased after a period of illumination (a be- 

 havior typical of dark-incubated cells which have received a considerable 

 dose of light), this increase takes place, not gradually over the whole illu- 

 mination period, but suddenly, when the light is turned off. This con- 

 firms the conclusion of Weigl and Calvin that products of photosynthesis 

 do not become available for respiration immediately after their formation. 

 However, it seems implausible that this "protection" could remain in force 



to 

 </> 



Ui 



at 

 *»- e 



< 

 I- 



< 



a. 



to 



tn 

 if) 



< ., 



20 



40 

 MINUTES 



60 



80 



Fig. 37D.19. Time course of tracer oxygen uptake bj^ a Chlorella suspension 

 after Brown 1953). Knop's solution used for culture and experiment. Gas 

 phase: ca. 2% 02(34)-enriched oxygen in nitrogen. Data for 02(34) only. 



indefinitely if illumination should continue. Rather, one must assume that, 

 after sufficiently long illumination, the photosynthesized materials will 

 begin drifting into the respiratory metabolism. It is, however, significant, 

 that switching off the light accelerates this transfer. Conceivably some en- 

 zyme (or enzymes) , involved in it, are kept in an inactive (perhaps, reduced) 

 state in the illuminated chloroplast. (For a specific suggestion along these 

 lines, made by Calvin ei al., see chapter 36, p. 1698.) 



These experiments gave no support to Warburg and Burk's concept of 

 strongly enhanced respiration in light. Since it was conceivable that en- 

 hancement could only be observed in intermittent light, as used by War- 

 burg, Burk et al., Brown made mass-spectroscopic runs also with this type 

 of illumination. Fig. 37D.20 shows a typical result: enhancement is 

 absent in intermittent as well as in continuous light. 



