THE MECHANISM OF PHOTOSYNTHESIS 



315 



synthetic apparatus is a reducing action in the immediate neighborhood 

 of the chloroph}^! (see Wassink and Katz, 1939, p. 152). 



TIME, rnin 



Fig. 5-12. Fluorescence-time curves of Chlorella suspensions. Gas phase: air (curves 

 1 and 3) or nitrogen (curves 2 and 4); normal (curves 1 and 2) or under complete 

 inhibition of photosynthesis by cyanide (curves 3 and -4) (light intensity 1.38 X 10* 

 ergs/cm2 • sec; Warburg Buffer No. 9, 29°C). (From Wassink and Katz, 1939.) 



4-3. TRACER EXPERIMENTS 



By means of tracer experiments, Calvin and Benson (1948) inde- 

 pendently reached the same conclusion as that given in the preceding 

 section. They followed the ab- 

 sorption of Ci^02 by a suspension 

 of Scenedesmus in darkness. These 

 suspensions slowly take up C^**02, 

 but if an illumination under a car- 

 bon dioxide-free atmosphere has 

 preceded, a strong and rapid extra 

 consumption of Ci^02 is observed 

 in subsequent darkness. From this 

 they concluded that green algae 

 have the ability "to accumulate a 

 certain amount of reducing power 

 during illumination in the absence 

 of carbon dioxide, which could later 

 be used for the reduction of carbon 

 dioxide " {ibid., p. 476 ; see Fig. 5-13). 



The previously mentioned experi- 



10 



50 



20 30 40 

 TIME, mm 



Fig. 5-13. Fixation of 0^*0-2 in Scenedes- 

 mus after dark period (a) and after pre- 

 illumination in the absence of COa (b). 



J. M n j_i (From Calvin and Benson, 1948.) 



ments on nuorescence, as well asthe 



tracer experiments just described, elucidate the nature of process (2) of 



Sect. 2, the light reaction proper. It appears to have the character of a 



