1706 CHEMICAL PATH OF CARBON DIOXIDE REDUCTION CHAP. 36 



increase in ATP indicates an excess of 7.5% (or 15%) of the photochemi- 

 cally produced over the simultaneously consumed ATP. 



Figure 36.28 illustrates the relation of the ATP concentration to light 

 intensity. Contrary to expectation, this concentration has a maximum at 

 a certain relatively low light intensity; at the higher light intensities it 

 settles to a steady level, which is still markedly higher than in darkness. 

 A further peculiarity is illustrated by fig. 36.29: the ATP \'alue increases 

 in the dark immediately after the cessation of illumination. This concen- 

 tration shows a characteristic transient fluctuation also at the beginning of 

 the illumination period (fig. 36.30) which should be considered in conjunc- 

 tion with the induction curves of gas exchange and fluorescence, described 

 in chapter 33, and with the transients in the concentration of tagged com- 

 pounds, observed by the Berkeley group (figs. 36.22 and 36.23). 



■ 10 I 



MINUTES 



Fig. 36.29. Changes in ATP level in Chlorella duiing and after 10 min. il- 

 lumination in the absence of CO2 (after Strehler 1952). 



The concentration of AtP in Chlorella can be enhanced also })V the ad- 

 mission of oxygen to anaerobically incubated cells (i. e., by stimulation 

 of respiration). The photochemical enhancement is much less sensitive to 

 a lowering of temperature than the respiratory enhancement, which seems 

 to indicate a direct relation of ATP formation to the primary photochemical 

 act, rather than to the enzymatic stages of photosynthesis. 



Strehler interpreted these results as supporting the hypothesis that for- 

 mation and utilization of ATP is a part of photosynthesis. Rather than 

 thinking of ATP as merely an "assistant" in the reduction of RCOOH to 

 RCHO, he suggested that direct photochemical hydrogen transfer from 

 water occurs only to an acceptor with a potential not highei- than 0.0 volt 

 (cf. chapter 35, section B4 for evidence which could be adduced in support 

 of this postulate), and that the further enhancement of reducing power 

 ("energy dismutation") is brought about by reoxidation of some of the 

 reduced products and storage of their oxidation energy as phosphate bond 

 energy. 



It may be useful to juxtapose here the different variants of the theory of 



