1668 



CHEMICAL PATH OF CARBON DIOXIDE REDUCTION 



CHAP. 36 



(36.5) Cj acceptor + C*02 > (phospho)gly eerie acid 



(36.6) Cs acceptor + C*02 > (oxalacetic acid) ^ ^ malic acid 



the first reaction being predominant in strong light, and the second one in 

 weak light. 



Gaffron and co-workers (1951) disagreed with this hypothesis and sug- 

 gested that (36.5) is the only carboxylation reaction directly related to 



Fig. 36.16. C*02 fixation in C2 and C3 compounds as function of time in 

 Scenedesmus at 15° C. The cells (10 fi\.) were supplied with an amount of C*02 

 sufficient for only 4 min. of photosynthesis; this e.xplains the decUne of activity 

 at longer times (after Benson et al. 1952). 



photosynthesis. They argued that the assumption of two different car- 

 boxylations will require five different photochemically induced reduction 

 steps (c/. section 12 below), which they considered theoretically implausible. 

 The evidence for an independent carbo.xylation leading to malic acid they 

 found inconclusive, even the low-light findings of Badin and Calvin (1950) 

 (since in very weak light, dark catabolic processes occur at rates comparable 

 with those of light-induced reactions). More recently (Bassham et al. 

 1954) this point of view was accepted also by Calvin and his group. This 

 meant the abandonment of the argument that any intermediate whose 

 tagging curve approaches the zero point on the time axis under a finite 

 angle, must be tagged by the uptake of external C*02, without any sizable 

 reservoirs of intermediates interposed between it and the medium. Malic 

 and phosphoglyceric acids consistently showed this behavior (cf. figs. 

 36.16 and 36.18) ; but so did several other compounds, including some sugar 

 phosphates {cf. fig. 36.17), which certainly could not be the direct products 



