1698 



CHEMICAL PATH OF CARBON DIOXIDE REDUCTION 



CHAP. 36 



+ 12H 



+3C02\C 



3C 



(pentose) 



Scheme 36.V. Mechanism of carbon dioxide reduction (single carbox- 

 ylation C, single reduction R, all other reactions are disproportionations of 

 sugars) (Bassham et al. 1954). 



The sugar transformations postulated in this scheme have been made 

 plausible by recent investigations of the chemistry of pentoses and heptoses 

 by Horecker and co-workers (c/., for example, Horecker 1952). 



13. The Lipoic Acid Hypothesis 



Calvin and Barltrop (1952) (c/. Calvin 1952^) suggested that the inhibi- 

 tion of respiration in light — or at least, the failure of fresh photosynthates 

 to be involved in respiration as long as illumination continues (which they 

 deduced from the lack of tagging in tricarboxylic acid in light) — can be ex- 

 plained by the assumption that a catalyst, needed for the respiration cycle, 

 is involved also in photosynthesis, and is kept in the reduced state as long 



