Chapter 36 

 CHEMICAL PATH OF CARBON DIOXIDE REDUCTION* 

 (ADDENDA TO CHAPTERS 3, 8 AND 9) 

 A. IsoTOPic Carbon Tracer Studies* 



Abbreviations used in this chapter: GA, glyceric acid; PGA, phosphoglyceric 

 acid; FA, pyruvic acid; PPA, phosphopyruvic acid; MA, maUc acid; and OOA, 

 oxalacetic acid. 



TP, triose phosphate; DHAP, dihydroxyacetone phosphate; GMP, glucose mono- 

 phosphate; FMP, fructose monophosphate; FDP, fructose diphosphate; RDP, ribu- 

 lose diphosphate; and SMP, sedoheptulose monophosphate. 



Also DPN and TPN, diphospho- and triphosphopyridine nucleotides (coenzymes 

 I and II, hydrogen carriers); coA, coenzyme A ("acetyl carrier"); LA, lipoic (or thioc- 

 tic) acid (hydrogen carrier); and ADP and ATP, adenosine diphosphate and triphos- 

 phate (high-energy phosphate acceptor and donor, respectively). 



The experiments and speculations described in this chapter are con- 

 cerned with the primary fixation of carbon dioxide in photosynthesis and 

 its subsequent reduction — topics that have been discussed in chapter 8 

 (sections 3 and 4) and chapter 9 (section 10) of Vol. I, respectively. Also 

 included are observations that throw some light on the sequence in which 

 different sugars, amino acids and other synthesized products appear in 

 photosynthesis — a matter discussed in chapter 3. 



Some data will have to be included in this chapter concerning the effect 

 of poisons on photosynthesis — a subject dealt with in chapter 12, and again 

 in chapter 37D (section 2a). 



1. Photosynthetic and Respiratory Fixation of C*02 



It was hoped at first that the application of tracer carbon would rapidly 

 lead to clarification of the hitherto mysterious chemical mechanism of car- 

 bon dioxide reduction. The task proved to be more difficult than was an- 

 ticipated for two reasons. In the first place, it has become apparent that, 

 whatever the intermediates and primary products of carbon dioxide trans- 

 formation in photosynthesis may be, they are very rapidly changed into a 

 variety of other compounds; or, at least, the tracer carbon incorporated 

 in them is rapidly redistributed through a multitude of metabolites. 

 Within a few minutes, C(14) makes its appearance in compounds of differ- 



* Bibliography, page 1710. 



1630 



