EVOLUTION OF THE CO2 REDUCTION MECHANISM 1689 



reduction has undergone rapid development. Chemical mechanisms based 

 on almost pure speculation (such as Baeyer's formaldehyde theory), or on 

 plausible analogy (such as Thimann's 1938 surmise that photosynthesis in- 

 volves a reversal of glycolysis, c/. p. 183), which had been current before 

 the new approach (and extensively used in the preceding parts of this 

 monograph), have been replaced by schemes based primarily on experi- 

 mental evidence, although still strongly influenced by analogies with other 

 better known metabolic mechanisms. The Berkeley group (Benson, Cal- 

 vin et al.) in particular, has proposed many such schemes. They were pri- 

 marily heuristic hypotheses, repeatedly altered to fit the growing body of 

 data. At this writing, some of the originally controversial questions have 

 been settled, and several steps in the reaction sequence have been firmly 

 established; others, while still speculative, have become at least highly 

 plausible. As this section Avas revised in 1954, it seemed tempting to dis- 

 card the chronological presentation of the several hypotheses adopted at 

 its first writing in 1950, and use only the picture suggested m the most 

 recent papers. However, retelling the gradual emergence of this picture 

 may not be useless, even if somewhat confusing. It is an interesting record 

 of gradual emergence of a landscape from the fog, through which at first 

 only one or two disconnected landmarks were visible. Furthermore, this 

 kind of presentation serves to underline the incompleteness and uncertainty 

 of the current mechanism — instead of making it appear final and unalter- 

 able. 



Since much of the study of the chemical mechanism of carbon dioxide 

 fixation in photosynthesis has been guided, consciously or subconsciously, 

 by what is known of the mechanism of carbon dioxide liberation in respira- 

 tion, it is useful to begin by saying a few words about the latter process. 



In Volume I (p. 224, scheme 9. II) we reproduced an early version of the 

 respiratory decarboxylation cycle. Since that chapter was first written, 

 this cycle has been amended and enlarged, and has acquired a more or less 

 definitive shape known as the "tricarboxylic acid cycle," the ''citric acid 

 cycle," or— most commonly— the "Krebs cycle." It is reproduced in 

 scheme 36.1. 



The main difference between this scheme and the earlier scheme 9. II 

 is the insertion, between pyruvate and succinate, of a sequence of Ce and 

 C5 tricarboxylic acids. Furthermore, the present cycle differs from the 

 earlier one in the mechanism by which pyruvate is fed into the cycle. In- 

 stead of a simple C3 + C3 condensation (pyruvate + pyruvate) starting 

 a new turn of the wheel, as assumed in 9. II, we now postulate a C4 + C2 

 condensation (oxalacetate + "active" acetyl, i. e., acetyle + coenzyme A 

 complex). Pyruvic acid now functions, in the steady state, only as source 

 of the acetyl (being converted into it by oxidative decarboxylation, with 



