APPLICATION OF PAPER CHROMATOGRAPHY 1065 



tion). Even leaving aside, as biochemically implausible, the once popular 

 Baeyer's mechanism, which envisaged a straight reduction of Ci compounds 

 (CO2 -^ HCOOH -^ CH2O) followed by condensation of formaldehyde to 

 glucose, one could imagine other mechanisms in which Ci fragments were 

 grafted onto a large carrier molecule, then reduced to the carbohydrate level, 

 and the reduction product split off from the carrier without the latter having 

 been directly involved in the reduction process. This, in fact, was the model 

 used in most chemical and kinetic speculations concerning photosynthesis, 

 such as the kinetic theories reviewed in chapters 27 and 28. The symbols 

 ACOo or {CO2I were used there to designate a CO2 molecule "grafted" onto 

 an acceptor molecule prior to being photochemically reduced; the acceptor, 

 A, was assumed to be regenerated at some stage of the process, without 

 change, and returned into the cycle. True, some evidence was found, 

 even prior to the radiocarbon studies, that the CO2 acceptor itself is a 

 product of photosynthesis. (For example, certain induction phenomena 

 could be attributed, in chap. 33, sect. C3, to the disappearance of the CO2 

 acceptor in the dark and its regeneration in light) ; but even this did not 

 prove that the acceptor molecule undergoes a continuous, cyclic transfor- 

 mation in light. Radiocarbon studies first proved that the carbon skeleton 

 of the acceptor molecule is assembled anew at the end of each cycle from 

 carbon atoms derived, in part, from the carbon dioxide molecules it has 

 taken up in the preceding turns of the cycle. (Without this throwing of 

 the new and the old carbon atoms into a common reservoir, the labelling 

 of the a and /3 carbons in PGA would not occur at all, or would occur much 

 slower than it actually does.) 



One consequence of this finding is that in any future analysis of the 

 kinetics of photosynthesis, one will have to consider the concentration of 

 the acceptor, not as a constant (Ao in chapter 27), but as a function of the 

 rate (and perhaps also of certain special conditions) of photosynthesis. A 

 further complication will arise if the acceptor (or its carboxylation product) 

 is produced also as intermediate (or by-product) of respiration (as seems 

 to be the case with PGA; cf. chapter 37D, section 3). 



The proof of regeneration of the carbon dioxide acceptor by a rapid 

 cyclic process can be considered as the second important result of the appli- 

 cation of carbon tracer to the study of photosynthesis (the first one having 

 been the identification of phosphoglyceric acid as the first intermediate). 



The study of the intramolecular distribution of labelled carbon was con- 

 tinued by Bassham, Benson, and Calvin (1950). 



Degradation experiments with labelled phosphoglycerate showed that 

 a and jS carbon atoms have approximately the same activity {e. g., in one 

 experiment, after 60-sec. photosynthesis in C*02, 55% of all C* present 

 in phosphoglyceric acid was located in the carboxyl, 25% in a, and 27% in 



