EVOLUTION OF THE CO2 REDUCTION MECHANISM 1695 



whether in weak or in strong light, both carboxylations must proceed at the 

 same rate (more exactly, two CO2 molecules must be taken up by the C2 

 acceptor for each CO2 molecule taken up by one C3 acceptor, cf. scheme 

 36.IIIA). However, in very weak light, the establishment of this steady 

 state may require a measurable time, during which the tagging of malic 

 acid will outrun that of all other compounds, including PGA. Another 

 possibility to be taken into consideration is that of tagged malic acid being 

 involved in respiratory, catabolic processes, i. e., serving as a "bridge" be- 

 tween photosynthesis and respiration. We will return to this important 

 possibility when discussing the interaction of respiration and photosynthesis 

 (chapter 37D, section 3), and its possible effect on quantum yield mea- 

 urements (same chapter, section 3). Here, we will continue the evolution 

 of the carbon dioxide reduction mechanism. 



It was reported in section 6 that the postulate of two different primary 

 carboxylations, both essential for photosynthesis, was opposed by Gaffron, 

 Fager and co-workers, who suggested that even Badin and Calvin's finding 

 of a preferential tagging of malic acid in very weak light can be explained 

 by transformations of the only primary tagged product, PGA (leading to a 

 marked storage of the tracer in a C4 side product when the main sequence 

 proceeds very slowly). 



The only way in which the C3 carboxylation could run steadily at a 

 rate higher than one half of that of the C2 carboxylation, without causing 

 malic acid to accumulate indefinitely, is for this acid to be drawn into cata- 

 bolic reaction by which it is again decarboxylated. This would be in agree- 

 ment with Calvin and Benson's conception of malic acid as a half-way 

 "bridge" between photosynthesis and respiration. 



Fager, Rosenberg, and Gaffron (1951) suggested a reaction scheme 

 making use of only one CO2 acceptor. The C2 acceptor was assumed to be 

 regenerated from the final product — hexose sugar — by splitting of the latter 

 into three C2 molecules (which must then be reduced to the level required 

 for an acceptor able to produce glyceric acid by carboxylation) : 



( + H2O) 

 (36.13a) C2H4O3 + CO.. , C3H6O4 Carboxylation of d acceptor 



(glycol?) to PGA 



(36.13b) C3H6O4 + 2[II] > CsHeOa + H-O Reduction of PGA to triose 



(36.13c) C3H6O3 > 3^C6Hi206 Dimerization to hexose 



(36.13d) }4 C6H12O6 > C2H4O2 Dissociation of hexose to biose, 



glycolaldehyde 



(36.13e) C2H4O2 + 2[H] > C2H4O3 Reduction of glycolaldehyde to 



glycol 



(36.13) CO2 + 4[H] > Ve C^HuO, + H2O 



