1664 CHEMICAL PATH OF CARBON DIOXIDE REDUCTION CHAP. 36 



In the 1947 experiments (table 36.1), the distribution of C(14) in glucose 

 was far from uniform even after a whole hour of photosynthesis; in 1948 

 experiments (also mentioned in section 1), uniform distribution was ap- 

 proached, in glucose, only after about two hours. The reason for this dif- 

 ference is not clear. 



Giljbs (1949, 1950) studied the distrilnition of C*(14) in glucose, fructose, dextrin, 

 starch, alanine, and mahc acid, in materia! from sunflower leaves exposed to C*02 for 

 1.5, 2, and 4 minutes, with rather confusing and irrei)roducibIe results. In fructose, 

 for example, the distribution was nonuniform after 1.5 and 2 minutes, but uniform after 

 4 minutes; in starch, on the other hand, the 3,4 positions were strongly favored even 

 after 4 minutes; in malic acid, three quarters of total C* were found in the carboxyl both 

 after 1.5 and after 4 minutes of exposure. \^'hen sunflower plants were exposed to 

 C*02 in darkness, C* was found to >90%in the carbo.xyl groups in alanine and malic 

 acid, and in 3,4 positions in sucrose, dextrin, and starch, after 16 or 27 hours of exposure. 



The rapid appearance of C* in the a and ^ positions in PGA is very 

 significant. It proves — almost beyond doubt — that photosynthesis involves 

 a cyclic process. An "acceptor" molecule. A, first takes up COo to form 

 PGA; the newly added carbon is located entirely in the carboxyl group. 

 The carboxyl-tagged PGA then luidergoes reduction and condensation, at 

 some stage of which the products divide into two parts; one is transformed 

 into permanent photosynthates (polysaccharides, proteins), the other is 

 reconverted into the carbon dioxide acceptor, A. The latter now contains 

 labelled carbon; it therefore gives, upon carboxylation, PGA molecules 

 with the tracer not only in the carboxyl group (7, or 3-position), but also 

 in the two other positions (a and jS, or 1 and 2). 



A cyclic process of this type was anticipated by biochemists, who saw 

 the mechanism of photosynthesis, almost a priori, as a reversal of that of 

 respiration, and therefore expected it to include a cycle in which hydrogen 

 atoms (supplied by donors identical with, or analogous to, reduced co- 

 enzymes I and II) and carbon dioxide molecules (supplied by appropriate 

 carboxylases) are first "grafted" on a "stock" (a C3 or C4 compound); 

 the C atoms in the product are then shuffled around (and H atoms thrown 

 in) until a triose molecule is synthesized and the "stock" regenerated. 

 Taking this type of mechanism for granted, the biochemists asked whether 

 the "catabolic" cycle will prove to be simply the known anabolic cycle 

 (Krebs cycle) run in reverse (which seemed to be the simplest hypothesis), 

 or different from it. The above-described early findings with C(14) 

 proved that the biochemists' main hunch was correct— a cyclic mechanism 

 of the postulated type does exist in photosynthesis ; but it seems to be dif- 

 ferent from any known respiratory cycle (e. g., it contains no tricarboxylic 

 acids as intermediates). 



It is useful to realize that a cyclic mechanism was not an a priori neces- 

 sity for photosynthesis (as it had not been an a priori necessity for respira- 



