THE CHEMISTRY OF PHOTOSYNTHESIS 155 



For this reason, only the first explanation is worthy of consideration. Kan- 

 dler (33) found that alcohol poisoning blocks the reduction reactions which 

 lead to the formation of sugar phosphates somewhat more rapidly than the 

 carboxylation to 3-phosphoglyceric acid. Thus, the statement made by 

 Ba.ssham et al. that the reduction of 3-phosphoglyceric acid may continue to a 

 certain extent in the alcoholic extract seems unjustified. By contrast, the 

 alcoholic extraction may give rise to some splitting of labile intermediate 

 products produced in the photosynthetic process to 3-phosphoglyceric acid 

 and cause further deviations from the real activity values. 



Though Kandler's investigations may upset the correctness of Calvin's 

 carbon cycle, many questions remain unanswered. The hypothetical labile 

 intermediate compound is difficult to visualize chemically. 



Tolbert and Zill (51, 52, 53) observed that a part of the fixed ^'*C is excreted 

 in the culture medium as glycolate. Calvin et al. (21) also found that glyco- 

 late appears among the first labeled products. Warburg (58) obtained, in 

 experiments with the addition of blue-green light, 7-values approaching 1.3 

 which corresponds to the formation of glycolic acid (CHiOH-COOH) from 

 CO2 and H2O (see § 36). As has been discussed in § 54, Moyse and Jol- 

 chine obtained labeled malic acid and other acids of the tricarboxylic acid 

 cycle from certain leaves. In § 61 we shall discuss important investigations 

 by Warburg from which it follows that ^^C-labeled amino-acids appear prior 

 to 3-phosphoglyceric acid. 



Gibbs (26) postulated that during photosynthesis COo is reduced first to 

 formate and then to the formaldehyde level and transferred to either a C5 or 

 a C2 acceptor to form a hexose or a triose respectively. ^^C-labeled formate 

 was added to Chlorella and after 4 min of photosynthesis the tracer in the 

 glucose was located in the C4 atom. This primary reduction to formate is 

 reminiscent of the early hypothesis of Willstatter and Stoll (62) according to 

 which formate bound to the Mg atom of chlorophyll should be intermediately 

 produced. 



§ 58 Some Final Remarks 



The use of heavy and radioactive isotopes as tracers has considerably en- 

 riched our knowledge of metabolic processes. However, it must be kept in 

 mind that the tracer methods may give rise to erroneous conclusions if the 

 interpretations of experimental results are incorrect or when certain neces- 

 sary experimental conditions are not fulfilled. We have met examples of 

 this in § 42 and § 48. The various investigations with labeled carbon for 

 determining substances produced in photosynthesis show relatively little 

 agreement. In principle, it is possible that labeled carbon appears in all the 

 products of intermediate metabolism when cells are exposed to an atmosphere 

 containing ^^COo. There is no irrefutable evidence that the labeled products 

 found concern photosynthesis only. Furthermore, there is still considerable 

 ignorance of and disagreement on the fate of ^^C02 in the dark. It is ex- 



