THE CHEMISTRY OF PHOTOSYNTHESIS 



143 



(§ 57) and Warburg (§61) show, however, that citric acid and other inter- 

 mediary products of the tricarboxyhc acid cycle do not belong to the pri- 

 mary products of CO2 fixation. Moyse and Jolchine (29, 41, 42) observed 

 that in certain leaves the tricarboxylic acid cycle proceeds in the light as 

 freely as in the dark. Besides the carboxylation of ribulo.se- 1,5-diphosphate 

 proposed by Calvin, they found irreversible carboxylation of phospho-enol- 

 pyruvic acid to produce oxaloacetic acid which is then reduced to mahc acid 

 (6, 55) (see §46). 



COOH 



CO'^POsHa + CO2 

 CHo 



COOH 



H.O 



^ CO 



+ H3P04 



phospho-enol- 

 pyruvic acid 



COOH 



I 

 CO 



CH2 



I 



COOH 



oxaloacetic 

 acid 



-\- 2H 



*■ 



CH2 



COOH 



oxaloacetic 

 acid 



COOH 

 CHOH 

 CH2 



COOH 



malic 

 acid 



With the use of ^^C02, the production of labeled malic acid was established. 

 Moreover, labeled citric acid, wo-citric acid and fumaric acid as well as 

 amino acids were found. These findings are in contrast to Calvin's views 

 regarding the blocking of the tricarboxylic acid cycle by the photoreduction 

 of lipoic acid (see § 55). 



Warburg, also, does not admit photosynthesis to be the simple reversal of 

 respiration. There is no definite evidence that the occurrence of the products 

 of the carbon cycle cannot be due to the induced respiration. In Figure 51 

 some relationships between the carbon cycle and respiration are depicted 

 schematically. 



§ 55 The Significance of Lipoic Acid 



When lipoic acid is added to a system of illuininated algae and quinone, the 

 rate of O2 production is considerably increased (15). It seems as if lipoic 

 acid is intercalated between the light activation reaction and the quinone 

 reduction. Calvin (20) originally assumed that lipoic acid acts as both a 

 hydrogen acceptor and an oxygen acceptor 



2[H]+ [0] + L<'l 



/ 



SH 



SOH 



