phate were ultimately converted to 3 molecules of pentose 

 phosphate. 



Other known metabolic pathways leading from PGA 

 (Figure 4) give rise first to phosphoenolpyruvic acid (PEP A), 

 which then may undergo further transformations, including 

 the following: (1) it may be carboxylated and transaminated 

 to give aspartic acid, (2) it may be carboxylated and reduced 

 to give malic acid, or (3) it may be dephosphorylated and 

 transaminated to give alanine. All these compounds are 

 labeled after short exposures of the algae to HC"0.s~ in the 

 light. 



The enzyme system of plants, which during respiration 

 brings about the oxidation of triose phosphate to PGA in 

 the glycolytic pathway, was known to produce ATP and 

 TPNH (or DPNH). If PGA is to be reduced to triose phos- 

 phate during photosynthesis, it follows that ATP and TPNH 

 must be supplied. We have already seen that these two co- 

 factors, and possibly others, are produced as a consequence 

 of the light reaction and the splitting of water. It might be 

 expected that, if the light were turned off from plants photo- 

 synthesizing in ordinary carbon dioxide at precisely the same 

 time that C^^02 is introduced, PGA would no longer be 

 reduced to sugar phosphates but would still be formed (if 

 no light-produced cofactors are required for the carboxyla- 

 tion reaction). Moreover, the PGA would still be used in 

 other reactions not requiring these cofactors. In Figure 5, 

 the radioautograph from just such an experiment, this pre- 

 diction proves to be correct. Labeled PGA is still formed 

 by the algae from C^^02 during 20 seconds in the dark, but 

 only a very little of the PGA is reduced to sugar phosphates. 

 At the same time, a large amount of alanine is formed from 

 PGA via PEPA in reactions that do not require ATP. 

 The trace of labeled sugar phosphates that does appear may 

 be due to the residual ATP, or some unknown cofactor, 

 which was formed while the light was on but which had 

 not yet been used up when the C^^02 was introduced. The 

 formation of malic acid and of alanine and aspartic acid 



17 



