THE PHOTOTROPHIC ASSIMILATION OF CARBON 39 



that the €3 acceptor is derived by spUtting and reduction 

 of oxaloacetic acid.^^ It will be seen from Fig. 9 that in this 

 cycle, which involves two successive carboxylations, the 

 three substances named are interconvertible by known bio- 

 chemical mechanisms. There is, however, no direct evidence 



C2 acceptor _^^.q^ phosphogly''^^ ^^^^ 



? '- ^CH20H.CH0(P).C00H 



reduction 



+ CO2 



.CO.COOH < CH3.CO.COOH 



splitting 



COOH.CH, 



oxaloacetic acid pyruvic acid 



FIG. 9. Scheme showing a possible manner of origin of the C^ 

 acceptor in photosynthesis from phosphoglyceric acid (after 

 ref. 30). 



for the existence of this cycle and it is open to criticism 

 on various grounds. ^^^ The evidence for the various other 

 schemes that have been put forward^^- ^1^' ^^^ is equally 

 unsatisfactory. 



A fundamental problem which is yet unsolved is that of 

 the manner in which the reductions involved in the carbon 

 dioxide fixation cycle take place. Chloroplasts separated by 

 suitable techniques from living cells, or, in the case of algae 

 such as Chlorella, intact cells, are capable of carrying out 

 the 'Hill reaction' in which, under the influence of light, 

 water is split resulting in the evolution of oxygen and the 

 reduction of an appropriate hydrogen acceptor. A, accord- 

 ing to the equation: 



A+H2O— ^-HgA+iOa . . . (5> 



Substances such as ferricyanide and quinone can act as 

 direct hydrogen acceptors in this reaction, but carbon 

 dioxide cannot.^*'^' ^^^' ^^^ The reduction potential devel- 

 oped is, in fact, insufficient for the direct reduction of the 

 latter substance to the level of carbohydrate. ^^^ However, 

 reduction coupled with carbon fixation by illuminated 

 chloroplast preparations from higher plants has been shown 

 to occur in the presence of a suitable hydrogen carrier. 

 Triphosphopyridine nucleotide (co-enzyme H) is such a 



4 



