228 REDUCTION OF CARBON DIOXIDE CHAP. 9 



adenosine diphosphate, phosphoglyceric acid, phytic acid, hexose diphos- 

 phate and inorganic phosphate.) This fraction was divided, by a seven- 

 minute hydrolysis, into a "labile" and a "resistant" part. In the ab- 

 sence of carbon dioxide, the relative amount of "resistant" phosphorus 

 was large in the dark, but decreased upon illumination (e. g., from 38% 

 to 10% of the total phosphorus content). In the presence of carbon 

 dioxide the change was in the opposite direction, e. g., from 10% "re- 

 sistant" phosphorus in the dark to 23% in light. 



The results under ^ and ^ prove that Chlorella does have a phos- 

 phorus metabolism — which is almost a trivial result, in consideration of 

 the universal participation of phosphates in the metabolism of most if 

 not all organisms. The results under 3 indicate, however, that the paths 

 of the phosphorus metabolism of Chlorella may be significantly different 

 from that of the animal tissues and bacteria. 



The result under 4 represents a failure to prove a photochemical con- 

 version of inorganic into organic phosphate. (It was hoped that, in the 

 absence of carbon dioxide, at least, high energy phosphates would accu- 

 mulate in light to an extent sufficient for analytical identification.) 



The results under 5 show that carbon dioxide has an influence on the 

 composition of the organic phosphorus compounds in Chlorella in the 

 dark, and that this composition is further affected by illumination — the 

 direction of the change being different in the absence and in the presence 

 of carbon dioxide. 



Although these results are interesting as first steps towards the inves- 

 tigation of the phosphorus metabolism of Chlorella, they obviously do not 

 represent effective arguments in favor of the "phosphate storage" hy- 

 pothesis of photosynthesis. Until more positive evidence is provided, we 

 are inclined to consider as more convincing a general argument against 

 this hypothesis, which can be derived from energy considerations. Pho- 

 tosynthesis is eminently a problem of energy accumulation. What good 

 can be served, then, by converting light quanta (even those of red light, 

 which amount to about 43 kcal per einstein) into "phosphate quanta" 

 of only 10 kcal per mole? This appears to be a start in the wrong direc- 

 tion — toward dissipation rather than toward accumulation of energy. 



The difficulty of the phosphate storage theory appears most clearly 

 when one considers the fact that, in weak light, eight or ten quanta of 

 light are sufficient to reduce one molecule of carbon dioxide (cf. Vol. II, 

 Chapter 29). If each quantum would produce one molecule of high- 

 energy phosphate, the accumulated energy would be only 80-100 kcal 

 per einstein — while photosynthesis requires at least 112 kcal per mole, and 

 probably more, because of losses in irreversible partial reactions. Of 

 course, one light quantum contains enough energy to produce two (or 

 more) molecules of high-energy phosphate — but this result is unlikely to 



