1708 



CHEMICAL PATH OF CARBON DIOXIDE KEDUCTIOX 



CHAP. 36 



"hydrogen adapted" algae, which do not produce free oxygen, but ne\er- 

 theless reduce CO2 to the carbohydrate level.) By encompassing all these 

 alternatives, one is brought to the most general picture — that of "energy 

 dismutation" (chapter 7, section 6 and chapter 9, section 7) by back reac- 

 tions between intermediate or final oxidation products and intermediate 

 or final reduction products of photosynthesis, with the one specific sugges- 

 tion that the energy of these back reactions is temporarily stored as phos- 

 phate bond energy. Energy storage in phosphate bonds is, however, not 

 the only conceivable mechanism of energy dismutation; and though the 

 available experimental indications of the participation of ATP in photo- 

 synthesis are suggestive, they are not yet conclusive. 



Table 36.IX 



Relative Size of Phosphate Reservoirs in the Ste.\dy State of Photosynthesis 



IN Scenedesmus (after Goodman et al. 1953) 



Compound 



Phosphoglj'cerate 



Hexose monophosphate. . 

 Uridine diphosphoglucose . 

 Sugar diphosphates ..... 



ADP 



ATP 



Goodman, Bradley, and Calvin (1953) initiated a systematic quantita- 

 tive study of the incorporation of P(32) tracer in tlifferent compounds by 

 dark and photochemical metabolism of Scenedesmus obliquus, using paper 

 chromatography to separate and identify the labelled compounds. After 

 about 30 sec. exposure to radiophosphate in the dark, 72% of incorporated 

 P* were found in ATP and 9% in ADP, with 11% in sugar diphosphates, 

 5% in the hexose monophosphate area, and 3% in PGA. After 30 sec. 

 in light, the P* distribution was much more uniform (39% in sugar diphos- 

 phates, 19% in hexose monophosphates, 179c i'l PGA, 15% in ADP, and 

 10% in ATP). This means that even 30 sec. is too long a time, in strong 

 light, to identify the port of entry of P*. The early P* labelling of PGA 

 in light suggests a close relation between photosynthesis and the organic 

 binding of mineral phosphate (which is indicated also by the above-de- 

 scribed analytical experiments). The considerable labelling of ADP is 

 worth noting, because the terminal phosphate in ADP is not usually con- 

 sidered as contributing to metabolic transformations. 



By longer exposures to P*04~, enough labelled compounds could be 



