PHOSPHORYLATION AND PHOTOSYNTHESIS 227 



Ruben suggested that the high-energy phosphates required for the 

 phosphorylation of {CO2} may be synthesized with the help of light 

 energy. Since not more than four out of eight or ten primary photo- 

 chemical oxidation products ( { OH } or Z) are utilized for the production 

 of oxygen, Ruben thought that the remaining ones may be utilized for 

 exergonic oxidation-reduction reactions (e. g., a direct or indirect re- 

 combination with the primary reduction products, {H} or HX) which 

 are coupled with the synthesis of high-energy phosphate esters. 



However, it is also possible that the high-energy phosphates used for 

 the phosphorylation of {CO2} are produced, without the help of light, 

 by oxidative metabolic reactions. (In other words, some of the com- 

 bustion energy of the products of photosynthesis may be borrowed in 

 advance to make photosynthesis possible.) 



Whether the high energy phosphates are synthesized at the cost of 

 light energy or oxidation energy, their role in photosynthesis, according 

 to Ruben, is a subsidiary one — to assist in two thermodynamically 

 difficult steps: in the carboxylation of an acceptor and in the reduction 

 of a carboxyl group to a carbonyl group. One could, however, also 

 attribute to the phosphorylation a more fundamental importance, in 

 analogy with the expansion of the phosphorylation theory of respiration 

 (page 226). One may assume that all light quanta utilized in photo- 

 synthesis (and all oxidation energy utilized in chemosynthesis) are first 

 converted into the energy of unstable phosphates, and that the transfer 

 of hydrogen from water to the {CO2} complex occurs by a sequence of 

 easy steps, each requiring not more than 10 kcal, and made possible by 

 a coupling, with the degradation, of these phosphates. Such a theory 

 was suggested recently by Emerson, Stauffer, and Umbreit (1944). They 

 attempted to support it by an experimental investigation of the phos- 

 phorus metabolism of Chlorella, which leads to the following results: 



1. Chlorella cells are capable of utilizing phosphorylated compounds 

 for respiration in the dark. 



2. Dried Chlorella cells can be used for the preparation of a "Lebedev 

 juice" which will catalyze the esterification of inorganic phosphate (in 

 the presence of glucose, fluoride and pyruvate). 



3. The phosphorylated compounds contained in Chlorella appear to 

 be different from those which commonly occur in animal and most bac- 

 terial cells. 



4. A 90-minute illumination of Chlorella, in the presence as well as in 

 the absence of carbon dioxide, does not change appreciably the relative 

 contents of inorganic and organic phosphorus in the cells. 



5. A significant change can be noted in the composition of the frac- 

 tion of organic phosphate which is precipitable by barium. (In other 

 materials, this fraction was found to contain adenosine triphosphate, 



