142 PHOTOSYNTHESIS 



followed by a dark process 



«(RH2) + C02+(^--i)02=(CH0H)+«(R)+(n-i)H20 (^3) 



where the reduction of CO2 is coupled with the reoxidation 

 of RH2 The equation a and p will represent the absolute 

 minimum when n = 2 (i.e. transfer of 4 equivalents of H). 

 If it were possible to define the number n in the equations 

 as represented, i.e. to decide if the third term in (^) were 

 either absent, molecular oxygen, or some equivalent oxidant 

 produced in a light reaction, the analysis of the mechanism 

 of photosynthesis would be much advanced. There is no 

 convincing direct evidence at the moment to suggest that 

 n is necessarily greater than 2. On the other hand such a 

 'chemosynthetic hypothesis' gives the most plausible method 

 of relating the present results of in vitro experiments with 

 the physiological process of photosynthesis. If, for example, 

 the view of Ochoa is accepted that the essential result of the 

 photochemical process is the reduction of the coenzymes 

 (representing R in the above equations) then, in order to 

 reduce the CO2 from the atmosphere to the state of carbo- 

 hydrate in the plant, a further supply of chemical energy is 

 needed. This further supply could of course only come from 

 light and thus some form of chemosynthetic coupling must 

 be postulated. This is because the reoxidation of the co- 

 enzymes is only appreciable around the range -0-3 volt 

 at pH 7. The reduction of CO2 would require it to be 

 in the range -0-5 volt. The definition of this amount of 

 extra energy makes Ochoa's experiments all the more sug- 

 gestive, for the gap to be bridged is within the range of 

 free energy increase by means of oxidative phosphoryla- 

 tions, represented by a potential of 0-2 volt, a possibility 

 suggested by Ruben and many others (see Rabinowitch, 

 vol. I). 



There is some experimental evidence for the supposition 

 that the reduction of coenzymes caused by the illuminated 

 chloroplast preparations is by no means a direct process. It 

 is also possible that it requires some extra energy. Such an 

 assumption would, for example, reconcile the experiment 



