138 PHOTOSYNTHESIS 



presence of the appropriate phosphokinase diphospho- 

 glyceric acid is formed, from which triose phosphate may 

 easily be formed with triose phosphate dehydrogenase 

 in presence of reduced DPN (reducing potential of about 

 0-32 volt) whilst inorganic phosphate is liberated. The triose 

 phosphate system is described in Chapter 6 and it is clear 

 that the energy involved in bringing the phosphoglyceric 

 acid into equilibrium with the coenzyme is derived from 

 the phosphate group. The free energy of hydrolysis of ATP 

 to ADP is taken as -11,300 cals. under standard con- 

 ditions. If no energy is lost by the spontaneous hydrolysis 

 of the diphosphoglyceric acid there could be a potential 

 shift of 0-2 volt when the reduction of phosphoglyceric 

 acid is compared with the reduction of diphosphoglyceric 

 acid. 



Thus given the appropriate catalytic mechanisms there is 

 the possibility that any of the steps in respiration may be 

 reversed, but such a reversal must always be accompanied 

 by a corresponding gain in free energy, the energy being 

 derived from the over-all respiration. 



Reduction of coenzymes by light 



The experiments with chloroplasts suggest that energy 

 partially equivalent to that derivable from respiration can 

 be obtained by transformation of light energy into a chemical 

 form; the available energy may be represented as a redox 

 potential. This discussion of reversibility of reactions in 

 connexion with respiration has been given to indicate the 

 far-reaching importance of Ochoa's suggestion, later con- 

 firmed by his experiments that a possible representation of 

 the process of assimilation of CO2 could be the reduction of 

 coenzymes by the illuminated chlorophyll containing system 

 of the chloroplasts. Experiments showed that no perceptible 

 direct reduction of TPN (as determined spectrophoto- 

 metrically) would occur simply in presence of illuminated 

 chloroplast preparations. This was to be expected from the 

 data on the production of oxygen, though of course there 

 is no justification for accepting the present experimental 

 measurements on the reducing properties of the chloroplasts 



