112 PHOTOSYNTHESIS 



results obtained by Calvin. On the other hand it has some- 

 times been maintained that in the normal green plant the 

 difference between the light and dark reduction process is 

 in some ways quite fundamental. 



The energy relationships of some of the processes that 

 have been referred to are summarized in a potential diagram 

 (Fig. 7.2 and Table 7.1; pp. 121, 122). 



The scale is in volts and refers to reactions in dilute 

 solution at pH 7. The 'reducing action' of the light cor- 

 responding to the quantum of wavelength 675 m/Li is here 

 represented as a difference of potential from the oxygen 

 electrode (which has a potential of = +o-8 volt at pH 7 when 

 referred to the standard H2 electrode at pH o). This choice 

 is completely arbitrary, but is directed by the fact that Og 

 is the only recognizable product of photosynthesis on the 

 oxidizing side. If for example we were to assume that H2O2 

 was the product of a light reaction then the reducing action 

 would be less by about o-6 volt. The important point to 

 note in the diagram is the relative positions of the O2, H2, 

 and coenzyme potentials in relation to those of some of the 

 systems that have been discussed. Dixon (1949) has given a 

 diagram of the free energies in terms of A^ and rH of a 

 number of biological systems. The hydrogen electrode sup- 

 plies the point of reference. For the energy corresponding 

 to the light process we have no standard of reference, but 

 it is plausible to take a point somewhere between the Og 

 electrode and the O2 over-potential corresponding to the 

 decomposition of H2O2 by the reaction HgOg = IO2+H2O, 

 AF = -25,100 cals. The value of a diagram of free energies 

 is that it gives the data required to determine how far a given 

 reaction will proceed if allowed to come to equilibrium. The 

 diagram can assist in the formulation of hypothetical reac- 

 tion schemes to guide our interpretation of experiment. The 

 photochemical process is only to be formulated in terms of 

 steady states, and the fact that it has not been possible to 

 obtain equilibrium data is reflected in our inability to fix a 

 reference point for the available light energy on the free 

 energy or potential diagram. 



