380 R. LUMUY AND J. D. SPIKES 



tioii is essentially independent of oxidant concentration over this 

 range with rate nearly identical for many different oxidants, although 

 this fact has not yet been unequivocally established. Since the over- 

 all reaction is not limited l)y oxidant concentration in this range, 

 it would appear that the free energy of acti\'ation of the oxidant 

 reduction process is probably ([uite low if not zero. This strongly 



.7 -6 -5 -4 -3 



LOG FERRIC YANIDE MOLARITY 



Fig. 2. Curves showing the relation between oxidant concentration and the rel- 

 ative values of the light-reaction-rate parameter A'l and the dark-reaction-rate 

 parameter ko for the Hill reaction of isolated chloroplasts as defined by Equation 2. 

 Open circles, manometric method; closed circles, potentiometric method; half- 

 solid circles, manganese dioxide method. 



suggests that the oxidant is reduced by a very high-energy inter- 

 mediate in the overall reaction — a fact consistent with the low speci- 

 ficity requirements for oxidants. 



The middle region demonstrates the simplest kinetics. It is thus the 

 most useful range for studying the Hill reaction, since we will prob- 

 ably find here the basic mechanism of the natural reaction least con- 

 fused by artifacts. 



When oxidant concentration is progressively increased above 



