1596 



PHOTOCHEMISTRY OF CHLOROPHYLL 



CHAP. 35 



dioxide in the first step of photosynthesis, rather than its supply to the 

 locus of this reaction. 



The same considerations apply to the Hill reaction; here, too, diffusion 

 of oxidant may determine the shape of the function H = /[oxidant]. (In 

 the case of Hill reaction in whole cells, the cell membrane is an impor- 

 tant diffusion barrier.) High concentrations of oxidants produce inhibi- 

 tion similar to that caused by high concentrations of carbon dioxide; with 

 some oxidants inhibition seems to occur even before saturation is reached. 



Clendenning and Gorham (1950) investigated the effect of quinone con- 

 centration of the rate of Hill reaction in separated spinach and wheat 



80 



Quinone 



. Anthraquinone 



I I L 



_L 



_L 



40 60 80 



% INCIDENT LIGHT 



100 



Fig. 35.21. Light curves of oxygen evolution by chloroplast fragments in light 

 with different quinones (after Aronoff 1946). 



chloroplast suspensions (fig. 35.22A). Maximum initial rate was observed 

 at 0.5-1 mg. quinone in 2 cc. (2-4 X 10 -- mole/liter). With 2 mg. quinone 

 the initial rate was somewhat lower, but the total yield higher — the reac- 

 tion proceeded steadily until about 90% of the stoichiometric oxygen 

 amount was produced. With 4 or 8 mg. quinone the initial rate and the 

 total yield both were much lower — an evidence of "self-inhibition" of the 

 quinone reaction by excess quinone. 



It will be seen in part C that similar observations were made by Clen- 

 denning and Ehrmantraut (1950) with live Chlorella cells. 



Holt and French (1946), using Hill's mixture (ferricyanide and ferric 

 oxalate), observed (acidimetrically) no effect of [FeCye-^] between 5 X 

 10~^ and 60 X 10"^ mole/liter on the initial rate of the Hill reaction. 

 Spikes, Lumry, Eyring and Wayrynen (1950), on the other hand, who meas- 



