KINETICS 



1599 



Hill's mixture, 85-95% of the oxidant could be utilized for oxygen produc- 

 tion in separated chloroplast fragments, and only 65-75% in crude sus- 

 pensions containing plasma and cell sap, obviously reflecting oxidant 

 losses by reaction with cellular material. 



Wessels (1954) considered more closely the second above-mentioned 

 factor in "maximum utilization" of the oxidant — the influence of back reac- 

 tions. This influence must depend on whether the experiment is carried 

 out in an atmosphere of practically constant oxygen content in a closed 

 system where the photochemically produced oxygen is permitted to accum- 

 ulate, or in a stream of oxygen-free gas. It was already noted in Hill's 



w 



15 20 25 30 35 40 45 50 55 



w. time in minutes 



Fig. 35.22C. Effect of light intensity on change of redox potential in chloro- 

 plast suspensions containing 2,6-dichlorophenol indophenol (^'c = —0.255 

 volt): solid hne, 15 and 12 klux; dashed line, 6 klux; dashed-dotted line, 2.3 

 klux; dotted line, 0.9 klux (Wessels 1954). 



early experiments that the reduction of ferric oxalate stopped short of com- 

 plete reduction, depending on the partial pressure of oxygen. Incomplete 

 decoloration of certain quinonoid dyestuffs also was observed before, and 

 we mentioned several times that the failure to observe Hill reaction with 

 many oxidants of low oxidizing power (such as the pyridine nucleotides) 

 probably was due to the photostationary state being almost entirely on the 

 side of reoxidation. 



Wessels observed, beside some cases of incomplete decolorization (e. g., 

 of toluylene blue) , a large number of systems in which the photostationary 

 redox potential indicated incomplete reduction. Of course, the potential, 

 being a linear function of the logarithm of the ratio [reductant] [oxidant], 

 never shows complete reduction (or oxidation) ; but when it differs from the 

 normal potential by more than, say, 0.15 volt, its use for the calculation of 



