1564 



PHOTOCHEMISTRY OF CHLOROPHYLL 



CHAP. 35 



The reaction with chromate was studied somewhat more closely. Oxy- 

 gen was evolved only in light ; this evolution was prevented if the chloro- 

 plasts were preheated to 50° C. for 15 min., showing that it involves an en- 

 zymatic reaction. The total amount of oxygen evolved was only about one 



150 



130 



no 



90 



E 70 



E 



50 



30 



/ 



Light 



Dark 



.• — • — •- 



/ 



Light. 



/ 



/ 



Added Hill's 

 solution 



J_ 



10 20 30 40 50 



TIME, minutes 



60 



70 



Fig. 35.17. Resumption of oxygen evolution by illuminated chloroplasts on addi- 

 tion of Hill's solution, following cessation of evolution from chromate (6.65 X 

 10-6 mole KjCrOi; O2 equivalent; 111 mm.'; pH 7.3; 0.1 Af phosphate; 16° C; 

 nitrogen atmosphere; 0.25 mg. chlorophyll) (after Holt and French 1948). 



half (45-75%) of that expected for the oxidation of water by Cr(VI), and 



the reduction of the latter to Cr(III) : 



(35.30) Cr04-2 + 5 H+ > Cr+=' + 2^ H2O + % O2 



Figure 35.17 shows that the cessation of oxygen evolution, after about one 

 half of the stoichiometric equivalent was produced, was not the result of 

 damage to the photochemical system, since addition of Hill's mixture led 

 to a resumption of oxygen liberation. On the other hand, oxygen libera- 

 tion could not be renewed by the addition of fresh chloroplasts. Incuba- 

 tion of chloroplasts in 2 X lO^^ jlf chromate solution for 30 min. in the 

 dark did not inactivate them. The total fraction of chromate reduced was 

 approximately the same for chromate concentrations between 1.5 X 10"=^ 



