174 



PROBLEMS OF PHOTOSYNTHESIS 



nature. The reaction is not inhibited by either lO"'' A^ phenanthroHne or 

 lO""^ A^ HCN. It was found that the carotenoid concentration decreases 

 during the reaction so that, according to Warburg, the reaction is an enzy- 

 matic oxidation of carotenoids by molecular O2. The enzyme responsible, 

 carotenoid oxygenase, is activated by the addition of small amounts of qui- 

 none. In lyophilized cells the reaction proceeds spontaneously. From ex- 

 periments with fresh cells it follows that the rate of O2 uptake depends upon 

 the amount of quinone and upon the pH (optimal 6.5). The rate rises with 

 increasing Oo pressure. 



4000 



4200 



Fig. 67. Oxygenase reaction and carotenoid concentration. 200 yul Chlorella in the n.an- 

 ometer vessel suspended in salt solution S. pH 3.8. In the side-arms no quinone or 2 mg 

 quinone. Air in the gas phase. Curve I : control without quinone, normal respiration. 

 Curve II: 2 mg quinone, incident intensity of white light 730 yul quanta/min. After 225 min 

 60 ix\ O2 used. Curve III : 2 mg quinone in the dark. After 225 min 370 yul Oo used (War- 

 burg et al., Zschr. Naturf.). 



The action of the oxygenase is light-sensitive. Less O2 is consumed in the 

 light than in the dark and the carotenoid concentration is not decreased as 

 much in the light as in the dark. The simplest explanation is that the oxy- 

 genase transfers O2 in the dark and removes it in the light. Figure 67 shows 

 the behavior of the carotenoids. A Chlorella suspension of 200 /xl cells in salt 

 solution S is divided into three samples. Sample I, which serves as a con- 

 trol, is shaken with air in the dark for 225 min and no quinone is added. 

 Sample II is shaken with air and strongly illuminated for the same time with 



