1624 



PHOTOCHEMISTRY OF CHLOROPHYLL 



CHAP. 35 



20 



18 



34 



30 



26 - 



TIP 

 i 



34 



• •,32 



°OoOOo 



Light 



_L 



30 



60 

 MINUTES 



90 



120 



Fig. 35.28. Isotopic tracer study of Hill reaction with quinone and oxygen in 

 Chlorella cells (Brown 1953). 1 mg. quinone tipped into cell suspension in 

 phosphate buffer as shown. Respiration poisoned at once. Only 02(32) evolved 

 in light ( 18 klux) until 87% of quinone reduced; 02(34) taken up and 02(32) evolved 

 afterwards in light as expected for Hill reaction with Oo as oxidant. 



dioxide produced by the decomposition of oxalate. This surmise is con- 

 firmed by the absence of oxygen liberation in red light, and by the inhibi- 

 tion of oxygen liberation by cyanide. 



Ehrmantraut and Rabinowitch (1952) also could not confirm the libera- 

 tion of oxygen by Chlorella in the presence of benzaldehyde; the findings 

 of Fan, Stauffer and Umbreit, mentioned at the beginning of Part C, must 

 therefore be considered as in need of renewed study. 



Isotopic oxygen tracer experiments by Brown (1953) seem to indicate 

 that oxygen can act as Hill oxidant in Chlorella cells— in other words, these 

 cells function as photocatalysts for the exchange of 0^* between O2 and 

 H2O (cf. section B4 (c)). In the presence of quinone (c/. figure 35.28) this 

 isotopic equilibration in light does not begin until practically all quinone 

 has been reduced (similar observations were described in part B for chloro- 

 plast preparations) . In the absence of quinone, one has the problem of dis- 

 tinguishing between (light-stimulated) respiration and the "Mehler reac- 

 tion," as possible mechanisms of isotopic equilibration. The best evidence 

 is obtained with species such as Anahaena, or Scenedesmus, in which respira- 

 tion can be poisoned by cyanide without affecting the isotopic exchange in 



