PHOTOCHEMISTRY OF LIVE CELLS 



1619 



by quinone (figure 35.25B) . However, even in this case, very little oxygen 

 was produced when 4 mg. quinone were added. This cannot be explained 

 by direct photolysis of quinone, and indicates the occurrence of a second 

 phenomenon — "self-inhibition" of the Hill reaction by quinone (also noted 

 with isolated chloroplasts, c/. section 4(d) above. 



z 

 o 



h- 

 o 



3 

 Q 

 O 



cr 

 a. 



UJ 

 X 



o 



U- 



o 



< 

 cr. 



Photosynthesis 



: O 



20 40 60 80 



RELATIVE LIGHT INTENSITY 



100 



Fig. 35.26. Light saturation curves for photosynthesis and the quinone reac- 

 tion in whole Chlorella cells (after Clendenning and Ehrmantraut 1951). (•) net 

 photosynthesis; (O) photosynthesis corrected for respiration. 



Maximum Rate in Strong Light. Figure 35.26 shows the light curves of 

 photosynthesis and of the Hill reaction (with quinone as oxidant) for two 

 aliquots of the same cell suspension. (10 mm.^ cells in 3.0 cc. and 3 X 10~^ 

 M quinone.) The two curves approach the same saturation level in strong 

 light, although more light is needed to saturate the Hill reaction. The rates 

 were measured in the first 30 min. of illumination, following a 20-min. dark 

 period after the addition of quinone. (We will see below that even brief 

 preillumination of cells without quinone produced an inhibition.) 



Clendenning (1954) found that at low temperatures (0-15° C.) O2 

 evolution with quinone as oxidant far exceeds that by photosynthesis. 

 This supports Franck's view (chapter 31) that at these temperatures car- 

 boxylation becomes the rate-Hmiting reaction in photosynthesis. 



