530 



PHOTOCHEMISTRY OF PIGMENTS IN VIVO 



CHAP. 19 



3 - 



c 



a 



■V 



K 



o 

 o 

 a 



o 2 



o 

 a. 



A 



20 



40 60 



Oxygon, % 



80 



100 



Fig. 58. — Rate of photoxidation as a function of oxygen pressure (after Franck 

 and French 1941). o, oxygen consumption in C02-starved live Hydrangea leaves 

 (after Franck and French); A, decUne of oxygen liberation of live Chlorella cells caused 

 by excess oxygen (after Warburg); «, oxygen consumption by horse serum in light, 

 sensitized by adsorbed porphyrin (after Gaffron). All three processes require a high 

 oxygen pressure to reach a full rate of photoxidation. 



Figure 58 shows oxygen consumption in the steady period of phot- 

 autoxidation as a function of oxygen pressure, while figure 59 represents 

 a similar plot for respiration. The latter indicates saturation at less 

 than 1% of oxygen in the atmosphere, whereas photoxidation requires 

 over 50%. (This fact was quoted above as a proof that photoxidation 



20 



40 60 



Oxygen , % 



80 



100 



Fig. 59. — Rate of respiration as a function 

 of oxygen pressure (after Franck and French 

 1941). Saturation is reached at a very low 

 oxygen pressure. 



