THE ENERGETICS OF PHOTOSYNTHESLS 



59 



TABLE 9 



Dependence of yields upon acceptor concentrations 

 (X = 5780 A, C„ = 1.54 X 10-3 mole/liter, jg" C) 



these investigations is that, under suitable experimental conditions, a one- 

 quantum reaction takes place in the photooxidation of chlorophyll solutions. 

 Burk and Warburg (11) found that the photochemical yield is smaller than 1 

 if higher light intensities are used. When pure pyridine is used, the yield is 

 less than 1, even when smaller intensities of light are used. The yields in- 

 crease when certain bases, such as piperidine, are added. Ethyl chlorophyl- 

 lide can be effectively replaced by pheophorbide in which case the quantum 

 yield averages 0.70 independently of the light intensity. 



§ 24 A Chemical Actinometer 



Based on the experiments discussed in § 23, Warburg and Schocken (34) 

 constructed a chemical actinometer. Instead of allylthiourea in acetone, 

 they used thiourea in pyridine, the pigment being either acetyl chlorophyl- 

 lide or protoporphyrin. 200 mg thiourea are dissolved in 5 ml pyridine in a 

 14 ml vessel. For measurements with red, yellow or blue light 2 mg crystal- 

 line ethyl chlorophyllide are added. For measurements with green or blue 

 light 10 mg crystalline protoporphyrin are added. The gas phase contains 

 pure Oo. The vessel is attached to the usual manometer and is shaken in the 

 dark at 20 ° C until there is constant pressure. The illumination sets in and 

 the Oo pressure decreases, as O-i is now used for the oxidation of the thiourea. 

 The pigment concentration must be high enough to ensure complete absorp- 

 tion of the radiation energy. 



The vessel constant A'q, is determined from equation 7. If Brodie's solu- 



tion is used, Pfj 

 then, at 20 ° C 



10000. If Vg = 9100 ^1, Vp = 5000 ^1 and cvo= 



Ko, = 0.893 mm-' 



0.092, 



In one investigation the decrease in pressure h = 39 mm, so that 



xo, = 39 X 0.893 = 34.8 ^1 



or, calculated in mole, 



X02 = 1.55 jumole 



