60 



PROBLEMS IN PHOTOSYNTHESIS 



The light energy absorbed during the experiment was 1.64 mole quanta, so 

 that 



<P 



1.55 



= 0.95 



The average value of <^ at various wave-lengths was 0.98. 



The quantum yield in photosynthesis can be easily determined with this 

 equipment when the intensity of the radiation is unknown. The uptake of Oo 

 is measured by means of this chemical actinometer. After replacing it by a 

 manometer vessel of the same dimensions, the Oo produced is measured in the 



R. 



:B; 



Fig. 28. Principle of the chemical actinometer. 



same time period. The amount of Oo absorbed is found at the quantum yield 

 1, whereas the photosynthetic Oo is produced at the quantum yield <p. Thus 



Oo produced lOo absorbed = (^:1 



or 



<p 



Go produced 

 Go absorbed 



The light absorption may be measured even more simply in a combined 

 experiment, using the actinometer solution. Burk and Warburg (11) use 

 two vessels: a 19 ml vessel A which can be fixed within a 120 ml vessel B 

 (Fig. 28).* Vessel A contains the cell suspension and vessel B contains 80 

 ml of actinometer fluid. Vessel B is attached to the manometer at C. The 

 light is directed upwards into vessel A. The fraction absorbed in A is a, 

 whereas the non-absorbed part 1 — a is absorbed by the actinometer fluid, 

 producing absorption of Oo. The vessels are shaken as usual. Vessel A is 

 filled with water and the amount of O2 used in vessel B is read off". The 

 pressure change is h mm, vessel B having absorbed the total radiation energy. 

 If vessel A contains the cell suspension and the pressure change over the same 



* Later Warburg el al. (29) enlarged the actinometer to a total volume of 184 ml and a solution 

 volume of 120 ml. The substances used were pure pyridine with 0.40 mg/ml pheophorbide a and b 

 and 30 mg/ml thiourea. 



