44 



PROBLEMS IN PHOTOSYNTHESIS 



ing the cell suspension and the control vessel containing white Chlorelln cells 

 are placed together in the sphere and are brought alternately into the path 

 of the light.* As in photosynthesis experiments, both vessels are shaken, the 

 light beam moving synchronously. Determination of the degree of absorp- 

 tion depends on three galvanometer recordings made when: 1) the white 

 cells are in the beam of light ; 2) the green cells are in the beam of light, and 

 3) the white cells are again in the beam of light. If, for example, the record- 

 ings are 100 scale units, 90 scale units and 100 scale units, the permeability 



a b 



F Sch 



Fig. 24. The measurement of light absorption with the Ulbricht sphere shown schemati- 

 cally. A'.- light source. B: diaphragm. W: cuvette with water. /.• interference disc. 

 L: lens. .SV movable mirror, a and b: manometer vessels, one containing green cells 

 and the other white cells. Sch: handle for moving vessel a or b into the light beam (Warburg 

 et al., Zschr. Naturf.). 



of the cell suspension is 90%, i.e., its absorption is 10%. The problem of 

 light absorption by thin cell suspensions is therefore satisfactorily solved by the 

 use of the Ulbricht sphere. 



If i^ is the intensity of the incident radiation — measured bolometrically — ■ 

 on the surface area F of the manometer vessel over the time period / sec, the 

 incident radiation energy E^ is 



= loFt 



If the fraction a of the incident intensity /„ — measured with the Ulbricht 

 sphere — is absorbed by the cell suspension, then we have for the absorbed 

 radiation energy E 



E = cxigFt 



* So-called white Chlorella cells are obtained when all the pigment of normal Chlorella cells is ex- 

 tracted with methanol. The concentration of the white cells in the manometer vessel must be 

 identical with that of the green cells. The use of the white cells results in a more uniform distribu- 

 tion of light within the sphere than when the control vessel contains water only. 



