VOL. 12 (1953) ENERGY CYCLE AND QUANTUM YIELD IN PHOTOSYNTHESIS 



349 



were centrifuged and taken np in a medium of the following composition : 7.25 g KH2PO4, 

 0.5 g Na2HP04, 2.5 g MgSO^^HaO, and i g NaCl per 1000 ml twice-distilled water 

 (pH 5.5). Heavy metal or other salts (especially of calcium) that deposit on a platinum 

 electrode are to be avoided, and may be removed, if necessary, by brief immersion of 

 the electrode in concentrated nitric acid or chromic acid, followed by distilled water 

 rinsing. 



The experimental vessel, filled completely with Chlorella suspension, was maintained 

 at constant temperature (20° C) in a water bath, and illuminated from below with 

 monochromatic light prepared and measured bolometrically as described by Warburg^. 

 Respiration-compensating light was supplied either by means of the same beam or by 

 means of a supplementary, nearly collimated beam of white light obtained from a 

 secondary lamp source, which, in the latter instance, was not measured The Chlorella 

 suspensions usually contained 5% COg, but 10, 20, and 50% CO2 were also studied, 

 supplied in Ng containing variable amounts of O2 up to 21%, but usually 0.1-2%. 



Oo-PRESSURE 



Since the electrochemical method is more sensitive at lower oxygen pressures (lower 

 current values), we first determined how far the oxygen pressure can be lowered before 

 a decrease in the normal, dark rate of oxygen consumption by the Chlorella becomes 

 notable. We found that, under our experimental conditions, especially with our great 

 stirring velocity, that the respiration remained constant down to at least o 05% of O2 

 saturation at i atm. Down to a pressure of 0.1% O2, there was thus, in practice, no 

 danger that a change in the rate of respiration could be caused by low oxygen pressure. 



We have tested this last important condition in special cases. Thus, in Curve i. 

 Fig 4, a known quantity of oxygen was added hy means of the microburette to a Chlorella 

 suspension held with a constant white light at respiration-compensation at 0.15% O2, 

 and after the given O2 addition compensation remained (respiration not increased). Two 



0.S 



60 ul Chlorella cells/l35ml 



20 "C 



White light CCuSO^ filter) 



i 



1.d X cell vol/h 



\^*-0.57xc.v/h 



Fig. 2. The photosynthetic cycle ,in Chlorella cells, without respiration compensation (galvanometer 



read visually). 



References p. 355. 



