160 



PROBLEMS OF PHOTOSYNTHESIS 



D. CARBON DIOXIDE 



§ 59 The Fluoride Reaction 



If stoichiometric O2 production is to take place after a dark period, the 

 presence not only of O2 but also of CO2 is necessary. Without CO2, re-illu- 

 mination would not yield any O2 (14, 22). Warburg el al. (4, 16) found that 

 Chlorella contains chemically bound CO2 which is stoichiometrically connected 

 with chlorophyll. Warburg calls this CO2 fraction the labile CO2 of photo- 

 synthesis. 



Fig. 60. Manometer vessel for the determination of labile carbon 

 dioxide. .S".- Chlorella suspension. F: NaF solution. 



53.0 A 



Fig. 61. Anaerobic removal of labile 

 carbon dioxide in the dark by NaF from 200 

 m1 Chlorella (pH 3.8, 20° C). Chlorophyll 

 content: 3.83 yumole. Curve I: with- 

 out NaF. Curve H with Vso ^V NaF. 

 Pressure difference: 49.5 mm. KcOi — 

 1.73 mm''. CO, removed = 49.5 X 1-73 = 

 85.7 fi\ = 3.86 yumole (Warburg et al., 

 Zschr. Naturf.). 



25 5 75 10 12.5 15 17.5 20 22 5 25 



mm 



The addition of NaF to Chlorella removes all the labile CO2. A conical 

 manometer vessel (Fig. 60) contains a Chlorella suspension (e.g., 200 ix\ cells 

 in 3 ml salt solution at pH 3.8). The gas phase contains pure, COs-free 

 argon and the side-arm a solution of NaF (e.g., 0.2 ml 0.2 N NaF at pH 3.8). 

 The fluoride is given to the cells in the dark and CO2 is developed. A clear- 

 cut end value is reached after about 20 min. As Figure 61 shows, the amount 

 of CO2 removed in the dark from 200 jul cells in 25 min is 85.7 ^1 = 3.83 

 )umole, i.e., about 43% of the cell volume (more than the amount of O2 

 erythrocytes can bind) . In the experiment shown in Figure 61 , the chlorophyll 

 content is 3.83 )umole* so that the stoichiometric relationship between chloro- 

 phyll and labile CO2 is obvious. It is possible to vary the chlorophyll con- 

 tent by cultivating at various light intensities. The end value of the CO2 re- 

 moved will, however, vary accordingly. These experiments prove that the 



* This high value corresponds, at 22.5% dry weight, to a chlorophyll content of 7.7%. 



