162 PROBLEMS OF PHOTOSYNTHESIS 



§ 60 The Reaction Equations of Photosynthesis 



The stoichiometric relationships between chlorophyll and O2 and between 

 chlorophyll and CO2 show that the O2 produced in the light originates from 

 the CO2 bound to chlorophyll. Warburg (22) shows this clearly in the fol- 

 lowing experiment: a cell suspension is divided into three samples. All 

 CO2 is removed from the first sample by NaF, i.e., 1 mole CO2 per 1 mole 

 chlorophyll. From the second sample only 0.5 mole COo per mole chloro- 

 phyll is removed and from the third sample no CO2 is removed. After- 

 wards the three samples are saturated with 10 vol % CO2. Illumination 

 shows that no O2 can be produced in the first sample, only 0.5 mole O2 is 

 produced in the second sample and 1 mole O2 per mole chlorophyll in the 

 third sample. When NaF is washed out of the first sample, so that CO2 can 

 combine again with chlorophyll, 1 mole O2 is produced per mole chlorophyll 

 upon illumination. The conclusion of this very instructive experiment is 

 that upon illumination the O2 produced originates only from the labile CO2, 

 i.e., from the CO2 which can be removed from chlorophyll by the addition of 

 NaF. All other forms of CO2 present in the cells (dissolved CO2, bicarbon- 

 ates) are not influenced by the action of light. As already shown in § 35, 

 O2 capacity and labile CO2 and their connections with chlorophyll can be 

 represented by the following reaction equations: 



Reaction 7 {light) : 



CO2* + Nhp + CO2 -* CO2 + C + O2 

 Reaction 2 (dark) : 



Vs C + Vs O2 ^ Vs CO2 + 70000 cal 



Reaction 3 {dark) : 



CO2 -* CO2* - 70000 cal 



Over-all reaction: 



V3 CO2 + Nhv -^ 1/3 C + 1/3 O2 



The production of the photolyte (reaction 3) is not a simple reaction in it- 

 self. It needs about 70000 cal/mole, an amount of energy produced by in- 

 duced respiration (reaction 2). 



Of the 110000 cal necessary to split one mole CO2, 70000 cal are supplied 

 by a respiratory process and the remaining 40000 cal are provided by one 

 mole quanta. All difficulties with respect to the quanta are eliminated by 

 this scheme. As Warburg (6) pointed out, all has been found experimentally 

 and measured in living Chlorella. Reaction 1 is measured by O2 production 

 and CO2 consumption in the light. Reaction 2 is measured by O2 consump- 

 tion and CO2 production in the dark. Reaction 3 is measured by the time 

 of recovery that elapses before the light is again able to produce as much O2 

 as in reaction 1. This recovery period lasts about 20 min in Warburg's 

 experiments (see § 41). 



