The Maximum Efficiency of Photosynthesis 111 



Thus by experiments above the compensation point it can be shown that the economic efficiency 

 of photosynthesis is high, even when the energy spent by nature for the maintenance of the energy- 

 transforming machine is subtracted from the thermodynamic gain. 



6. The idea has recently been discussed that nature may possibly utilize the 

 energy of respiration by diverting it in the light to photosynthesis, away from 

 functions it has in the dark 14 . If so, then the efficiencies computed for the absorbed 

 light energy should be different at different degrees of compensation. When res- 

 piration is compensated sixfold, it could contribute to the production of 1 mole of 

 Oo only 112,000 6 == 18,000 cal., that is, about 11% of the absorbed light energy 

 of 4 quanta of red light. But when respiration is but once compensated, it could 

 contribute about 65" ,, of the absorbed light energy. The fact that the same efficien- 

 cies of light are observed for uncompensated as for sixfold compensated cells seems 

 to preclude the possibility that the energy of respiration contributes significantly 

 to photosynthesis. 



The foregoing discussion shows how firm has become the foundation for the 

 new efficiency determinations based on experiments above the compensation point. 

 Most objections, procedural as well as theoretical, that have been raised concerning 

 high photosynthetic efficiencies since 1923, have in one way or another centered 

 around respiration. By compensating it, these objections have lost not only their 

 quantitative significance but even their qualitative meaning. 



6. The Two-Vessel Method 



By the two-vessel method, introduced in 1924 ", the O2 exchange as well as the 

 CO2 exchange of cells can be determined manometrically in the presence of inde- 

 finitely high pressures of CO-j. Equal amounts of cells are placed in two vessels, in 

 each of which vf/vg, the ratio of the volume of the liquid phase to that of the gas 

 phase, is different. Then the same gas exchange produces different pressure changes 

 in the two vessels and from these different pressure changes the quotient y = 

 CO2/O2 and the gas exchanges xo» and XCO2 ma Y be calculated. 



It is a necessary condition of this method that the gas exchange of the cells in the 

 two vessels be exactly the same for the whole duration of an experiment. But owing 

 to the principle of the method there must be differences of volume in the two 

 vessels. It is an important question whether these differences of volum can 

 possibly induce secondary differences in the gas exchanges of the cells in the two 

 vessels, during the course of an experiment. 



When in two vessels, with equal amounts of cells, 5 and 7 ml. of liquid are placed, the total 

 volume of each of the two vessels being 14 ml., vfJvg in the one vessel is 0.56 and in the other 

 1.0. This is an appropriate methodological difference. However, the concentration of the cells are 

 different. The rate of the intermittency cycles of the cells in the two vessels could be different 

 and so therefore the light actions could be different. In the course of long experiments autoinhibi- 

 tion of the cells might conceivably occur and cause different decreases in the gas exchanges in the 

 two vessels. 



Such sources of error are avoided when the volumes of the liquid phases are made equal. Then 

 the total volumes of the vessels must be different to obtain the necessary differences in vy/vq. For 

 example, 18 and 14 ml. total volume and 7 ml. of liquid volume is an appropriate arrangement, 

 vf/vg being in the one vessel 0.64 and in the other 1.0. But then the gas exchange of the cells 

 produces different partial pressures of CO2 in the two vessels, with conceivable consequences 

 on the cell metabolism. We prefer this inequality to the inequalities of cell concentrations, be- 



