104 The Maximum Efficiency of Photosynthesis 



To measure the entering light, a bolometer was constructed with a light-absorb- 

 ing area greater than the cross section of the light beam used. The total light inten- 

 sity entering the vessel was then obtained by one single bolometric measurement, 

 and the difficult and inaccurate Integration of light intensities over the cross section 

 of the light beam was avoided. 



When the foregoing technique had been worked out, a series of 37 efficiency determinations 

 were performed in 3 regions of the visible spectrum, and the manometric and bolometric readings 

 obtained were presented. If the number of quanta absorbed by the Chlorophyll of the cells was 

 compared with the O2 developed, the average quantum number of the series was 4, per molecule 

 of O2. Thus, in the red at 660 m/(, where the molar quantum energy is 43,000 cal., about 



112,000 



v 100 — fi"i / 



4x 43,000 X lüü - b5 '° 



of the absorbed light energy was transformed into chemical energy. 



It may be emphasized that it is more important to compare the absorbed light with the Oo 

 developed rather than with the CO2 consumed. The energy gained per molecule of O2 is very 

 much the same for most any of the possible end products of photosynthesis, whereas the gain of 

 energy per molecule of CO2 absorbed could be quite different for various possible end products. 

 As an extreme example, one may note that if one of the end products of photosynthesis were 

 oxalic acid, as in the equation 



2H 2 + 4 C0 2 : nhv = 2 (COOH) 2 + 2 , 



the gain of energy per molecule of oxygen developed here would obviously be four times the 

 gain of energy per molecule of carbon dioxide consumed. 



Efficiencies as high as 65°,, have thus far not been obtained in any endothermic photochemical 

 reaction. Only one other photochemical reaction is known 2 to have an efficiency approaching that 

 of photosynthesis, the ozonization of molecular oxygen by the wavelength 207 m//, 



3 O2 + 1 hv = 2 O3, 



68,000 

 with an efficiency of 100 = 50%. But this exceptionally high elficiency was obtained 



with quanta of the high energy of 137,000 cal./mole, whereas the high efficiency of photosynthesis 

 was obtained with quanta of the low energy of 43,000 cal./mole, in a reaction that thermodynamically 

 requires at least three such quanta. 



Because the photosynthetic efficiency reported in 1923 was very high, doubts arose as to whether 

 the results were correct. Some investigators, in fact, working with different methods 3 , succeed- 

 ed only in realizing much lower efficiencies. The doubts increased when 4 ' 5 an error was 

 claimed to have been discovered in the method of 1923. It was alleged: that when Chlorella, pre- 

 viously kept in the dark, was illuminated for short periods (e.g., 5 — 10 min.), CO2 burst out of the 

 cells, and was reabsorbed when the cells were darkened again; that a large part of the pressure 

 changes observed in 1923 in light and in dark were outbursts and inbursts of CO2 and that only 

 a fraction (one-third to one-half) could have been due to photosynthesis; and that the assimila- 

 tory quotient, y = CO 2 exchanged/O-2 exchanged, was not approximately minus one (— 0.91) 

 as determined by gas analysis in 1923 for experimental periods of several hours, but that its real 

 value must have approached r 4. Therefore, the real minimum quantum requirement of photo- 

 synthesis must have been not 4 but nearer 12, and the real maximum efficiency not 65° but 

 nearer 22° . This allegation seemed to have been further confirmed by experiments performed 

 not in acid medium as in 1923 but at pH 9 in carbonate buffer; this kept the pressure of CO2 

 invariant and therefore manometrically avoided the supposed errors caused by the outburst and 

 inburst of CO2. In such nonphysiological buffers, which had been introduced in 1918 1 for simpli- 

 fication of certain types of experiments, but avoided for determinations of maximum efficiency, 

 quantum requirements of 10 — 12 per molecule of O2 were reported . 



However, the proposed "CO2 outburst", at variance with the experience of the 

 previous Century and a half, was never actually demonstrated in published ex- 

 periments involving quantum yield measurements. In the only completely detailed 

 efficiency experiment in which the light-dark time course was reported 4 (p - 821 >, 

 calculations, not performed by the authors, show that the pressure changes for 



