1944 KINETICS OF PHOTOSYNTHESIS CHAP, 37D 



was superimposed on it for periods of 30 or 60 min. The suspensions were 

 dilute, absorbing only 20% of the green beam. An "over-all" requirement 

 of 5 quanta per molecule of oxygen was observed at the beginning of the 

 runs, declining to 4 or 3.5 in the subsequent hours (fig. 37D.25). 



Similar results were obtained also without a respiration-compensating 

 background light, but the rates were less steady (fig. 37D.26) and calcula- 

 tion had to be based on the comparison of light and dark periods immedi- 

 ately following each other. Variations in the color of background light or of 

 the measured beam had no effect. The action spectrum appeared constant 

 throughout the visible region — including 436 mfx, where carotenoids 

 account for a considerable fraction of total absorption (in disagreement 

 with Emerson's action spectrum of Chlorella in fig. 30.1). 



These experiments were considered by Warburg as the first definite 

 proof that the minimum over-all quantum requirement of photosynthesis is 

 less than 4 ; 3.5 quanta was suggested as a possible true minimum. 



The ratio ACO2/AO2 = —1.2, found in these experiments, was taken as indication 

 that carbon dioxide was reduced only to the average level of glyceric acid (L = 0.83 

 according to eq. (5.13), p. 103, corresponding to -l/Qp = -1.2). Nevertheless, War- 

 burg and Geleik used 112 kcal/mole as the energy stored per oxygen molecule evolved. 

 This is permissible, because, as noted on p. 216, the heat of combustion of all saturated 

 carbon-hydrogen-oxygen compounds is approximately the same per oxijgen atom con- 

 sumed (about 100 kcal/mole O2). The lower energy of combustion of an "under- 

 reduced" compound (such as glyceric acid) is therefore compensated by the smaller 

 amount of oxygen needed for its combustion. 



The maximum energy conversion yield of 77%, reported in this paper, 

 was raised to practically 100% in the next paper, by Warburg, Geleick 

 and Briese (1951). This boost resulted mainly from the unexpected find- 

 ing that the minimum quantum requirement, as determined by the new 

 procedure, was strongly dependent on carbon dioxide concentration. A 

 very high concentration (9-11% CO2) was used before; it was now found 

 that an optimum existed at 5% CO2. Another way to improve the yield 

 was to substitute, during the measuring period (even if it lasted for several 

 hours), distilled water for nutrient salt solution. In water, the yield was 

 high from the beginning, without the prolonged "induction period" seen 

 in fig. 37D.25. 



When these changes in procedure were combined with those described 

 earlier — particularly, low cell density (incomplete absorption) and inter- 

 mittent illumination— the calculated quantum requirement went down 

 to about 2.8, corresponding to practically complete conversion of light 

 into chemical energy (112 kcal:2.8 = 40 kcal; equal to the energy of 1 

 einstein of red quanta at 713 m^, or to 95% of the energy of one einstein of 

 quanta at 680 mpi) . 



