88 The Quantum Efficiency of Photosynthesis 



IV. Again Nos. 3 and 5, but no white light (under compensation point) pH 4.9 



V. Again No. 7, but with half light intensity (Actinometer, 2.75 fi\ O2 per minute), pH 9.2. 



10' red light — 24 mm 

 10' dark — 28 mm 

 10' red light — 24.5 mm 

 10' dark — 27.0 mm 

 10' red light — 23 mm 



Light action 10' 27.5 — 23.8 = + 3.7 mm --- 2.42 /A 



1 10- 2.75 



= = 11.3 



v 2.42 



The total duration of these experiments was 8 hours. 



IX. Summary and Conclusion 



Since development of the new methods and procedures described, in a sequence 

 of thirty experimental days, almost without exception quantum efficiencies of 3 to 5 

 quanta per molecule of O2 produced by the action of red light have been obtained. 



The simultaneously observed quotients of Q 2 for light action lay between —0.8 



anc l _1.3, which means that the quantum efficiencies for CO2 consumption in red 

 light were essentially the same as those for O2 production. 



These results were obtained not only with low light intensities below the com- 

 pensation point and for short periods of time (minutes), but also with light intensi- 

 ties well above the compensation point (several fold), and in experiments lasting 

 many hours. It is important to emphasize that with the same cell Suspension the 

 same quantum yields may be obtained both below and far above the compensation 

 point. 



The new results resolve several uncertainties left open by the experiments of 

 1923. At that time the light intensities were so low that only a fraction of the respi- 

 ration was compensated by the light. Thus the objection could never have been 

 refuted that light inhibited respiration anticatalytically, that is, without expenditure 

 of energy. But now, in the experiments above the compensation point, this question 

 is eliminated, and chemical energy, corresponding to positive 2 production and 

 CO2 consumption, is in fact clearly gained. 



