The Maximum Efficiency of Photosynthesis 127 



ko, = 1.09 

 no light action 



> 100 



But when a 100-w. incandescent lamp was turned on at a distance of 12 in. from vessel 34, 

 the respiration was compensated by the white light. 



This experiment shows, that at adequately low CO-j pressure there was no light 

 action by JJ. 



Acknowledgments 



We wish to acknowledge with deep appreciation the facilities and laboratory accommodations 

 for research and class demonstrations provided during the later phases of this work by Professor 

 E. S. Guzman-Barron and by Dr. Charles Packard, Director, Marine Biological Laboratory, 

 Woods Hole.We wish to thank Dr. Ernest Kun and Messrs. Jack Dureil, Richard Klein, Burlyn 

 Michel, and Martin Schwanz of the Physiology Class of this Laboratory for valued aid. Mrs. 

 Lois B. Macri, Mrs. Clara F. Smith, and C. R. Newhouser, of the National Cancer Institute, 

 provided extensive technical assistance. 



Summary and Conclusions 



Experiment No. 2 shows that the quantum requirement in carbonate buffer at pH 

 9 was 2-3 times the quantum requirement in acid culture medium saturated with 

 5% CO-2 in air (pH 5). This result was obtained for aliquots of the same cell Sus- 

 pension. It invalidates many efficiency experiments of the past, where for 10 years 

 the conventional medium for quantum requirement determinations has been car- 

 bonate buffer, and the view has been widely adhered to that the minimum quantum 

 requirement could be realized in carbonate buffer. But in no experiment in carbo- 

 nate buffer at pH 9 have we observed lower quantum values than 8, the average 

 being about 10. 



Experiment No. 9 shows that in acid culture medium the quantum requirement 

 is different according as the medium is saturated with 5% C0 2 or freed as nearly as 

 possible of CO>. When the culture medium was largely freed of CO^, no 2 was 

 produced by a light intensity that in the presence of 5% CO2 was highly efficient. 



In all experiments carried out by the prescribed procedure in acid culture me- 

 dium at pH 5, and saturated with 5% CO> in air, low quantum requirements were 

 observed.The results of Expts. Nos. 1 to 9 are summarized in the following table 

 in sequence. The asterisks mean that the respiration was overcompensated. In the 

 doubly asterisked experiments the respiration was more than 6-fold compensated. 



It may be seen from Table I that in acid culture medium saturated with 5",, 

 CO2 in air the quantum requirements per molecule of O2 produced were between 

 2.5 and 4.8, on the average near to 4. The reality of 3 in several instances cannot be 

 excluded. Four-quanta in the red at 660 mu mean 65" „ efficiency and 3-quanta 

 would mean 87% efficiency in the transformation of light to chemical energy. 



