The Maximum Efficiency of Photosynthesis : A Rediscovery 99 



gen — the photochemical reaction (decomposition of water)*; but there are many 

 different dark reactions that consume carbon dioxide to form a great variety of 

 products. Thus if, as an extreme case, oxalic acid were formed from water and 

 carbon dioxide y for this Compound would be - - 4 and the quantum requirement 

 for carbon dioxide would be only one-fourth of the requirement for oxygen. 



Conclusion 



It follows from the data obtained** that in thespectral region 630 to 660 mu no 

 more than 4 quanta are required to produce one molecule of oxygen gas. A require- 

 ment of 3 quanta is open to serious consideration, although thus far the average 

 value in our experiments has been nearer 4 than 3. 



A quantum requirement of 4 means that the efficiency of the energy transfor- 



, 112,000 100 



mation in our experiments was approximately - — — T^^p =65 percent, whereas 



the quantum number 3 would mean an efficiency of about 85 percent. 



In any event, under favorable conditions the greater part of the absorbed light 

 energy can be transformed in green cells into chemical energy, and this is, to quote 

 the man who laid the foundation of quantitative photochemistry, "a marvellous 

 achievement of nature-". 



Addendum 



Quantum requirement determinations were demonstrated to, and carried out with 

 the aid of, the students of the physiology class at the Woods Hole Marine Biological 

 Laboratory on July 19 and 20. Requirements of 3 to 5 quanta per molecule of 

 oxygen gas produced in photosynthesis were observed on these two days. The 

 respiration was overcompensated several fold by light of unmeasured intensity 

 from an incandescent lamp used with or without red filter (cut-off at ^ 560 m//). 

 In two closely comparable, consecutive experiments, quantum requirements of 

 3.0 and 3.5 were obtained from measured red light (630 — 660 m//). The aid 

 of students Jack Dureil, Richard Klein, Burlyn Michel, and Martin Schwartz 

 in carrying out various of the class experiments is acknowledged gratefully. 



References 



1 Franck, James, and Loomis, Walter E., Photosynthe- 4 Reprinted in Katalytische Wirkungen der lebendigen 

 sis in plants. Arnes, Iowa: Iowa State College Press, Substanz. Berlin: J. Springer, 1928. 



1949. 

 2 Warburg, Emil, Photochemic. Handbuch der Experi- 

 mentalphysik 18 (1928), 655. 6 Warburg, Otto, and Schocken, Victor, Arch. Bio- 



5 Amer. J. Bot. 35 (1948), 194. 



6 Warburg, Otto, and 

 3 Warburg, Otto, Biochem. Z. 152 (1924), 51. chem - 21 ( 1949 )> 363 - 



* Zusatz 1961. „Decomposition of water" ist beim heutigen Stand der Wissen- 

 schaft zu streichen. 



** Reported in greater experimental detail in Biochem. et Biophys. Acta, October 1949 (Meyer- 

 hof-Festschrift volume); Arch. Biochem., September 1949; and at meeting of Society of General 

 Physiology, Woods Hole, Massachusetts, June 1949. 



