6 The Maximum Efficiency of Photosynthesis : 

 A Rediscovery* 



Dean Burk, Sterling Hendricks, Mitchell Korzenovsky, Victor Schocken 

 and Otto Warburg 



It was reported in 1923 4 that in photosynthesis 4 light quanta, absorbed by Chloro- 

 phyll in green cells, can produce one molecule of oxygen under favorable condi- 

 tions. This means that with red light about 65 percent of the absorbed radiant energy 

 can be transformed into chemical energy. Quanta of red light are of low energy 

 (~ 44,000 cal/mole) and several such quanta must therefore cooperate to develop 

 one molecule of oxygen, which requires ~ 112,000 cal/mole. No analogy is known 

 in the nonliving world, where most photochemical reactions, although simpler 

 and effected by quanta of higher energy, are far less efficient in energy transfor- 

 mation, and are indeed usually exothermic. It became clear that an unknown 

 principle, active in nature, awaited elucidation by physics and chemistry. 



It is well known how the anticipated study of this new principle was interrupted 

 about 1938, when almost every investigator in the field came to accept as the maxi- 

 mum efficiency of the energy transformation low values of 20 to 25 percent, that 

 is to say, quantum values of 10 to 12 with red light. The development of this 

 revocation has been extensively summarized in a book just published in the last 

 few months by James Franck and Walter Loomis 1 and contributed to by a 

 series of investigators. 



We have reinvestigated the problem and have rediscovered the high efficiency 

 of energy transformation in photosynthesis. The data obtained are conclusive. 



In the course of this reinvestigation the methods have been simplified and im- 

 proved in such a way that determinations of quantum requirements are no longer 

 the privilege of the few. On the contrary, anyone who has a suitable light source 

 and simple manometric equipment can now determine quantum requirements and 

 convince himself that in green cells the greater part of the absorbed light energy 

 can be transformed into chemical energy (see Addendum). 



Improvements in Technique 



Culture of cells. A strain of Chlorella pyrenoidosa, used in various institutes for many 

 years and originally isolated and identified by Florence Meier, of the Smith- 

 sonian Institution, was cultivated in 300-ml gas wash bottles of the Drexel type. 

 The culture medium was 5 g MgS0 4 ■ 7H 2 0, 2.5 gKN0 3 , 2.5 gKH 2 P0 4 , 2 gNaCl, 

 and 5 mg FeSOa • 7 HoO, dissolved in 1000 cm 3 of nonsterilized well water (pH 

 4.5 to 5.0). Approximately 100 mm 3 of cells suspended in 200 cm 3 culture medium 

 was added to each bottle, and aerated with 5 percent CO2 in air so rapidly (/*»> 500 

 ml/min) that no Sedimentation of the cells occurred. They were illuminated with a 

 100-w incandescent lamp that raised the temperature in the bottles to 25 — 30° C. 



* Republished from Science 110 (1949): 225 by permission. 



