QUANTUM YIELD 1953 



In experiments in which unmeasured, white background light compen- 

 sated respiration, and a measured green beam (546 m^u) was added for one 

 minute at one minute intervals, the differences between the total gas ex- 

 changes in the two vessels in 20 dark periods and 20 light periods, corre- 

 sponded to a requirement of 1.29 quanta per molecule oxygen, and to a ratio 

 — ACO2/AO2 = 1.29. In a similar experiment with a measured blue 

 beam (436 m/n), the requirement was 1.15 quanta per oxygen molecule, and 

 1.0 quanta per CO2 molecule. A similar result was obtained with red cad- 

 mium light, X 644 mjLt, superimposed on compensating white illumination. 



When the gas exchange in continuous white light (steady respiration + 

 steady photosynthesis) was subtracted from the net gas exchange in an 

 equal period of alternating light, an over-all quantum requirement of 3-5 

 quanta per oxygen molecule was calculated for the additional light, in agree- 

 ment with the earlier results of Warburg and Burk. (The rate of gas ex- 

 change in the background light was found in these experiments to be the 

 same before and after a period of additional alternating illumination, thus 

 making the calculation unambiguous.) 



The conclusion Warburg and co-workers drew from these observations 

 was that the light process in photosynthesis can be separated from the dark 

 process by minute-to-minute alternations of light intensity. At the begin- 

 ning of the light periods, oxygen is liberated and carbon dioxide is absorbed 

 (in equal amounts) , with a quantum requirement of only one quantum per 

 molecule ("one-quantum process of photosynthesis"). Since the energy 

 of 1 red quantum (40-45 kcal) is insufficient to bring about the elementary 

 process of photosynthesis (which requires 112 kcal), Warburg suggested 

 that the extra "respiration" which develops after a minute or two of ex- 

 posure to light and is carried over into the first minute or two of darkness, 

 re-oxidizes two thirds of the carbohydrates (or other photosynthetic prod- 

 ucts) synthesized in light, and that the energy of this autoxidation is 

 stored in an unknown chemical compound. The latter thus becomes able 

 to reduce one molecule of carbon dioxide to carbohydrate, and to oxidize one 

 molecule of water to oxygen, with the help of a single additional light quan- 

 tum. This concept Warburg, Burk et at. called the "new theory of photo- 

 synthesis." 



It is misleading to refer to the suggested mechanism as "one-quantum 

 mechanism of photosynthesis." The minimum quantum requirement, 

 according to Warburg, Burk et al. is 2.8, not 1. That the energy of one 

 quantum is used in a way different from that in which the energy of the 

 other 1.8 quanta is used, does not change the fact that the combined action 

 of 2.8 quanta is needed to achieve complete photosynthesis. It has been 

 suggested by many before Warburg that, in photosynthesis, the energy of 

 several quanta is brought to bear on a single molecule of carbon dioxide (or 



