THE LIGHT FACTO It IN PHOTOSYNTHESIS 1049 



in the red averaged 59.3 per cent and in the green 68.0 per cent, a ratio 

 of 1.15 for green to red Hght. On the basis of quantum energies for the 

 wave-lengths employed, 5400 A (5900 to 4900 A) and 6600 A (7000 to 



o 



5900 A), the ratio should have been 0.82. From this Wurmser concluded 

 that the utilization in green light is greater than would be predicted and 

 indicated that there was a more complete utilization of the absorbed 

 energy in this region of the spectrum. This agreed qualitatively with 

 his idea that there is an abrupt transition in the efficiency in going from 

 the shorter to the longer wave-lengths, for there is enough energy in two 

 quanta of blue light to carry out photosynthesis whereas in red light three 

 quanta would be necessary. At about X5000 A there should be an abrupt 

 change in the number of quanta required accompanied by an abrupt 

 change in the energy efficiency of the process. 



Both Briggs (8) and Warburg (134) have criticized Wurmser's method 

 of determining the energy efficiency. This criticism is based on the way 

 in which he measured the light absorbed. This was determined by 

 measuring the transmission through the algal film before and after it 

 had been bleached by intense illumination. These criticisms have been 

 refuted by Wurmser (149). It may be concluded from the data available 

 that the photochemical action in the photosynthetic process is determined 

 by the number of quanta rather than by the total energy in the light 

 absorbed by the plant. 



ENERGY TRANSFER 



Since more than one quantum of energy is necessary for the photo- 

 synthetic reaction, it would be important to know in what manner the 

 energy transfer takes place. In order to obtain enough energy to drive 

 the reaction it would be necessary for the reacting system to absorb the 

 number of quanta required either simultaneously or consecutively. 

 Either of these mechanisms for the absorption of the requisite number of 

 quanta would be, however, very unlikely unless the life of the activated 

 process be relatively long. Even though this be the case it is improbable 

 that such high yields would be obtained. 



Although the life of most activated molecules is very short (10~^ sec.) 

 that of an activated molecule of oxygen has been found by Mecke and 

 Childs (76) to be of the order of 7 sec. Whether there is any relationship 

 between the necessity of oxygen for photosynthesis, the extremely long 

 life of the activated oxygen molecule and the possibility of energy 

 exchange between the chlorophyll molecule and oxygen, as is evidenced 

 by the quenchmg of the fluorescence of chlorophyll by oxygen (Kautsky, 

 49), will remain for future experiments to decide. 



A more probable explanation for the accumulation of the energy 

 necessary for photosynthesis is that the carbon dioxide molecule is 

 reduced in a step-wise fashion, and that each step requires only the 

 fractional amount of energy necessary for the total reduction. These 



