THE LIGHT FACTOR IN PHOTOSYNTHESIS 1047 



Table 5. — Quanta of Energy for Different Wave-lengths in the Visible 



Region of the Spectrum 

 Wave-length, A Quantum Energy, Nhi^ (cal. sec.) 

 4000 71 , 200 



5000 56 . 900 



6000 47.400 



7000 40 , 700 



8000 35 . 600 



great enough to supply the heat required by the reaction. Since photo- 

 synthesis is an endothermic reaction, 



CO2 + HoO = l/a:(CH20)x + O2 - 112,000 cal., 



the energy quantum must be great enough to supply this energy. By 

 a comparison of the energy of the reaction with the values given in Table 

 5j it is seen that in the visible region of the spectrum no quantum is equal 

 to 112,000 cal., the amount necessary for the postulated photosynthetic 

 reaction. This is one of the most difficult problems of a theoretical 

 nature concerning the mechanism of this process. 



The question is whether or not the process is dependent only on the 

 thermal energy supplied by the light or whether it is a typical quantum 

 process in which the amount of action is proportional to the number of 

 quanta absorbed. If the reaction is dependent only on the thermal 

 energy supplied by the light, the energy efficiency of the process should be 

 dependent on the number of calories absorbed irrespective of the wave- 

 lengths of light. If it is purely a quantum process the amount of reaction 

 should depend on the number of quanta absorbed and the energy effi- 

 ciency should increase at the longer wave-lengths. 



Warburg and Negelein (134, 137), using the alga Chlorella, have 

 attempted to determine which of these two processes fits the facts. They 

 have determined the energy efficiencies with different wave-lengths of 

 light. Their results are assembled in Table 6. In these experiments the 

 arrangements were such that all of the incident light was absorbed by 

 the organism and thus the uncertainties attending calculations of light 

 absorbed by living organisms were eliminated. By extrapolation of the 

 assimilation values obtained at known light intensities to zero light 

 intensity, they made an effort to obviate variations due to light intensity. 



On the basis of these experiments Warburg (136) concludes that 

 the number of quanta necessary to decompose a molecule of carbon 

 dioxide in the photosynthetic reaction is about 4. However, the number 

 of quanta necessary in blue light is greater, about 5. This Warburg and 

 Negelein attribute to the absorption of a part of the blue light by the 

 yellow pigments. But Warburg considers that it is more correct to 

 employ in the calculation only the energy absorbed by the chlorophyll. 

 When this is done he finds that the number of quanta used in blue light is 

 "less than 5 and more than 3, probably 4 quanta." 



