THE ENERGY RELATIONS IN PHOTOSYNTHESIS 335 



ably the most exact bearing on this subject. Suffice it to recall that from 

 these experiments it is concluded that for the reduction of one molecule 

 of carbon dioxide there are required four quanta in red and yellow light 

 and five quanta in the blue. 



Nevertheless, Weigert considers that his scheme represents the actual 

 state of affairs. He bases this largely upon the high temperature coefficient 

 of the photosynthetic process. The temperature coefficient has been ex- 

 tensively investigated and it is evident that for purely photochemical re- 

 actions the photosynthetic process exhibits some decided abnormalities. 

 Thus under conditions of high light intensity and high carbon dioxide-con- 

 centration, the temperature coefficient varies as folloYv^s for each 10° : 



5.4-10° 10-20° 20-30° 



Qio 4.3 2.1 1.6 



With low light intensities the temperature coefficient decreases, and 

 between 25 and 32° it is unity, indicating that under these circumstances 

 the photosynthetic process is independent of temperature. Such is the 

 case for most photochemical reactions. In these the energy requirements 

 of the chemical reactions following the primary reaction is less than the 

 energy of the absorbed quanta; i.e., the number of quanta absorbed, per 

 unit time, regulates the rate of formation of the photochemical end prod- 

 ucts. Thus the rate of reaction is independent of the temperature of the 

 system. 



In the photosynthetic process the outstanding facts are that this is an 

 endothermic reaction and that the absorption of energy quanta takes place 

 in a long wave length region of the spectrum. Thus the two quanta calcu- 

 lated as necessary for the reaction do not suffice to cover the total energy 

 requirements. In order that the process may take place at all the deficit of 

 energy must be supplied by the system. This according to Weigert's 

 theory would result in a cooling of the system and an automatic inhibition 

 of the whole process. Thus, at higher temperatures, the supplying of the 

 deficit is more easily accomplished than at lower temperatures and on this 

 basis Weigert accounts for the abnormally high temperature coefficient 

 at low temperatures. Moreover, the fact that with low light intensity 

 the temperature coefficient approaches unity is explained by the fact 

 that under these conditions, the rate of the primary process, as the 

 slowest one in the system of step-reactions, determines the rate of the 

 total reaction. 



The theory of \\'eigert is of interest in connection with that proposed 

 by Spoehr and McGee -^ on the relation between photosynthesis and 

 respiration. They have considered the probability that a portion of the 

 energy required for the reduction of carbonic acid is derived from the 

 oxidation of carbohydrates in the chlorophyllous cells. If the energy 

 liberated by the oxidative processes first appears in the form of light 



"Spoehr and McGee, Carnegie Inst, of Wash., Pub. Xo. 325, 76 (1923). 



