THE PRODUCTS OF PHOTOSYNTHESIS 199 



1-modification predominating. Wherever diere is a possibility of two 

 isomers being formed in this type of reaction, the reaction goes asym- 

 metrically.'^^ As yet we are not in possession of any definite knowledge 

 on the basis of which we can explain this difference in behavior of com- 

 pounds the free energy of which, as far as we know, is very nearly the 

 same. Doubtless the reactions constituting the breakdown of hexose 

 sugars are of a catalytic nature and it is also highly probable that the 

 catalysts in the form of enzymes are, like the carbohydrates, of unsym- 

 metrical structure. We have considerable chemical evidence that the 

 asymmetrical nature of a catalyst may exert a decided influence on the 

 course of the chemical reaction it is catalyzing. Thus, as has been men- 

 tioned, an asymmetrical catalyst may bring about the reaction of only one 

 of two space isomers or, what amounts to about the same thing, react with 

 one at a very much greater rate than with the other. However, the kinetics 

 of these reactions can as yet not be explained satisfactorily, and this must 

 probably await a better understanding of the mode of reaction of carbon 

 compounds and this in turn the nature of the carbon atom. 



On the behavior of asymmetric catalysts in inducing reactions which 

 show preference or selection for asymmetric compounds we must prob- 

 ably rely for an explanation of the fact that the carbohydrates formed in 

 photosynthesis are also asymmetric, or more properly, that of the relatively 

 large number of hexose sugars only a few are formed in the photosynthetic 

 process. Such a conception is strengthened by the fact, established by 

 Willstatter and Stoll and others, that photosynthesis is dependent upon 

 the action of an enzyme. In what step of the series of chemical reactions 

 comprising photosynthesis, the enzyme exerts its asymmetric influence it 

 is still impossible to say. There is some evidence for believing that the 

 first step in photosynthesis consists in the union of carbon dioxide with 

 a substance in the leaf. If this substance is already asymmetric it is 

 highly probable that its asymmetry will be imposed upon the product re- 

 sulting from the action of light on the carbon dioxide compound. But 

 until we are in possession of more precise knowledge regarding this first 

 step in photosynthesis and the subsequent photochemical reactions the 

 discussion of the origin of the asymmetric compounds in leaves is largely 

 speculation. 



The same may be said for most attempts at explaining the origin of 

 life on the basis of our present knowledge of physics and chemistr}^ One 

 of the characteristics of all living things is that they are composed of 

 asymmetric compounds and the behavior of these organisms or the enzymes 

 which make possible many of their life activities, is governed by the space 

 characteristics of the food material. By far the great majority of forms 

 of life known to us derive their energy from carbon compounds. The 

 only means which we know of by which carbon compounds capable of 

 sustaining life are formed is through photosynthesis. This process re- 



^^-Neuberg, et at., Biochem. Zeit., 67, 32 (1914) ; 90, 388 (1918) ; 91, 257 (1918) ; 

 92, 96, 111 (1918); 112, 313 (1920). 



