MECHANISM OF PHOTOSYNTHESIS I4I 



(c) REDUCTION OF CARBON DIOXIDE 



Theory of van Niel 



The simplest assumption that could be made concerning 

 the mechanism of photosynthesis on the basis of the general 

 scheme of van Niel would be that the assimilation of CO2 

 to give carbohydrate requires only the four equivalents of 

 H resulting from the production of one molecule of oxygen 

 per (CHOH) group. This assumption has strong support 

 from the fact that the photosynthetic quotient approaches 

 unity under a variety of conditions. Further support might 

 be derived from the fact that the tangent to the origin of 

 the light curves of photosynthesis and of the chloroplast 

 reaction in terms of oxygen production appear to have the 

 same value, indicating that the amount of light energy 

 required for the production of one oxygen molecule from 

 water is the same whatever the amount of chemical energy 

 fixed in the form of a reduced compound. If more than the 

 four equivalents of H were required to reduce one mole- 

 cule of CO2 we might hope to find a system which would 

 produce oxygen with less light energy than is required for 

 photosynthesis. 



A chemo synthetic mechanism in addition to the photochemical 

 process 



In spite of the fact that this simple assumption appears to 

 have good experimental support, the evidence derived from 

 comparative studies with different organisms would suggest 

 that it would be more correct to assume, that more than the 

 four equivalents of H in the form of a reducing agent are 

 involved for each molecule of carbon dioxide. This would 

 lead to the hypothesis that in the green plant the photo- 

 chemical processes are completed by a dark process equiva- 

 lent to chemosynthesis. If molecular oxygen were concerned 

 in this the reaction with CO2 might be of the following 

 type:— 



n(R)+«(H20)+light=«(RH2)+(in)02 (a) 



