PHOTOSYNTFiESIS 



and fermentation and, on the other, to a clearer knowledge of the 

 essence of photochemical reactions in complex molecules. Twenty- 

 five years ago there was an overabvmdance of uncorrelated general 

 observations and only a few \ery simple pictures of the mechanism 

 of photosynthesis. The latter were hardly supported by any straight- 

 forward experiments or permissible analogies. I need mention only 

 the concepts of a chlorophyll carbonic acid complex, of the release 

 of oxygen from this complex, and of the formation and polymerization 

 of formaldehyde, which are still faithfully reproduced in most textbooks 

 of botany. About 1930, van Niel (19) pointed out that photosynthesis 

 should be considered as a coupled oxidoreduction comparable to 

 other reactions of this kind. This approach to the problem proved 

 very fruitful and was soon generally adopted. Today, in 1945, we 

 can divide the process of photosynthesis into several partial reactions, 

 each with its particular problems, none of which seems to present 

 insurmountable conceptual difficulties. By drawing upon analogies 

 with other metabolic processes and from the results of direct experi- 

 ments it is possible to build up a theoretical picture which, though 

 incomplete in many places, satisfies the essential requirement that 

 the main observations can be correlated. The mystery of photo- 

 synthesis is mainly gone, and this in a rather fundamental sense. We 

 are now quite certain that photosynthesis promoted by chlorophyll in 

 visible light has nothing to do with the origin of life. Instead, it must 

 be regarded as a rather late achievement of the living cell. It is a 

 unique combination of a few reactions found only in the green plant, 

 with important devices characteristic of any kind of metabolism in 

 living cells. 



With the acquisition of pigments, the early living cells became 

 able to accelerate the reaction between hydrogen donors and acceptors 

 by absorbing radiant energy. The biological currency, H and OH, 

 the constituents of water, became available in larger quantities because 

 of the interaction between the irradiated dye and water. Despite the 

 photochemical reduction of carbon dioxide no true gain in free energy 

 of the complete system was yet possible, since the resulting "hydroxyl- 

 ated" counterparts {cf. reference 20) could only re-form water with 

 some valuable hydrogen donor. Any increase in the amount of 

 organic matter depended on the presence of inorganic hydrogen 

 donors such as free hydrogen, hydrogen sulfide, sulfur, ferrous iron, 



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