PHOTOSYNTHESIS 



value of eight and the obser\-ed one of ten we attribute to losses due to 

 inactive chlorophyll. The simplest explanation, then, for the fact 

 that photosynthesis proceeds either all the way from carbon dioxide 

 to carbohydrate \%-ith a permanent gain of 120 kcal. per mole, or not 

 at all, is that the photochemically produced hydrogen donors and 

 acceptors disappear by back reactions whenever the process becomes 

 artificially inhibited. Despite the most drastic effects of certain poisons 

 on the rate or the t\-pe of the reaction, there is no permanent shift of 

 the assimilatory quotient.* Hence, all but the final reaction products, 

 oxygen and starch or sucrose, must react back to form water. 



It is very unlikely that this disappearance of intermediates 

 should proceed by an exact reversion of the photochemical process. 

 A chemiluminescence with a 100% yield can certainly not occur. In 

 other words, we may expect a special kind of oxidoreduction \N-ithin 

 the assimilatory system (7) (compare the last part of this article, pages 

 43 et seq.). 



One way of avoiding immediate back reactions hes in the use of 

 radiant instead of thermal energ^^ A Hght quantum heats up, as it 

 were, a single molectile Ln an otherwise cool enwonment. Once the 

 structure of the absorbing molecule allows for a sp>ontaneous con- 

 version of electronic excitation energy into chemical energy (that is, 

 a change of structure to a configuration with a higher free energ\- con- 

 tent), the new tautomer has a good chance to sur\-ive until some 

 catal^'tic action makes use of its potential energ\\ The new tautomer 

 will live longer the higher the wall of activation energ\^ between the 

 original and the photochemically changed structure. In other words, 

 its lifetime depends on how much of the available light energ\' (in the 

 case of chlorophyll, this is always ca. 40,000 calories and is independent 

 of the size of the lieht quanttma absorbed) is expended in forming the 

 new structure. Actually, the ver\' first photochemical product aj>- 

 pears to be short-lived. It probably contains still much of the original 

 40,000 calories. J. Franck assimies, further, that a reduced inter- 

 mediate hydrogen donor comparable to, let us say, the reduced p>Ti- 

 dine nucleotides is not formed. Rather, the carbox\i group becomes 

 reduced direcdy as a consequence of the photochemical reaction taking 

 place in the chlorophyll-protein complex to which the essential car- 



* Assimilatory quotient = (oxxgen liberated") /(carbon dioxide consumed) 

 or (hydrogen absorbed) /(carbon dio.xide consumed). 



39 



