PHOTOSYNTHESIS 



emerge gradually, since about 1930, from the discussions accompanying 

 the newly gained experimental data (van Nicl, Stoll, Emerson, Wohl, 

 Franck, and others), the interrelation between the different types of 

 carbon dioxide assimilation has struck some investigators quite sud- 

 denly. In their enthusiasm over this fact they are likely to overlook 

 the peculiar problems which {photosynthesis offers in contrast to, and 

 distinction from, the other metabolic processes. To them, now, all 

 is very simple. The light absorbed by chlorophyll is used to mobilize 

 hydrogen. Water is decomposed into H and OH. And once hy- 

 drogen is available the reduction of carbon dioxide proceeds as a dark 

 reaction exactly as in the cases mentioned above. This has led to 

 some strange expressions and statements, such as "photosynthesis in 

 the dark" or "light per se is not essential for photosynthesis." 



Lately, the analogy between thermal and photochemical re- 

 actions has been pushed still further, and attempts have been made to 

 bring the so-called energy-rich phosphate bond into the picture. Be- 

 fore we analyze these attempts we must say a few words about inter- 

 mediates. 



In Search of Intermediates 



In taking apart the mechanism of photosynthesis, we shall in 

 all probability find two types of intermediates: on the one hand, the 

 enzymes necessary for the transfer of hydrogen and the removal of 

 oxygen; on the other, the final acceptors, the precursors of carbo- 

 hydrates and of oxygen. The former we may compare to the pyridine 

 nucleotides, flavoproteins, and cytochromes, the latter, to the deg- 

 radation products of glucose, with no clear counterpart to what has 

 been called the "photoperoxide" or "peroxide" or "moloxide" or 

 "hydroxylated compounds" (van Niel) in photosynthesis, that is, the 

 hypothetical substance which decomposes with the evolution of oxygen 

 (we are pretty certain that it is not hydrogen peroxide and perhaps it 

 is no peroxide at all) . 



Most of the intermediate catalytic steps in respiration and 

 fermentation have been shown to be reversible — even the decarboxyla- 

 tion of pyruvate, if it proceeds with the formation of acetyl phosphate 

 (11). The only requirement is the coupling with another reaction, 

 furnishing the energy to reverse the particular step. Without such a 

 coupling the breakdown processes continue unchecked until the specific 



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