CHLOROPHYLL AND ENERGY 179 



gluco sides, it is evident that only the half-way stage has 

 been reached. 



Attempts have been made to go a little further by varying 

 the oxidants. The ideal would be to use carbon dioxide itself, 

 but it has not yet been possible to achieve this directly. It has, 

 however, been possible to fix it on pyruvic acid by using a 

 mixture of pyruvate, with carbonate as the source of carbon 

 dioxide, triphosphopyridine nucleotide (TPN) as the source of 

 energy, salts of manganese as catalysts and malic diastase. 

 This diastase normally transforms maUc acid into pyruvic acid 

 by a decarboxylation, i.e., by hberating in the form of carbon 

 dioxide the atom of carbon which forms the acid radical 

 - COOH, so that we pass from a tetracarbonate molecule to a 

 tricarbonate molecule. This decarboxylation produces energy, 

 for it oxidizes the fourth valency of an atom of carbon. The 

 Hill reaction accomplishes the reverse; one atom of carbon, 

 the four valencies of which are saturated with oxygen, liberates 

 one of them to fix on the pyruvic acid which becomes mahc 

 acid. 



Wood and Werkman showed that plants, and even animals, 

 are capable of fixing carbon dioxide on pyruvic acid. This is 

 therefore not a specific reaction of photosynthesis, but is 

 accompUshed by borrowing the necessary energy, not from an 

 internal oxidation, but from light. Is there also a reaction 

 peculiar to photosynthesis, characterized by the utilization of 

 light only? 



In any case, the speed of liberation of oxygen in this 

 reaction is rather interesting since, under the best conditions, 

 one molecule of oxygen per molecule of chlorophyll is released 

 every thirty or forty seconds; in the cell, this speed may 

 increase to one molecule every fifteen or twenty seconds. 



Since all the valencies of carbon in the carbon dioxide are 

 saturated with oxygen and the fixation cannot take place 

 without liberating at least one valency, it seems obvious that 

 the Uberated oxygen comes from the carbon dioxide fixed. To 

 check this point. Holt and French, in 1948, used radioactive 

 oxygen and found that the liberated oxygen came, not from 



