CHLOROPHYLL AND ENERGY 181 



the bond energy rises to 16,000 calories. When it is fixed 

 on one atom of nitrogen, for example in arginine-phosphate, 

 or on another phosphoric acid (pyrophosphate) the energy 

 capable of being hberated is between 10,000 and 12,000 

 calories. 



There are therefore two main types of phosphoric bonds 

 — the first of low energy, less than 5,000 calories, the second 

 exceeding 10,000 calories. 



Here, one of the essential substances of both the animal 

 and vegetable organism — adenylic acid — intervenes. It is one 

 of the constituents of nucleic acid and does not exist only in 

 the voluminous molecule of this fundamental part of the cell 

 nucleus, but is also found more or less isolated, and we are 

 beginning to perceive the large and important part that it 

 plays. 



Ribose, a glucoside with 5 atoms of carbon, is attached at 

 one end of its carbon chain to an organic base, adenine, and 

 thus become adenosine. At the other end of the carbon chain, 

 phosphoric acid is fixed and we have adenylic acid, which is 

 also called adenosine-monophosphate and commonly desig- 

 nated by the initials AMP. 



This bond between phosphoric acid and ribose through 

 the intermediary of an alcohol radical is low in energy. But 

 on this phosphoric acid another phosphoric acid is fixed and 

 sometimes even a third. These two new bonds, between 

 phosphoric acids, are pyrophosphate bonds and consequently 

 of high energy, and we have adenosine diphosphate (ADP) 

 and adenosine triphosphate (ATP). 



In the hving cell ATP seems to be the pivot of the exchanges 

 of energy. It is, in fact, capable, when necessary, of hberating a 

 rather large quantity of energy by abandoning one or even 

 two phosphoric acids and being transformed into ADP or 

 AMP. On the other hand, it is capable of storing a large 

 quantity of energy by the formation of ATP. This possibihty 

 of acting in the two directions according to circumstances 

 gives adenosine triphosphate primordial importance in all the 

 intermediate metabohsm. 



