METABOLISM 65 



product of changes taking place in the tissue sub- 

 stance itself. 1 



These reactions, taking place constantly in the 

 tissues, are obviously of a very different sort from 

 the exchange of gases (breathing) to be observed in 

 higher plants and animals. They constitute the 

 true respiratory process. But the term respiration 



1 Chemists have discovered, indeed, that dry oxygen, at low tempera- 

 tures, is the greatest retarding agent in combustion. We are familiar 

 with the fact that iron rusts much more quickly if wet than if dry, and if 

 kept perfectly dry, will not rust at all. The rusting is an oxidation or 

 slow combustion resulting from the combination of the metal with oxygen 

 to form iron oxide. In this case the reactions are perhaps as follows 

 (Matthews) : 



I. Fe + 2 H 2 O = Fe(OH) 2 + 2H 

 II. 4 H + O 2 = 2 H 2 O, or 



2 H + a = H 2 o 2 



Fe = FeO + HzO. 



In the first equation the iron combines with the water to form ferrous 

 hydrate and hydrogen. The latter would immediately reduce the former 

 to metallic iron again if there were not oxygen present with which it 

 can combine to form hydrogen peroxide, which, giving up its extra atom 

 of O, forms ferric oxide, or iron rust, and water. The oxygen acts thus 

 not as a direct combining agent with the iron, but rather as a sort of 

 depolarizer to take off the nascent hydrogen, and the oxidation of the 

 iron is effected by the hydrogen peroxide. 



It is supposed that in animal and plant tissues much the same sort 

 of thing takes place, but with infinitely more complicated reactions. 

 The essential point, however, is that the oxygen does not combine 

 directly with the carbon element of the protoplasm to form COj, but, 

 where water enters into reactions with the substances composing the 

 tissues, it acts as a sort of depolarizer to combine with the hydrogen 

 liberated. The CO 2 probably arises independently as a by-product 

 in the shifting and rearrangement of various components of the sub- 

 stances making up protoplasm. "It is a sort of receipt for a given 

 amount of energy released by chemical decomposition." 

 F 



