RESPIRATION 295 



only go on in the living cell. Other similar instances 

 could be quoted. 



The probable course of events in respiration is that the 

 oxygen in some way unites with the protoplasm, rendering 

 it unstable, and initiating a series of decompositions which 

 result in the successive formation of many bodies of less 

 complex composition, each successive decomposition pro- 

 ducing simpler ones, till finally carbon dioxide and water are 

 formed. Simultaneously, reconstruction of the protoplasm 

 goes on, many of these residues being in whole or in great 

 part built up again into its substance, together with new 

 material supplied to it in the shape of food. If the 

 temperature is low, the breaking down of the protoplasm 

 proceeds but slowly, and reconstruction is rapid, so that 

 under these conditions the quantity of oxygen absorbed 

 or fixed as intramolecular oxygen by the protoplasm is 

 greater than the quantity of carbon dioxide formed by its 

 decomposition. At a higher temperature decomposition is 

 much more easily carried on, and its products are more 

 numerous and simpler. The decomposition and recom- 

 position go on side by side, simpler bodies being gradually 

 produced, either by their splitting from the protoplasm 

 directly, or by their being formed at the expense of the 

 more complex decomposition -products during processes of 

 slow oxidation in the substance of the protoplasm, till 

 finally a certain production of carbon dioxide and water is 

 arrived at. So long as the protoplasm remains alive the 

 amount of these is relatively small, reconstruction con- 

 tinually taking place. When, however, the protoplasm 

 dies, simple bodies, such as carbon dioxide, water, and 

 possibly ammonia, in addition, are produced abundantly 

 from the decomposition which attends its death. 



If the self-decomposition of protoplasm during life 

 involved such a splitting-up as would lead to the formation 

 of nothing but these, nearly all the potential energy of the 

 cell would be liberated. We have seen, however, that this 

 is not the case, but that a good deal of the energy set free 



