CHAPTER VII 



THE LIBERATION OF ENERGY AND THE EXCRETION 

 OF WASTE 



THE maintenance of life involves a continual ex- 

 penditure of energy. This energy is derived directly 

 or indirectly from the potential energy stored in 

 organic compounds manufactured during photo- 

 synthesis and the further constructive activities of 

 protoplasm. The potential energy becomes kinetic 

 in the satisfying of the oxygen affinities of the ele- 

 ments of the organic compounds, as we have already 

 seen in Chapter IV. The supply of oxygen is derived 

 from the air and the process of intaking of oxygen, 

 decomposition of organic compounds and excretion 

 of the simpler and more or less fully oxygenated 

 compounds are known as respiration and excretion. 

 Let us look at these processes rather more in detail. 



The primary composition of the atmosphere must 



be our first concern. It consists essentially of three 



gases, nitrogen, oxygen and carbon dioxide. In 



Composi- an average sample of air these gases occur in the 



air? following (approximate) percentages, viz., nitrogen, 



79*02 per cent. ; oxygen, 20' 95 per cent. ; carbon 



dioxide, 0*037 per cent. 



Detailed research has shown that all varieties of 

 protoplasm ultimately die in the absence of free oxy- 

 gen, and animal protoplasm is more sensitive in this 

 respect than plant protoplasm. There are, however, 

 some organisms of lower rank which can, for a con- 

 siderable time or during their entire life, exist in the 



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