100 RESPIRATION 



But the analogy between a fire burning and the oxidation 

 of protoplasm is not an exact one. In a fire the law of "mass 

 action" holds — the quicker oxygen be supplied, the faster 

 burns the fire. In living tissue the rate of oxidation is not 

 increased in accordance with the law of mass action. Only 

 if the carbon dioxide be removed as rapidly as it is formed is 

 this the case. 



Every boy is perfectly aware that he is warm. When he 

 goes to his chilly bed in a winter's night he knows that next 

 morning the bed and the bedclothes will feel warm. He has 

 been acting as a warming pan. Exercise which involves 

 increased respiration makes a boy still warmer, and the fact 

 that during a football match he is panting is an expression 

 of the fact that he is using up his energy and becoming hotter 

 during the course of the game. It is a common expression 

 to hear that people put on thick clothing to keep the cold 

 out. As a matter of fact they put on thick clothing to keep 

 the heat in. 



In the steam engine the combustion ultimately produces 

 a certain amount of ashes, for the coal is not wholly consumed ; 

 and so would it be if instead of eating our food we dried it 

 and burnt it — the result Avould be ashes. Carbon dioxide 

 and water are being given off by all plants and animals, but 

 the "ashes" of plants are more or less used over again and 

 built up into new compounds in their bodies. In animals, 

 however, the so-called "ashes," represented by urea and uric 

 acid, etc., are excreted by the kidneys and the sweat-glands. 

 Plants have no organs comparable with kidneys and sweat- 

 glands. 



Plants are built up more by carbohydrates than by com- 

 pounds of nitrogen, and it is these substances which are 

 mainly used up in plant respiration. In animals, however, 

 the proteins ^\dth their nitrogen content are more dominant 

 and the combustion of these necessitates special organs such 

 as contractile vacuoles in the unicellular forms, flame-cells, 

 the "kidneys" of many worms, and kidney-tubules which rid 

 the animal body of its waste matter or "ashes." Most of the 

 substances produced by the oxidation or burning of proteins 

 are poisonous to all living cells. Therefore the animal throws 



