62 THE NATURE OF LIFE 



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burned but the fact that it stops when 

 the organism is killed by heat indicates \ 

 that it is due to enzymes. ! 



In many animals the tissues are sup- 

 plied with oxygen by oxygen carriers in I 

 the blood which take up oxygen in the j 

 lungs or gills and convey it to the cells. j 

 In mammals an iron compound, hemo- 

 globin, performs this office, in the horse- 

 shoe crab it is a copper compound; in 

 certain other marine animals compounds 

 of vanadium or manganese. All of these 

 are admirably adapted to the role of oxy- 

 gen carriers and it is remarkable that so 

 many different metals are employed. 



The cells of the body are able to take 

 their oxygen from these compounds and 

 hence do not need free oxygen. Many 

 bacteria, protozoa and other organisms 

 get along without free oxygen and to 

 some of them it acts as a poison. 



Of late the belief has been gaining 

 ground that living cells can get the oxy- 

 gen they need by decomposing water. 

 This leaves hydrogen, which may unite 



