24 THE FUNCTIONS OF ANIMALS 



the final products of these reactions. But whereas all 

 green plants and some bacteria are able to build up these 

 complex compounds for themselves from water and the 

 carbon dioxide of the atmosphere, only a few (green) 

 animals have this power ; all the others depend for their 

 carbon supplies on the sugar, starch, and fat already 

 formed in the tissues of other animals, or of plants. As 

 regards nitrogen, most plants take this from nitrates and 

 the like, absorbed along with water by the roots ; whereas 

 animals obtain their nitrogenous supplies from the complex 

 proteins formed within other organisms. Most plants, 

 therefore, feed at a lower chemical level than do animals, 

 and it is characteristic of them that, in the reduction of 

 carbon dioxide, and in the manufacture of starch and 

 proteins, the kinetic energy of sunlight is transformed by 

 the living matter into the potential chemical energy of 

 complex food-stuffs — a process in which the green " chloro- 

 phyll " plays an essential part. Animals, on the other 

 hand, get their food ready made ; they take the pounds 

 which plants have, as it were, accumulated in pence, and 

 they spend them. For it is characteristic of animals 

 that they convert the potential chemical energy of food- 

 stuffs into the kinetic energy of locomotion and other 

 activities. In short, the great distinction — an average 

 one at best — is that most animals are more active than 

 most plants. 



Catalysis in vital processes. — Though much energy 

 is set free in the reactions in which organic compounds 

 combine with oxygen to yield carbon dioxide and water, 

 yet at ordinary temperatures these reactions may proceed 

 with imperceptible slowness. Thus a coal fire has to be lit, 

 i.e. brought to a temperature at which the oxidation is so 

 rapid that energy is set free in quantities sufficient to 

 prevent the temperature from falling until the coal is 

 completely burnt. For most organic compounds, such 

 as the principal food-stuffs, the temperature at which they 

 ordinarily burn is far above that endurable by living 

 protoplasm. It is therefore necessary, for the oxidation 

 of the food-stuffs by living cells, that the inertia of the 

 reactions should be so far lowered that they maintain 

 themselves at tolerable temperatures ; this is achieved by 



