THE BIO-CHEMISTRY OF ANIMALS AND PLANTS 677 



they occur there in far larger proportion than in the plant, 

 whereas the latter contains a preponderance of carbohydrates 

 and other substances which are free from nitrogen. Here we 

 are at the outset confronted with a fundamental difference 

 between animals and plants, which seems to find an explana- 

 tion, if we consider that the nitrogen supply of plants (apart 

 from atmospheric nitrogen) is not at all abundant, whilst on 

 the other hand in the carbonic acid of the air, which by the 

 aid of the energy of sunlight they are able to transform into 

 carbohydrates, plants possess a seemingly inexhaustible supply 

 of carbon. Nitrogen-free carbon compounds accordingly perform 

 functions in the plants for which there is no analogy in the 

 animal organism, e.g. cellulose, in the cell membrane. It can, 

 however, be easily demonstrated that a sufficient supply of 

 nitrogen is one of the essential factors for plant life, and that 

 the protoplasm of the plant contains proteins as certainly as 

 does the protoplasm of an animal ; further, that the properties 

 and behaviour of the vegetable proteins are very nearly allied 

 to those of animal origin. 



It is a fascinating problem, which the comparative study of 

 vegetable and animal biology has helped to solve, to investigate 

 the great cycle of nitrogen in Nature. Here again we meet 

 with another fundamentally important difference of animal and 

 vegetable metabolism. The animal organism relies as the only 

 source for its protein anabolism either on protein itself or on 

 its direct cleavage products. It is unable to form protein 

 material from inorganic compounds, nor does the animal cell 

 possess the property of utilising other organic nitrogenous 

 compounds, which have no direct relation to proteins. The 

 animal organism, in short, depends entirely for its source of 

 nitrogen on the vegetable world. 



The raw products available to the plant for the synthesis 

 of proteins are, mainly, the free nitrogen of the atmosphere, 

 ammonia salts, nitrates and nitrites in the soil and organic 

 nitrogenous substances of various kinds which are furnished 

 by the decomposition and decay of animals and plants, such 

 as amides, amines, amino acids, and amino salts of organic 

 acids. 



In the protein synthesis of the plant from these materials 

 a most important role is played by bacteria, those micro- 

 organisms which take an intermediate place between animals 



