268 Biological Chemistry. 



synthetical character, affords, therefore, the most suitable 

 subject for the first investigations in biochemistry. 



In contradistinction to the animal body, the plant can, 

 as a rule, utilize as foods simple substances like the carbon 

 dioxide of the air, and nitrogen from inorganic salts such 

 as nitrates. From suitable simple chemical compounds it 

 can build up complex products such as starch and the 

 proteins. As it moves but little, and its temperature is < 

 not necessarily above that of its surroundings, its energy 

 needs are small. The synthetical processes tend, therefore, 

 to predominate over the degradative processes, although 

 the latter are by no means negligible. Furthermore, there 

 is not in the plant the morphological differentiation which 

 exists in the higher animal and which is associated with 

 a differentiation of function. More distinct chemical pro- 

 cesses probably take place within a given plant organ than 

 occur within a single animal organ, and this fact alone 

 renders the study of biochemical changes in plants exces- 

 sively difficult. For the reasons stated above, it is not 

 illogical to choose the higher animal in the first instance for 

 the study of biochemical changes in spite of its great mo: 

 phological complexity. 





D 



s 



Part L The Total Metabolism and the Balance-Sheet 

 of the Organism. 



As already stated, the greater part of the food inges 

 by the animal undergoes oxidation within the organi 

 providing thereby a source of energy by means of whi 

 the temperature of the body is maintained and muscular 

 work can be performed. A relatively small part of the 

 ingested material is required, however, for the replacement 

 of tissue wasted, and in the case of the young animal, for 

 the growth of new tissue. No diet can, therefore, be con- 

 sidered satisfactory which does not fulfil the twofold func- 



