METABOLISM 535 



All these data are obtained by a series of analyses : 



I. The Energy Metabolism. 



As synthetic agent the plant in forming the complex compounds 

 from inorganic materials absorbs energy from the sun's rays. It is 

 represented by the analysis of its tissues in terms of carbohydrate, fat, 

 protein. In animals the energy metabolism is determined by the 

 analysis of its food in terms of fat, protein, carbohydrate and the 

 amount of heat given off by the animal. The animal is placed in a 

 special form of calorimeter through which air of a constant temperature 

 can be passed, measured and analysed. The experiments are compli- 

 cated and difficult to perform. 



II. The Carbon Metabolism of the Carbohydrates, Fats and 

 Proteins. 



In plants this is determined by the analysis of the tissues, the 

 carbon being derived from the atmosphere. 



The carbon metabolism in animals is determined by the analysis 

 of the inspired and expired air. The animal is placed in a suitable 

 chamber, fed upon a given food, the volume of gas which enters and 

 leaves is measured and samples of each are analysed. The carbon in 

 the urine and excreta should also be estimated. 



As in the above experiments special apparatus is required and the 

 analyses are difficult to perform. Reference must be made to the 

 special books upon these subjects. 



III. The Total Nitrogen Metabolism. 



The total nitrogen metabolism in plants and animals is ascertained 

 by the analysis of the nitrogen in the soil, or in special culture solutions, 

 of the nitrogen in the plant tissue, of the nitrogen in the food and 

 in the excreta of animals. These analyses are usually effected by 

 Kjeldahl's method. 



IV. The Nitrogen Metabolism of Individual Compounds. 

 The total nitrogen value is the sum of the amounts of nitrogen in 



the various compounds. It can be subdivided into numerous groups : 



Nitrates \ Nitrates 



Nitrites Nitrites 



Ammonia |- in the soil Ammonia 



Other nitrogenous I Amides 



compounds J Amino acids \ m the P lant 



Proteins 

 Purines 



etc. 



Amides > Ammonia 



Amino acids I Urea 



Proteins | in the food Uric acid 



Purines f of animals Purines 



Creatine etc. 



etc. J 



in the excreta of 

 animals 



Each of these groups is estimated in the various tissues. 



