534 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



LIFE-CYCLE OF THE COMPOUNDS. 



Our knowledge of the complete course of the changes which the 

 various organic compounds undergo in their synthesis and degradation 

 in both plants and animals is so insufficient that it is not possible to 

 draw up a full and proper balance-sheet, such as the following : 



> ? > amino acids -^ proteins -> amino acids > 



Amines + CO 2 



Hydroxy acids, etc. -f NH 3 -> ? 



Fatty acids + NH 3 



H,P0 4 



H 2 SO 4 



But a balance-sheet representing the beginning and the middle and 

 the end can be more or less satisfactorily drawn up. The elements 

 in the soil and the amount of protein, fat and carbohydrate in the plant 

 can be determined. The amount of these constituents in the food of 

 animals and the amount of carbon dioxide, ammonia, urea, etc., in their 

 excreta can be determined. Thus, a known weight of carbohydrate 

 or fat eaten by an animal is represented by the corresponding weight 

 of carbon dioxide evolved through the lungs, unless some is retained 

 in the body, in which case the body weight of the animal will increase. 

 Similarly a known weight of protein or nitrogenous food will be 

 represented by the corresponding weight of nitrogen in the urine. 

 The actual amount of each constituent ingested by an animal is the 

 amount consumed less the amount contained in the faeces, which con- 

 sists of undigested food, i.e. food which has not entered the circulation. 

 When the intake and the output of the constituents is the same, 

 then the animal is said to be in nitrogen, phosphorus, etc., equilibrium ; 

 when less is put out than is taken in, then there is retention or assimi- 

 lation and increase of body weight ; when the output is greater, then 

 there is loss of weight 



In addition to these factors, the energy factor can also be ascer- 

 tained. Each compound has a certain caloric value and can evolve 

 on complete combustion a definite amount of heat. This serves to 

 maintain the body temperature. The total heat evolved by the body 

 corresponds to the heat value of the food. The physiological heat 

 value of 



I gm. of protein is 4-1 large calories l ; I oz. = 116 cal. 



I gm. of fat is 9-3 ; I = 263 / 



I gm. of carbohydrate is 4-1 ; I =116 



1 A large calorie is the heat required to raise 1000 gin. of water from o-i ( = 1000 

 small calories). 



