BIOLOGICAL ENERGETICS 



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insight into energy metabolism as the direct chemical approach described 

 above, it nevertheless has important practical applications. 



The heat of combustion of a food is determined by use of the bomb 

 calorimeter. This is a heavy steel cylinder or "bomb" surrounded by 

 water in an insulated container. The sample plus oxygen at high pres- 

 sure is placed in the tightly closed cylinder and ignited by a spark. The 

 amount of heat liberated as the sample burns is determined by noting 

 the exact rise in the temperature of the water and making suitable cor- 

 rections for heat taken up by the bomb itself. The results are expressed 

 as calories liberated per mole or per gram of substance burned. 



When pure chemicals are examined in this way, the heats of com- 

 bustion are found to vary according to the composition of the sample. 

 The figures given in Table 16-4 show that higher percentages of carbon 

 and hydrogen are associated with higher heat values, whereas the opposite 

 is true for oxygen. The reason for this, of course, is simply that sub- 

 stances like glucose with a high oxygen content are in effect already 

 partly oxidized. 



The fuel values of foods likewise depend on their elementary composi- 

 tion, but information of this sort is not usually available. Instead, 

 foods are usually analyzed for their contents of carbohydrate, fat, pro- 

 tein, minerals, and moisture. These percentages constitute the proxitnate 

 composition. The heat value of a given food can be calculated easily 

 from its proximate composition, if the heat contributed by each of the 

 major components is known. Minerals and moisture, of course, contribute 

 nothing in this regard. Carbohydrates, proteins, and fats in the bomb 

 calorimeter give about 4.1, 5.7, and 9.5 Cal. per gram, respectively (see 

 Table 16-4). However, not all of this energy is available to the animal 

 body partly because foods are not completely digested and absorbed and 

 partly because they are not always oxidized completely in the body. 

 Proteins in particular are oxidized to carbon dioxide, water, and free 

 nitrogen in the calorimeter, but in the body the nitrogen is converted 

 into excretory products (urea, creatine, etc.), which are themselves or- 

 ganic substances capable of being burned, and releasing additional heat. 



Table 16-4 



Relation of chemical composition to heat of combustion of various substances 



Substance Composition Heat of combustion 



C% H% 0% N7o (Cal. per gram) 



Glucose 40.0 6.7 53.3 3.73 



Sucrose 42.1 6.4 5L5 3.96 



Starch 44.4 6.2 49.4 4.22 



Alanine 40.4 7.9 35.9 15.7 4.35 



Casein* 53.1 7.0 22.5 15.8 5.85 



Stearic acid 76.0 12.8 11.2 9.53 



Animal fats (av.) 76.5 12.0 11.5 9.50 



* Also contains 0.8% S and 0.8% P, 



