108 GENERAL ZOOLOGY 



tion since the inorganic constituents of the diet are incapable of 

 further oxidation. Organic foodstuffs contain potential energy 

 in a chemical form in an amount equivalent to the amount of 

 energy expended in the original synthesis of the foodstuff from 

 simple elements, just as a bent spring possesses potential energy 

 in proportion to the amount of work expended in bending it. 

 If one end of the spring is released, it straightens out and in 

 doing so its potential energy is converted into kinetic energy. 

 In a similar way, when a complex organic substance, such as 

 protein or carbohydrate, is oxidized, the complex spatial arrange- 

 ment of the atoms in the molecule is destroyed in the formation 

 of simpler molecules, in which process energy is released. Thus 

 a molecule of carbohydrate possesses potential energy as long 

 as the atoms of carbon, hydrogen, and oxygen maintain a certain 

 structural and spatial relation to each other in the molecule. 

 When this relation is disturbed by an oxidation process, the 

 structure collapses, energy is liberated in the form of heat, and 

 the carbon, hydrogen and oxygen emerge from the reaction as 

 carbon dioxide and water. 



The energy value of foods is measured in calories. A small 

 calorie or gram-calorie is the amount of heat necessary to raise 

 the temperature of 1 gm. of water 1°C. By means of the 

 calorimeter it has been determined that, on the average, 1 gm. 

 of fat yields 9,300 gram-calories, 1 gm. of carbohydrate 4,100 

 gram-calories, and 1 gm. of protein 5,778 gram-calories. Except 

 for protein these values represent the actual energy values of 

 these substances to the body. Owing to the fact that protein 

 is not completely oxidized in the body (1 gm. of protein yielding 

 3^ gm. of urea), to obtain the calorific value of 1 gm. of protein, 

 it is necessary to subtract the energy value of ]^ gm. of urea, 

 which is 841 calories, from 5,778 calories, leaving 4,937 calories. 

 Very likely this value is too high since all of the nitrogen of 

 protein metabolism is not eliminated as urea. If allowance is 

 made for nitrogen losses in forms other than urea, the actual 

 calorific value of 1 gm. of protein is reduced to 4,100 gram-calories. 



The energy requirements of a man can be determined by 

 means of a respiration calorimeter. In its most complete and 

 elaborate form the respiration calorimeter is a small room in 

 which an individual may remain for long periods in comfort, and 

 so arranged that the amount of oxygen consumed from the air 



