62 INTRODUCTION TO GENERAL PHYSIOLOGY 



The Supply of Energy 



Up to the present we have chiefly considered the thinj 

 necessary to make new structures or to replace those worn away. 

 Only incidentally have questions concerning energy been touched 

 upon. 



It was stated above that, in the adult, there is very little loss in 

 the wear and tear of the cell machinery in its normal work. There 

 is some loss by destruction of cells on the surface of the body and 

 elsewhere. 



The fact that the machinery does not wear away in its normal 

 function has been shown most clearly in the case of muscular 

 work. Since the cell structures contain nitrogen, if there were 

 disappearance of their material there would be found an increased 

 amount of nitrogenous compounds in the urine, since this is the 

 way in which they are got rid of. The most careful investigation 

 has shown that no increase is to be detected. 



The necessity of taking more food than that indicated by such 

 considerations as those above is a matter of general experience, so 

 that there is another purpose which actually requires the provision 

 of something other than the material itself. As already pointed 

 out, this is energy, which has been lost in the performance of work, 

 and must be replaced. 



The food we take is, chemically, of such a kind that by oxida- 

 tion, or burning with oxygen, energy is given off. If we burn 

 sugar or fat in the air (E., p. 189), we notice that heat is produced, 

 and if we collect the gas given off, we find that carbon dioxide and 

 water have been formed. We saw previously that these are also 

 produced by the living organism when it makes use of the same 

 substances. The chemical energy which appears as heat when the 

 substance is burned in air is therefore available for the needs of the 

 organism when similarly burned in its cells. But the mechanism 

 of the living cell is so arranged that it can seize upon this energy 

 before it has become degraded to heat, and so make more 

 economical use of it. 



In the preceding chapter it was shown that any form of energy 

 can be converted into heat, and therefore measured in terms of heat 

 units. This is a matter of some convenience in respect of materials 

 used for food. We can burn them with oxygen and measure the 

 heat produced, thus obtaining their value as sources of energy, on 

 the assumption, of course, that they are such as the body is capable 

 of using in this way. Charcoal or coal gas are useless, because the 

 living cell does not possess the means of burning them. In the 

 case of carbohydrates and fats, the values are those actually 

 obtained by the organism, since these are completely oxidised by 



