CHAPTER III 



NUTRITION 



FROM the preceding chapter we have learned that respira- 

 tion is a destructive process consisting either in the break- 

 ing up of complex compounds into simpler ones, or in the 

 physiological oxidation of various combustible substances. 

 With the important exception of the nitrogen, sulphur, and 

 iron bacteria, which derive their energy from simple com- 

 pounds, all living organisms depend for their chief supply of 

 energy upon complex compounds existing in nature only as 

 the result of the constructive activities of living organisms. 

 In the last chapter we assumed the presence of these com- 

 plex compounds, examining only the means of deriving 

 energy from them. 



Energy is needed for construction, to do work. Only so 

 much energy can be liberated by complete combustion or 

 complete decomposition as was employed in construction. 

 Theoretically just as much energy should be liberated in the 

 combustion of a starch grain as was needed to make it, but 

 the cell is not a perfect machine; not all the energy or 

 power used goes into the finished product, some is expended 

 in overcoming the internal resistance of the machine, some 

 is radiated, or "lost" in other ways. There is waste of 

 energy and of material in every machine, the product does 

 not represent the total expenditure of material and energy. 

 As it costs a certain amount of energy to keep an engine 

 going without its doing any other work, and a larger 

 amount to make it do work as well as go, so it costs a 

 certain amount of energy to keep an organism alive and 

 more to make it do anything. The sum of the energy ex- 

 pended in making the engine go, plus the amount expended 

 in making it do work, equals the amount of energy which 

 must be developed to run it, if there is no loss by radia- 



