FOOD DIGESTION AND RESPIRATION 41 



Nevertheless, as every one knows, a fairly large amount of 

 food is necessary to the adult, especially if he is doing hard work. 

 The last remark indicates the purpose of this food. In fact, by 

 far the greater part of the food taken, both by the adult and the 

 growing organism, is for the purpose suggested, not for growth 

 or maintenance. To do things, to cause changes, requires energy ; 

 and, when energy has been used, it must be replaced if more 

 work is to be done. This is the chief function of food. 



We will discuss the two uses of food in turn, taking first that 

 for growth and maintenance. 



Since the object of this is to make new substance or to replace 

 what has been lost, it is clearly necessary to know what is the 

 chemical composition of protoplasm, and of the various structures 

 made by it. We may, indeed, to begin with, take the general 

 composition of the organism as a whole. We find that it is 

 composed of organic and inorganic substances (E., p. 181). The 

 latter are not present in very large amount, but are of some 

 variety, and of great importance. Organic compounds, as the 

 student will not need to be reminded, are the compounds of carbon. 

 Of these there are an enormous number known, and a very large 

 number are produced by living beings. The other chemical 

 elements making up these latter carbon compounds are nitrogen, 

 hydrogen, oxygen, and to a less extent, sulphur and phosphorus. 

 Iron and magnesium are found in two very special compounds, as 

 we shall see presently. 



The four elements, oxygen, hydrogen, sulphur, and phosphorus, 

 are obtained in the course of taking as food those substances which 

 are necessary as sources of carbon and nitrogen, since animals 

 cannot build up their structure from elementary carbon and 

 nitrogen. Plants cannot utilise the former, but a few exceptional 



micro-organisms can take nitrogen from the atmosphere and form 

 compounds useful for food to higher organisms. 



Source of Carbon the Sugars 



While there is a large variety of organic compounds which 

 serve the animal for this purpose, it has been found that none 

 simpler than the sugar called glucose is of use. Green plants, on 

 the other hand, are able, by making use of the sun's energy, to 

 produce glucose for themselves from the carbon dioxide of the 

 atmosphere. Our study of energetics has shown us that, since 

 carbon dioxide cannot be further burned up, it must be converted 

 into a compound that can be so oxidised, if it is to serve as a 

 source of energy. It must have energy supplied to it for this 



