CH. v] THE SOURCES OF FOOD 119 



both forms of organisms oxidise this to carbonic 

 acid and water, which are then excreted into the 

 surrounding air or water. In oxidising this carbona- 

 ceous food heat, or some other form of energy, is 

 produced. The carbonaceous food consists of carbon 

 and hydrogen, or of these elements and oxygen as 

 well. All forms of carbohydrate and fat are built up 

 of chains of compact little groups of atoms ; thus 

 grape sugar has the following composition : 



COH . (CH . OH) 4 . CH 2 . OH. 



The proteid is much more complex than any other 

 substance known to us. It is built up from amino- 

 acids, which are chains or rings of little groups of 

 carbon and hydrogen atoms as in the case of the 

 carbohydrates, but in addition to these the amino- 

 acid contains a group of nitrogen and hydrogen 

 atoms the amino group, NH 2 and this is associated 

 in a certain position with another group, the carboxyl 

 one, COOH. Amino-acids are piled on amino-acids 

 to form the giant proteid molecule, and this differs 

 from the other food-stuffs, not only in its greater 

 complexity, but also in the fact that it contains 

 nitrogen. In order to manufacture their proteid 

 food all organisms require nitrogenous food-stuff. 



Two sharply contrasted modes of nutrition have 

 forced themselves on our attention the holophytic 

 and the holozoic. In the former the carbonaceous 



