6 BIOLOGICAL PROBLEMS 



burned up, as we say. This combustion is effected 

 by living organisms, and the results, both in the 

 chemical nature of the products and in the amount of 

 energy obtained, are identical with what would have 

 been the case if the food had been burned under a 

 steam boiler. Without oxygen, therefore, we could 

 obtain no energy. 



All three classes of food fat, carbohydrate, and 

 protein are used for this purpose. Carbohydrate 

 and protein, weight for weight, are equivalent; fat 

 is of higher value in proportion to its weight. The 

 fact that protein contains nitrogen is of no importance 

 in this respect. By far the largest part of our food is 

 needed for the purpose of supplying energy. We 

 should notice that the precise chemical constitution 

 of the food does not matter. The fact is very striking 

 in the case of certain micro-organisms, which are able 

 to burn paraffin, methane, or hydrogen as food for 

 energy purposes. We may compare the structure of 

 living cells to the mechanism of a petrol motor. Work 

 is done by the motor by making use of the energy 

 afforded by the combustion of the fuel with oxygen 

 from the air. The fuel enters in no way into the 

 chemical composition of the motor, and may be either 

 petrol, benzene, alcohol, or other volatile combustible 

 liquid, or even gas. Iron is useless. 



To give an aspect of reality to this energy value 

 of food, samples of various food-stuffs, each of 100 

 calories, have the following weights: 



Casein or egg white (pure proteins) ... 24.5 grin. 



Fat ... 13.5 ,, 



Cane-sugar (carbohydrate) ... ... 24.5 ,, 



Meat 46 ,, 



Bread 37-5 



Oatmeal 28 ,, 



->ii lie ... ... ... ... ... ... 1 43 



Beer ... ... ... ... ... ... 1300 ,, 



