104 BIOLOGY AND HUMAN LIFE 



relation to the protoplasm. We must therefore guard against 

 thinking of food as related especially to the tissues or organs. 

 There is no such thing as brain food or muscle food ; all food is 

 protoplasm food or it is not food at all. 



We may summarize the material requirements of a living 

 body in this way: 



1. Water. The chemical changes that go on in living pro- 

 toplasm can take place only in the presence of water. In the 

 larger organisms water is also the chief medium for the 

 transportation of materials within the body. 



2 . Oxygen. Although this is not usually regarded as part of the 

 food, it is an essential part of the income of every cell. It is the 

 chemical union of oxygen with other substances (organic) that 

 sets free the energy by which the protoplasm does all its work. 



3. Protein. Building material for the construction of new 

 , protoplasm or for the replacement of protoplasm that has 



been destroyed (oxidized) in the course of the activities of the 

 protoplasm is supplied only by the amino acids of proteins. 



4. Fuel foods. In addition to the proteins that are oxi- 

 dized there is usually some other organic material that is 

 oxidized. Two classes of compounds commonly furnish this 

 fuel: namely, {a) carbohydrates and (b) fats. 



5. Salts. Various mineral or inorganic compounds serve in 

 the protoplasm as activators or regulators. 



6. Vitamins. In some of the higher animals an essential 

 part of the income consists of organic substances that influ- 

 ence growth and nutrition. Three groups have been recog- 

 nized, which are called for the present A (fat soluble), B (water 

 soluble), and C (water soluble). 



THE MEANING OF FOOD 



A. Organic substances 



1. Building material for protoplasm (for growth) 



Proteins (consisting of amino acids) 



2. Energy or fuel material (for movement and other processes) 



Carbohydrates Fats 



Starches ; sugars 



