

PREVIEW 325 



food, water, and oxygen from its environment. 1 Both the non- 

 living and the living machine do the same thing with fuel or food. 

 They oxidize it and make use of the energy thus released. They 

 both receive heat as a direct result of this oxidization or burning. 

 In addition, however, the living organism may use food to repair 

 parts that have broken down or even build new parts. Thus 

 food may be denned as something that can be used by the 

 body of a plant or animal to release energy, or to form material 

 for the growth or repair of that body. However, we must not 

 think of our body as taking the foods and burning them directly, 

 thus providing us with heat and energy to do work. It is a much 

 more complicated process than this might sound. Our digestive 

 organs first have to break down the food materials into such forms 

 that they can be taken into the blood and carried to all parts of 

 the body. The millions of cells of which the body is composed 

 must be given more material which will form new living matter. 

 These cells must also be provided with food material which is 

 oxidized to release energy when muscle cells move, or gland cells 

 secrete, or brain cells work. 



Experiments have proved that an animal may be provided with 

 what seems to be the foods necessary to burn and make tissue, 

 and yet it will die. Professor Mendel of Yale and his students 

 have shown that unless animals receive proteins that contain 

 certain so-called amino acids they will die, although their diet 

 is apparently sufficient in quantity and quality. It has been 

 found that in certain proteins there are these amino acids which 

 are used by the animal to build up its tissues. So important are 

 these substances that the Germans have called them " building 

 stones," for without them no growth can take place. Animal 

 proteins appear to have more of these amino acids than do plants. 

 Hence we see the value of a mixed diet which includes both plant 

 and animal foods. Milk appears to have both the necessary 

 amino acids and certain other substances called vitamins of which 

 we shall hear more later. Certain mineral salts, as calcium, iron, 

 sodium, and potassium, are also needed by the body. 



1 Animals and some plants get organic food from their environment ; but green 

 plants make organic food from materials which they get from their environment. 

 h. bio — 22 



