CHAPTER XII 

 WHAT FOOD IS 



74. The material needs of protoplasm. Several classes of 

 materials seem to be necessary to keep protoplasm working. 

 Water, for example, which we have seen to be necessary for 

 the sprouting of seeds as well as for the absorption of mineral 

 matters by the roots of a plant, is a constant and necessary 

 factor in the activity of live protoplasm. It is thus a necessary 

 part of the income of every plant and every animal. Within 

 the cellsj too, water makes possible the movements of the 

 various substances, and their chemical action and reaction with 

 one another. In the larger plants and animals the transfer of 

 materials between various parts of the organism takes place 

 through liquid media, as blood, sap, bile, milk, and these 

 consist very largely of water. 



Certain minerals seem to be necessary parts of the income of living 

 things. Some of these salts, through their chemical actions, appear to 

 start other chemical processes, and are therefore called activators. 

 Other salts, or elements, appear to modify certain chemical processes 

 (just as the bromide used by the photographer makes the development 

 proceed more slowly), and are called regulators. 



We have already seen that living things generally use oxygen 

 in the course of their activities. 



75. What food is. In addition to the water, oxygen, and 

 various mineral salts, every living thing uses various substances 

 as material out of which protoplasm is constructed by the 

 process of assimilation, and it uses substances that can be 

 oxidized within the cells and thus yield energy. 



Whether everything that an organism takes in from the 

 outside is to be called food or not is altogether a matter of 



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