CHAPTER VII 

 METABOLISM 



The term mctaholism, whether used of animals or of plants, 

 refers to the entire series of chemical changes and processes 

 involved in the activity of the living organism. It may be 

 divided roughly into constructive and destructive metabolism. 

 The former process begins, in plants, with the production of 

 simple carbohydrates by photosynthesis, and is concerned with 

 the construction therefrom of the more complex plant foods and 

 with their storage, their digestion, their assimilation into the 

 living protoplasm, and the growth of new tissues which they make 

 possible. The latter process involves a breaking down of the 

 living substance thus built up, with the consequent production 

 of waste materials and a liberation of the energy which is neces- 

 sary for the activity of the organism. 



Plant Foods. — We have already discussed the production of 

 glucose by photosynthesis. Glucose is the basic plant food from 

 which are ultimately derived all others — the more complex carbo- 

 hydrates, the fats, and the proteins — which support the life of 

 animals and plants. 



Before we inquire into the characteristics of these various 

 food types and their mode of origin, however, we should consider 

 just what is implied by the term food itself. A broad definition 

 would make "food" include everything taken into the body of 

 the organism which is essential to its life and continued activity. 

 Water, carbon dioxide, and the various essential mineral salts 

 would thus be considered as the food of plants, and indeed it is 

 the last of these which in ordinary speech are most commonly 

 referred to as "plant foods." From such a conception of food 

 as this, however, has arisen a fallacious distinction sometimes 

 drawn between animals and plants, namely that the former 

 require organic food and the latter only inorganic. A more 

 strict and perhaps from our point of view a more useful employ- 

 ment of the term "food" limits its application to anything which 

 supplies either of the two fundamental needs of the organism — 



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