CHAPTER X 



THE LIVING CELL AND ITS FOOD 



Plant and Animal Cell Alike. — At the very beginning of our 

 study of the chemical processes of plants and animals we must 

 emphasize the important fundamental fact that, as living 

 organisms the plant and the animal are alike. Each individual 

 plant and each individual animal is composed of an aggrega- 

 tion of simple cells, and the fundamental chemical processes of 

 these living cells are alike. The life of the complex individual 

 is but the total of the lives of all the cells of which it is composed. 

 Thus, to understand the chemical processes of the individual 

 we must first understand the chemical processes of the living cell. 



We do not wish to give the impression that the whole ques- 

 tion of life is simply one of chemical reaction. We may under- 

 stand the chemical reactions which take place together with the 

 physical conditions under which they occur without being able 

 to explain the phenomenon of life itself. As a great German 

 biochemist has said,^ " Physiology cannot really be identical with 

 the chemistry and physics of living organisms." Furthermore 

 it is only a small part of the reactions involved in living organ- 

 isms which we as yet do understand or which are so clearly demon- 

 strated that they are proper subjects for discussion in this study. 



Composition of the Cell. — Biologically considered the living 

 cell is characterized by containing the substance known as pro- 

 toplasm. This substance is essential to living matter. Chemi- 

 cally its composition is not definitely known, but it has been 

 defined as a solution of protein containing also carbohydrates, 

 fats and inorganic salts, either in solution or colloidal suspen- 

 sion. Thus all living matter contains these three groups of 

 organic compounds, carbohydrates, fats and proteins. 



^ Czapek, "Chemical Phenomena in Life," 1911. 



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