CHAPTER II. 



THE INFLUENCE OF PHYSICO-CHEMICAL LAWS ON 

 PHYSIOLOGICAL PROCESSES: ENZYMES. 



Having learned of what materials the cell is composed, we may 

 proceed to enquire into the chemical and physical reactions by 

 which it performs its functions. The cell, either of plants or 

 of animals, may be considered as a chemical laboratory, in which 

 definite reactions are constantly going on, being guided, as to 

 their direction and scope, by the physical conditions under which 

 they occur. A study of the material outcome of these reactions 

 constitutes the study of metabolism, to which special chapters 

 are devoted further on. At present, however, we must briefly 

 examine the physico-chemical conditions existing in the cell 

 which may give the directive influence to the reactions. Why 

 should certain cells, like those which line the intestine, absorb 

 digested food and pass it on to the blood, whilst others, like those 

 of the kidney, pick up the effete products from the blood and 

 excrete them into the urine? We must ascertain whether these 

 are processes depending on purely physico-chemical causes, or 

 whether they are a function of the living protoplasm itself, a 

 vital action, as we may call it. In general it may be said that 

 the aim of most investigations of the activities of cells is to find 

 a physico-chemical explanation for them, and it is one of the 

 achievements of modern physiology that some should have been 

 thus explainable. A large number, however, do not permit of 

 such an explanatic-n, and this has induced certain investigators 

 to believe that there are some animal functions which are strictly 

 vital and can never be explained on a physical basis. The ' ' phys- 

 ical" and the "vital schools" of physiologists are therefore 

 always with us. 



From the standpoint of physical chemistry, the cell may be 

 considered as a collection of two classes of chemical substances, 



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