PHYSIOLOGY OF THE CELL 27 



are disrupted and oxidized the potential energy is transformed into kinetic 

 energy which manifests itself for the most part as heat. To the sum total 

 of all the chemic changes occurring in tissues and foods the term metabolism 

 is given. 



There is, however, much difference of opinion as to the extent to which 

 the living material is metabolized and to the actual disposition of the food 

 materials, and especially the proteins, or their cleavage products, the amino- 

 acids. Thus Voit contended that the tissue molecules are comparatively 

 stable in composition and under ordinary conditions of nutrition do not 

 undergo any material change during either rest or activity, and that metabo- 

 lism is confined to the food materials occupying spaces in and around 

 the living cell. The cause which initiates this metabolism is unknown, 

 but is supposed to reside in the cell, if it is not a property of the cell itself. 

 Because of the fact that but a very small amount of sugar or fat enters into 

 the composition of bioplasm it is generally admitted that these foods are 

 metabolized in the tissue spaces and in the manner just alluded to. The 

 problem, however, is different in the case of the proteins. Voit contended, 

 as previously stated, that the proteins of the tissue molecules, which he 

 distinguished as tissue proteins, do not metabolize to any appreciable extent 

 and confined practically all protein metabolism to the food proteins, now 

 distinguished as amino-acids, circulating in the tissue spaces and which he 

 characterized as circulating proteins. Even in starvation the tissue pro- 

 teins, as such, do not metabolize until they have been disintegrated in con- 

 sequence of chemic changes and transformed into circulating proteins. 



Pfliiger, however, asserted that the circulating proteins cannot be metabo- 

 lized as such but that they must first be built up into tissue proteins. The 

 metabolism of protein is, therefore, confined, in this view, to the molecules of 

 the living material. It is possible, however, that both views are correct and 

 that in the physiologic condition the activity of the tissues is attended by a 

 partial destruction of the protein molecules which is followed in turn by con- 

 struction out of amino-acids during the subsequent rest, but that the greater 

 part of the protein metabolism takes place outside the cell, though in 

 contact with it. 



Though the cell is, therefore, the seat of two opposing processes, assimila- 

 tion and dissimilation, it retains under normal conditions an average physio- 

 logic state, and so long as this is the case it is in a condition of nutritive 

 equilibrium and capable of performing its various functions. 



Though the foregoing statements are applied to the individual cell they 

 are equally applicable to the body as a whole, inasmuch as the organs and 

 tissues of which it consists are composed of cells. The body grows in size 

 and maintains, its nutrition, by the introduction of food materials which are 

 utilized in part, for the repair of the tissues which have undergone molecular 

 disintegration in consequence of activity, and in part for the liberation of 

 energy. As a result of the disintegration or the metabolism of tissue and 

 food materials, products such as carbon dioxid, urea, etc., are formed which, 

 apparently of no further use, are discharged from the body by eliminating 

 organs as the kidney, lungs, skin, etc. Assimilation and dissimilation are 

 constantly taking place. If the food assimilated and metabolized exactly 

 replaces the tissues dissimilated and the food metabolized the body will retain 

 a condition of nutritive equilibrium. 



