CHAPTER V 



CONVERSION OF CHEMICAL ENERGY INTO 

 MECHANICAL ACTIVITY 



PART of the energy that is obtained by oxidation of 

 the organic materials described in Chapter I is trans- 

 formed by the cells into various biological processes. 

 The function of the cell is to act as an energy transformer.* 



Many of these transformations cannot yet be given con- 

 cretely enough for discussion in this book but the simpler 

 transformations of chemical energy into mechanical action 

 can be briefly described. 



There are two ways in which mechanical action can be 

 produced. The first is by change in osmotic pressure trans- 

 ferring water from one place to another. The transference 

 of water causes a change in pressure (or volume) with the 

 result that some form of movement takes place. The second 

 method is by a change of surface tension at surfaces of separ- 

 ation so that the surface is pulled on with greater or less force 

 which in turn may produce movement. 



Cells are distinctly demarcated from their surroundings, 

 hence one can apply the laws described in Chapters II and III 

 to the osmotic movement of water between the cell and its 

 surrounding medium or to the changes in surface tension 

 between the same two structures. 



PHYSICO-CHEMICAL RELATIONS OF CELLS TO THEIR 

 SURROUNDINGS 



It is true that the cells themselves and sometimes their 

 surrounding media are heterogeneous systems, but for pur- 

 poses of description we can simplify the subject by treating 

 them as separate phases, that is we can consider the reactions 

 between the cells and their surroundings as the interchange 

 between two separate fluids. 

 The points to be considered are four in number : 

 i. What are the chemical differences between cells and their 

 surroundings ? 



* B. Moore in Recent Advances in Physiology, Edited by L. Hill. 

 Edward Arnold, 1906, pp. 1-41. 



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