CHAPTER II 

 REACTIONS IN HOMOGENEOUS SYSTEMS 



LIVING cells are not uniform in structure and the 

 different parts are distinguishable from one another. 

 Therefore we must study how chemical reactions 

 proceed in a uniform medium and how surfaces of separation 

 influence these reactions. 



A mass of material of uniform composition is known as a 

 phase and reactions in such a medium are known as reactions 

 in a homogeneous system, but when the material consists of 

 parts with different compositions we have to deal with sur- 

 faces of separation : reactions in a system of more than one 

 phase are known as reactions in a heterogeneous system. 



The organic chemical compounds described in Chapter I 

 are a source of energy which can be set free by oxidation and 

 used in cells for carrying out their various processes. 



Transformation of chemical energy into mechanical action 

 is brought about in two ways. 



The first way is by producing pressure which can be used 

 to produce movement. For biological purposes this process 

 is best understood by studying reactions that occur in a 

 single phase, and any separation to form a closed space may 

 be regarded as an artificial barrier. In the present chapter 

 we will discuss the nature of chemical reactions that take 

 place in a single phase. 



The second way in which chemical energy can be trans- 

 formed into mechanical activity is by ajtering the tension 

 or pull at a surface of separation. This process involves the 

 presence of separate phases and we shall deal with the reactions 

 in heterogeneous systems in Chapter III. 



As living cells are composed of watery solutions we can 

 confine our study of reactions in homogeneous systems to 

 reactions in solutions in water. In order to do so we can 

 make use of the hypothesis of Van't Hoff that substances 

 in solution behave as if they were gases occupying the same 

 volume as the solution.* 



* J. H. Van't Hoff, Zeit. f. physik, Chem., 1887, vol. i, p. 481. 



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