124 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



chemical reactions which might be coupled to the transport of K and Na across 

 cell membranes. 



THEORETICAL SECTION 



Classification of Transport Processes 



Due to our ignorance of the physical chemistry of cell membranes, an un- 

 equivocal classification of transport processes across such structures is not, at 

 present, possible. The most frequent general terms which have been applied to 

 cellular transport processes are the words 'passive' and 'active'. This classifi- 

 cation is essentially energetic, the word passive connoting a process driven by 

 an electro-chemical potential gradient of the transported molecules themselves, 

 and active suggesting transport requiring free energy from reactions occurring 

 in the cell. However, the words are also sometimes used to imply the molecular 

 mechanism of transport, thus the term 'active transport' may be used to indi- 

 cate that chemical reactions are involved. This ambiguity may lead to serious 

 confusion. Thus, transport in the direction of decreasing electro-chemical 

 potential may be termed 'passive' energetically but still involve chemical re- 

 actions with components of the cell surface and thus be mechanistically 'ac- 

 tive'. 



Even when the words active and passive are restricted to their energetic 

 meaning, ambiguities are unavoidable. If some chemical reaction contributes to 

 the driving force for transport of one ion across a membrane, in principle, the 

 transport of all other ions will be affected according to the reciprocal relations 

 of irreversible thermodynamics (14, 40, 68). For example, consider a membrane 

 which generates an electrical potential difference between identical outside 

 solutions by coupling of reactions in the membrane with the transport of one 

 ionic species. Then, the driving force for transport of all ions across the mem- 

 brane depends, in part, on free energy made available from the reactions 

 occurring within the membrane. Thus, no ion can be said to traverse such a 

 membrane 'passively', if by that word one means not involving free energy 

 made available by membrane reactions. It is clear that any writer using the 

 words active and passive in the description of transport processes must opera- 

 tionally define the terms in context if he is to avoid confusing the reader. Some 

 authors have accomplished this successfully (43, 113), although amplifications 

 of the definitions have become necessary (114). 



Somewhat arbitrarily, the use of the words active and passive will be avoided 

 entirely. Instead, transport processes will be loosely classified into two large 

 groups according to the molecular mechanism involved. Defects are also pres- 

 ent in this approach. They mainly derive from the fact that it is not possible 

 to define specific molecular mechanisms of transport across biological mem- 

 branes in terms of definite experimental operations at the present time. Despite 

 this difficulty, a mechanistic classification will perhaps provide greater clarity 



