MECHANISMS OF ION TRANSPORT 



35 



according to the rules outlined, but the number of electrical charges 

 carried by each ion must be taken into account. In general, at 

 equilibrium : 



1 1 



where x and y are the valencies of the mobile cation and anion 

 respectively. From this it follows that bi-, tri- and multivalent ions 

 are accumulated in excess of univalent ions in Donnan systems. If 

 for example, potassium and calcium ions are present, each at the 

 same concentration, on the outer side on a membrane enclosing a 

 Donnan system containing indiffusible anions, and accumulating 

 potassium ions to a concentration of 0-OlM, calcium ions are 

 accumulated to a concentration of 0-lM, 



The valency of immobile ions in a Donnan system does not 

 affect the position of equilibrium, since it is the total number of 

 electrical charges, rather than the number of particles which is 

 relevant. For the same reason, a particular Donnan system 

 accumulates cations or anions, but not both. In the presence of 

 immobile anions and cations, the behaviour of the system is 

 determined by the resultant electrical charge. The walls and 

 protoplasts of plant cells carry an overall negative charge at pH 

 values more alkaline than about 4 and therefore behave as Donnan 

 systems containing indiffusible anions. For a further discussion of 

 Donnan equilibria and their estabUshment in biological systems, see 

 Vervelde (1953). 



F. Adsorption 



Solutes tend to accumulate at interfaces if surface tension is 

 thereby lowered, and this process is called "adsorption". Both 

 cations and anions are adsorbed from aqueous solutions on to the 

 surface of many cell constituents, including cellulose and proteins. 



The relationship between amount of solute adsorbed and 

 concentration in the aqueous phase is indicated approximately by 

 the equation: 



