MECHANISMS OF ION TRANSPORT 



39 



and to be less easily reversed. When the dye crystal violet is adsorbed 

 by charcoal, the coloured anion is adsorbed and the solution 

 becomes more acid. In these circumstances, the dye can be readily 

 washed out again with dilute acid. If, however, the solution is 

 brought to neutrality, the dye forms a condensation product which 

 involves chemical combination and is then not easily removed. 



Chelation is a well-known mechanism of chemical combination 

 between certain organic substances and bi- or trivalent cations in 

 which the ions are held partly by co-ordinate bonds in undissociated 

 complexes. One of the best-known synthetic chelating agents is 

 ethylene diaminetetra-acetic acid (Fig. 10), and amongst the 



COOH 



HOOC 



COO" "OOC 



Fig. 10. Ethylene diaminetetra-acetic acid 



naturally occurring chelators are citric acid, pyridoxal, nucleic 

 acids and proteins. 



Actively metabolizing cells are unique in their capacity to 

 incorporate ions, especially anions, into organic cell constituents 

 through enzyme reactions. Nitrate, sulphate and phosphate are 

 converted into permanent structural components, and such incor- 

 poration continues as long as the cell grows. In addition, both 

 cations and anions are held temporarily in more labile complexes, 

 e.g. with enzymes and metabolic intermediates. This process is 

 referred to as "labile chemical binding". 



Chemical combination may instigate salt absorption by creating 

 and maintaining a concentration gradient along which ions can 

 diffuse. This mechanism resembles facilitated diffusion in that a 



