CHAPTER 5 



SALT ABSORPTION AND 

 METABOLISM 



In all cases osmotic exchanges are correlated and 

 regulated by the vital activity of the organism. Both 

 the formation of the plasmatic membranes as well as 

 any modification of their diosmotic properties which 

 may later arise are the work of the living organism, 

 which also initiates those intra- and extracellular 

 changes to which the continuance of exosmosis and 

 endosmosis is to be ascribed. 



W. Pfeffer. 

 The Physiology of Plants (1900). 



A. Mechanism Linking Passive Processes to Metabolism 



Although it was known from the beginning of the nineteenth 

 century that plants discriminate between ions presented to them, the 

 imphcation of this fact in relation to the mechanism of absorption 

 escaped notice for many years. It was not until investigations of 

 the effects of such factors as temperature and oxygen pressure on 

 salt uptake were made during the present century that a dependence 

 of absorption on metabolic energy became widely accepted. Sub- 

 sequently, analyses of the uncontaminated sap from coenocytes 

 and giant algal cells showed that salts are accumulated to con- 

 centrations higher than those which occur in the medium — a process 

 which clearly requires expenditure of energy. 



Following the recognition that salt absorption and metabolism 

 are closely connected, various attempts were made to link respiration 

 to physical mechanisms of ion transport in ways which could 

 facilitate continuous absorption. If, for example, salts are utilized 

 or osmotically inactivated within cells, absorption can continue 

 indefinitely by diffusion through permeable membranes, along 

 metabolically maintained concentration gradients. 



Several schemes have been proposed through which ionic 



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