CHAPTER 3 



MECHANISMS OF ION TRANSPORT 



The reasoning of one who desires the possible ultimate 



reference of all that takes place in the organism to 



simple chemical and physical causes is as devoid of 



true logic as is that of the peasant who, on seeing a 



locomotive for the first time argues by analogy that a 



horse must be concealed inside. 



W. Pfeffer. 



The Physiology of Plants (1900). 



A. Definitions 



Some confusion has arisen in the past among students and 

 investigators of saU absorption through differences in terminology. 

 To avoid ambiguity, a number of words will first be defined, and 

 then used only in that sense in the discussion which follows in 

 subsequent chapters. Uptake, intake and absorption are used 

 synonymously, and do not imply any particular method of entry of 

 salts. When these expressions are used, net movement is implied 

 unless otherwise indicated in the text. Gross uptake is sometimes 

 referred to as influx and the difference between gross and net uptake 

 as efllux (syn. outflux). Efflux, like influx, may occur by purely 

 physical processes such as diffusion and exchange, or depend upon 

 metabolism {active transport). In the former case, it is called leakage 

 and in the latter, excretion or extrusion. Active transport into 

 vacuoles, or into the cavities of non-living cells in the stele of roots, 

 is sometimes called secretion by analogy with similar processes in 

 animals. 



Accumulation implies movement of ions against a concentration 

 gradient, generally as a result of active transport. It can also occur 

 passively, e.g. by the establishment of Donnan equilibria or by 

 adsorption. When ions are absorbed and at once incorporated 

 irreversibly into organic constituents of a cell, an element may be 

 accumulating in the organism, but the process is not referred to as 

 salt accumulation. 



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