74 



MINERAL SALTS ABSORPTION IN PLANTS 



exchange may be linked with metaboHsm. Brooks (1929) suggested 

 that cations and anions might be absorbed continuously in exchange 

 for ionic products of metabolism. If there is, for instance, a con- 

 tinual production of hydrogen ions inside a cell so that their 

 concentration on one side of a cation-permeable/anion-permeable 

 membrane is maintained at a higher level than on the other side, 

 cations, e.g. potassium ions, can be absorbed against the existing 

 concentration gradient (Fig. 26a). A steady state is reached when 

 the rate of diffusion of hydrogen and potassium ions across the 

 membrane in opposite directions is equal to the rate of production 

 of hydrogen ions in the cell. A similar system can be visualized for 



[HCO,] 



HCO3 



[Hco;] 



[cr] 



(0) (b) 



Fig. 26. Absorption by ionic exchange 



a. Membrane, cation permeable, anion impermeable, b. Membrane, anion 



permeable, cation impermeable. For explanation, see text; 



the accumulation of anions, involving an anion permeable cation- 

 permeable membrane, and metabolically produced anions, such as 

 bicarbonate (Fig. 26b). Brooks supposed that if the surface 

 membrane has a mosaic structure with cation and anion permeable 

 areas, the two systems may be combined, and by simultaneous 

 production of hydrogen and bicarbonate ions, accumulation of a 

 neutral salt be achieved. Sollner (1932) pointed out that such a 

 mechanism will not function unless the cation and anion permeable 

 regions are electrically insulated from one another. More recently, 

 he devised a model system in which this condition is met (Sollner, 

 1955), and simultaneous accumulation of anions and cations by 

 exchange was demonstrated. Conceivably anions and cations might 

 be accumulated initially into separate electrically insulated compart- 

 ments in the cytoplasm of plant cells, and subsequently mixed in 



