32 MINERAL SALTS ABSORPTION IN PLANTS 



cally mass flow is possible at absolute zero because the energy 

 involved comes from outside the system (cf. diffusion). 



In spite of its name, electro-osmosis is an example of mass flow 

 (Fig. 6b). When one aqueous solution is separated from another 

 by a membrane possessing charged pores, and a potential difference 

 exists between the two solutions, mass flow of water and dissolved 

 substances occurs. The water molecules are thought to acquire an 

 electrical charge, opposite to that possessed by the pore walls, and 

 then to move by electrical attraction towards the side of the 

 membrane having the same charge as the walls. In so doing they 

 carry along molecules of dissolved solutes. 



D. Ion Exchange 



If a solution of a dissociated salt (M"^A~) is separated from 

 distilled water by a membrane which is permeable to both ions, 

 diffusion will occur until the concentrations of salt on the two sides 

 are equal (Fig. 7a). Should the membrane be impermeable to either 

 cations or anions, no diffusion of solute will occur (Fig. 7b). Now, 

 if two solutions containing different salts (M|A7 and Mj A2) are 

 separated from one another by either a cation-permeable/anion- 

 impermeable or cation-impermeable/anion-permeable membrane, 

 one of the two ion species in each case is free to move across in 

 exchange for an ion of hke charge (Fig, 7c, d). Equihbrium is 

 estabhshed when the ratio [M|]/[M 2] (in the first case) or [A7]/[A2] 

 (in the second case) is equal on the two sides. At equilibrium, 

 exchange does not stop but equal relative amounts of the two 

 cations or anions move in each direction, so that the ratio remain 

 constant. The total concentration of salts on either side is not 

 affected by exchange, even when a concentration gradient exists, 

 because movement of one ion species, and therefore diffusion of 

 salt, is prevented. Exchange involves equivalent electrical charges 

 so that two univalent ions exchange for one bivalent, three for one 

 trivalent ion, and so on. 



When a solution of salt containing a radioactive isotope is 

 separated by an ion exchange membrane from a non-radioactive 

 solution of the same salt, it is possible to calculate the rates of 

 exchange of ions between the two solutions from measurements of 

 the changes in radioactivity with time, on either side (Ussing, 



