40 



MINERAL SALTS ABSORPTION IN PLANTS 



high degree of specificity may be exhibited through the chemical 

 reactions which control binding, but it differs in being directly 

 dependent on metabolic energy (cf. active transport). 



H. Active Transport 



When ions are moved across a membrane by a mechanism which 

 is directly dependent on metabolic energy, they are said to be 

 transported "actively". Most of the hypotheses advanced to explain 



External m 

 solution 



Mj Internal 

 solution 



•CO, 



GUIACOL 



(a) 



(b) 



Fig. 1 1 . The carrier concept 



a. General concept of a carrier mechanism. For explanation, see text; 



b. Diagrammatic representation of a model carrier mechanism (Osterhout 



and Stanley, 1932). For explanation, see text. 



active transport involve reversible binding of ions to a constituent 

 of the membrane which acts as a "carrier". Although it is very 

 likely that something of this kind happens, the operation of carrier 

 systems has not yet been demonstrated unequivocally. 



According to the "carrier" concept (Fig. 11), an ion reacts with 

 its carrier (X) at, or near, the outer surface (MJ of the membrane. 

 This reaction might involve adsorption, exchange adsorption, or 

 some kind of chemical combination. Neither the carrier nor the 

 ion-carrier complex {IX) can move into the medium, but IX is 

 mobile in the membrane, and moves to the other side (M2). Here it 

 breaks down, releasing the ion into the internal solution and forming 



