42 MINERAL SALTS ABSORPTION IN PLANTS 



In order to understand any carrier mechanism which may be 

 operative in plants, it is necessary to know: 



(i) the mechanism of movement of the ion-carrier complex ; 



(ii) the nature of the substances involved, and 



(iii) the reactions they undergo. 



One suggestion regarding (i) is that carrier and ion may form a 

 lipid soluble complex which can diffuse across a lipoprotein 

 membrane along a concentration gradient (Fig. 12a). Another 

 possibility is that the carrier or part of it is capable of rotating in 

 some way in the membrane, and thus of transferring ions bound at 

 one surface across to the other (Fig. 12b i, ii). Some device would 

 be necessary to ensure that the binding site returns to the outer 

 surface unloaded to receive another ion. Alternatively, the carrier 

 might be a strongly surface-active substance which slides along the 

 walls of water-filled pores in the membrane with the polar head to 

 which ions are bound in the water phase, and the lipophilic tail 

 associated with Hpid materials in the membrane (Fig. 12c). Phos- 

 phatides appear to be particularly well suited for such a purpose, 

 and they are known to be constituents of biological membranes. 



As an alternative to the passive propulsion of ion-carrier 

 complexes by physical processes, mechanisms more directly 

 dependent on metabolism have been suggested. If, for example, 

 the carrier forms part of a contractile molecule of protein, transport 

 may be accomplished by rhythmic contraction and expansion of the 

 polypeptide chain (Fig. 12d). Again, contractile proteins in a 

 membrane might procure transport of bound ions from one side 

 to another by causing vesiculation (pinocytosis). When the process 

 is completed, the droplet within the membrane may disintegrate to 

 release the bound ions (Fig. 12e). 



In the active transport of salts, the movement of one ion cannot 

 be considered without reference to that of another of opposite sign. 

 One possibility is that each is absorbed independently by an exchange 

 process involving metabolically-produced cations and anions (Fig. 

 13 a, b). Linked absorption of two ionic species need not necessarily 

 involve the active transport of both. If, for example, cations are 

 actively transported across a membrane which is impermeable only 

 to cations, uptake of anions might occur passively along the electrical 

 gradient created by accumulation of the positively charged ions 



