76 MINERAL SALTS ABSORPTION IN PLANTS 



of cations was that suggested by Osterhout (1936) from observations 

 with Valonia. He proposed that an acidic substance (HX) located in 

 the outer protoplasmic membrane combines with an entering base 

 according to the equation 



K0H + HX-^KX + H20 



The neutral, undissociated complex (KX) diffuses across the proto- 

 plasm and is decomposed at the inner surface where the sap is more 

 acid than the external medium (cf. Fig. lib, p. 40). The necessary 

 pH gradient might be maintained by production of carbon dioxide 

 in respiration. The failure of Valonia to absorb sodium as rapidly 

 as potassium by this mechanism was attributed to the lower mobility 

 of the sodium ion-carrier complex in the membrane. 



Various objections can be raised to Osterhout's hypothesis: 



{a) It is difficult to adapt the mechanism satisfactorily to account 

 for the simultaneous accumulation of anions and cations in 

 vacuoles. 



{b) Excised roots can accumulate cations when the medium is 

 more acid than the cell sap (see p. 55); 



(c) absorption seems to be more closely related to oxygen 

 absorption than carbon dioxide production, since absorption 

 stops in higher plant cells under anaerobic conditions, 

 whereas carbon dioxide production does not; 



{d) absorption sometimes occurs more efficiently than is expected 

 on the basis of the 1 : 1 exchange of cations and carbon 

 dioxide postulated (see p. 80). 



Jacobson and Overstreet (1947) enunciated a general mechanism 

 for active transport involving acidic and basic carrier molecules for 

 cations and anions respectively, which avoids some of the objections 

 to Osterhout's hypothesis. They suggested that ions combine with 

 unspecified carrier molecules (HX, YOH) in the cell membrane, 

 according to the equations: 



M+ + HX^MX + H+ 

 A- + YOH^YA + OH- 



These reversible reactions are thought to be involved both in 

 the uptake of ions on one side of the membrane and their release on 

 the other. The carriers are presumed to be synthesized metabolically 



