SALT ABSORPTION AND METABOLISM 75 



the vacuoles, but there is no evidence at present that this occurs. 

 Briggs (1932) suggested that cations and anions might be accumu- 

 lated separately in distinct phases of the cell — for example, cations 

 in the cytoplasm and anions in vacuoles. While it is true that an 

 excess of mobile cations over mobile anions is maintained in cyto- 

 plasm by the establishment of Donnan equilibria, the fact that both 

 anions and cations are accumulated in vacuoles renders this 

 hypothesis inadequate to account for one of the most characteristic 

 features of salt absorption in plants. 



Continuous uptake by adsorption can be linked with metabolism 

 either through synthesis of new adsorption sites during growth, or 

 by vacation of occupied sites as a result of metabolic utilization of 

 ions. The clearest examples of labile, metabolism-dependent, 

 binding of cations are derived from work with micro-organisms. 

 Pulver and Verzar (1940) observed that when yeast cells were 

 placed in a potassium-free medium, about one-third of the potassium 

 diffused out rapidly. Upon subsequent incubation with glucose, the 

 ions were reabsorbed during a brief period in which glucose was 

 taken up (cf. Fig. 21a, p. 64). Cowie et al. (1949) suggested that in 

 Escherichia coli intermediates of sugar breakdown, perhaps glucose- 

 6-phosphate and fructose-6-phosphate, may be involved as binding 

 substances, 



B. Active Transport 



1. Acid-base Carriers 



None of the above mechanisms is adequate to explain the rapid 

 transport of ions from one side to another of a membrane with a 

 high resistance to passive penetration. Where such membranes are 

 present carrier mechanisms are usually invoked (see Chapter 3, pp. 

 40-41). In general, metabolism might be involved in: 



(a) the synthesis of carrier molecules, 



ib) complex formation between the ion and its carrier, 



(c) transport of the ion-carrier complex, 



{d) breakdown of the complex, or 



{e) the movement of the unloaded carrier back to the outer 

 surface of the membrane. 



One of the first precise mechanisms proposed for active transport 



