I04 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



formation of complexes between the ions and protoplasmic constituents ('ion 

 binding compounds' or 'carriers'). Osterhout (37) suggested in 1935 that electro- 

 lytes enter by combining reversibly with a constituent HX of the protoplasm. 

 In 1937 van den Honert (43), studying the absorption of phosphate by sugar 

 cane, concluded "that the phosphate is adsorbed by the surface layer of the 

 protoplasm of the root cells and subsequently carried in by a mechanism re- 

 sembling a constantly rotating belt conveyer, which removes its charge from 

 the surface, depositing it inside and returning empty to be charged again." 



In 1942 Wohl and James (46) presented a scheme of the operation of carriers 

 based on theoretical considerations. Like van den Honert's simile of the con- 

 veyer belt, their model stressed the breakdown of the ion-carrier complex as 

 much as its initial formation. Jacobson and Overstreet (26) in 1947 and in 

 subsequent papers (27, 38) used the concept of ion binding compounds in their 

 studies on ion absorption by barley roots. 



Our own approach (16) to kinetics and selectivity in ion absorption is based 

 on the carrier hypothesis. The ion, "SI, is conceived as combining with a carrier 

 R, which resides in a membrane not permeable to free ions. It is the complex, 

 MR, rather than the free ion which actually traverses the diffusion barrier. 

 At the inner surface of the membrane there occurs a change, probably en- 

 zymatic in nature, in the carrier as a result of which the ion is released to the 

 region beyond: 



R + M outside ;^ MR; MR ^=^ R' + :M inside, 

 ko k4 



k being the rate constants of the respective reactions. 



This scheme of transport by means of the formation of a labile intermediate 

 complex between the substance transported and the active agent is treated in 

 terms of the familiar mechanism of enzymatic catalysis involving intermediate 

 compound formation between enzyme, E, and substrate, S, resulting in the 

 formation of product, P: 



ki k;, 



E + S ;==^ ES ; ES ;=^ E -f P 

 ko kj 



If the breakdown of the complex is essentially irreversible (k4 is negligible) and 

 the overall velocity of the reaction is proportional to the concentration of the 

 complex, then the Michaelis-Menten equation (34) should apply: 



V = V (S)/K, + (S), 



where v is the observed velocity at substrate concentration (S), V the maximal 

 velocity attainable when all of the active agent is saturated with substrate and 

 Ks the 'Michaelis constant' representing the substrate concentration at which 

 half the maximal reaction velocity is attained. 



