MECHANISMS OF ION TRANSPORT 45 



molecule, capable of transporting both cations and anions simul- 

 taneously (Fig. 13f). Measurements of the passive permeability of 

 membranes and the potential differences existing across them may 

 help to distinguish between the possibilities listed, but there is not 

 yet general agreement about the situation in plants. In yeast, and 

 bacteria, cation movements seem to involve exchange and to be 

 largely unrelated to transport of anions (mechanism a), while in 

 roots, mechanism d has received strong support from Lundegardh 

 (1954). 



Even though the whole concept of carrier systems is still in 

 doubt, there has been no lack of speculation about the nature of 

 carrier molecules. Enzymes ("permeases" or "translocases"), 

 structural proteins, peptides, amino acids, ribonucleic acid, phos- 

 phatides (e.g. lecithin), pyridoxal, cytochromes and sugar phosphates 

 have been suggested as possible carriers. The manner in which some 

 of them might function in the transport of salts is discussed below 

 (Chapters 5 and 6). 



The relationship between external concentration and active 

 transport involving a carrier, resembles that between concentration 

 and adsorption, and can similarly be represented by the Freundlich 

 and Langmuir equations. Active transport may be treated kinetically 

 using a modification of the Langmuir equation which was appHed 

 by Lineweaver and Burk (1934) to enzyme kinetics. If an ion 

 combines with a carrier to produce an intermediate complex which 

 subsequently breaks down, the reactions can be represented thus: 



ki kg 



I+X^ XI ^X' + I 



where /, X, X^ and XI are the ion, ion carrier, carrier precursor, 

 and ion-carrier complex respectively (cf. Fig. 11a), and k^-^ are the 

 rate constants of the reactions, as indicated. In such a sequence, 

 the rate of transport (v) is related to the concentration of the ion 

 [/] as follows: 



v = (K[7])/(/^, + [/]) 



where V is the maximum rate of transport when the carrier system 

 is fully saturated. K^ is a constant, corresponding to the Michaelis 



