MECHANISMS OF ION TRANSPORT 41 



a carrier precursor (X^) which is incapable of leaving the membrane, 

 or of reaccepting an ion. X^ is transported back across the membrane 

 and reconverted to X, where at M^ it can combine with another ion. 

 Thus a limited number of carrier molecules are capable of trans- 

 porting an indefinite amount of salt. Efficient active transport 

 requires the presence of a rather impermeable membrane, since 

 otherwise ions transported by a carrier mechanism against a con- 

 centration gradient diffuse in the opposite direction and 

 accumulation is prevented or reduced. 



The functioning of a carrier mechanism for ions can be demon- 

 strated with a simple chemical model (Fig. lib). An aqueous 

 solution (/) containing potassium chloride and potassium hydroxide 

 is separated from water (//) by a layer of guiacol. If carbon dioxide 

 is bubbled through //, potassium ions migrate across the guiacol 

 and eventually may attain a higher concentration in // than in /. 

 The explanation is as follows: potassium hydroxide reacts with 

 guiacol (HG) where the latter is in contact with /, according to the 

 equation : 



KOH + HG-^KG + HaO 



Potassium guiacolate (KG) forms, diffuses through the layer of 

 guiacol along a concentration gradient, and is decomposed at the 

 surface in contact with //, as follows : 



KG + H2CO3 ->KHC03 + HG 



Guiacol is thus reformed, while potassium ions accumulate in // as 

 potassium bicarbonate. In this model, guiacol molecules act as 

 carriers, and transport continues as long as potassium ions remain 

 in solution / and the pH gradient is maintained. 



A similar model can be designed to accumulate anions, e.g. 

 chloride across a layer of a suitable organic base (YOH) as a result 

 of the reactions : 



HC1 + Y0H-^YC1 + H20 on one side 

 and YC1 + K0H->Y0H + KC1 on the other. 



In this case transport occurs from a more acid to a more alkaline 

 solution. It is not possible to combine the two mechanisms in a 

 model which will accumulate both ions of a neutral salt in the same 

 solution because cations and anions move in opposite directions 

 in the presence of a pH gradient (cf. p. 74). 



