SALT ABSORPTION AND METABOLISM 85 



to another oxidized substance (A) with release of the ion thus 



(i) XM->X+M+ + ie) 

 (ii) U + (e)+i/+->i^//2 



The oxidized carrier X returns across the membrane and becomes 

 reduced to reform the active carrier according to the equation: 



IAH2 + X-^X- + H+ + IA 



Under aerobic conditions, hydrogen ions combine with oxygen and 

 electrons to produce water; other hydrogen acceptors may be 

 implicated under anaerobic conditions. 



It should be noted that in contrast to the Lundegardh hypothesis, 

 the redox pump will transport ions from a more reducing to a less 

 reducing region of the cell membrane. It is therefore an especially 

 appropriate mechanism for transporting cations in the outward 

 direction from cells, as occurs, for example, in yeast, certain sea- 

 weeds, and animal tissues. Active excretion of a particular cation 

 for example, sodium, might be accompanied by either active or 

 passive absorption of another cation such as potassium, and it could 

 be in such a way that the low sodium and high potassium content of 

 certain plant and animal tissues is maintained (see Chapter 9, p. 

 154). Conway suggested that uptake of alkali cations may be 

 achieved also by exchange for hydrogen ions actively excreted by 

 the redox pump. Anion transport is excluded from Conway's 

 scheme since, if exchange of cations occurs there is no net transfer of 

 positive charges. Anion accumulation must therefore depend on a 

 separate mechanism, involving either anion exchange, or con- 

 comitant net absorption of cations. 



The redox pump is more acceptable than Lundegardh's 

 hypothesis in that it is not so rigidly linked to a particular carrier 

 molecule. It could operate in anaerobic as well as aerobic organisms 

 and it accounts more adequately for the selective absorption of 

 cations. It is unsatisfactory in explaining the simultaneous absorp- 

 tion of cations and anions and in its simple form it suffers from the 

 same limitation of efficiency as the Lundegardh mechanism. 



3. Phosphorylation Mechanisms 



a. General. In view of the increasing amount of evidence that 

 "energy-rich" organic phosphates such as adenosine triphosphate 



M.S.A.P. 



