92 NITROGEN METABOLISM 



reactions between cellular constituents and the substance 

 being transported across the cell membrane [20]. This 

 implies that the latter contains a substance or carrier which 

 reacts with the transported substance to form a relatively 

 stable product which traverses the cell membrane and then 

 either undergoes chemical decomposition or takes part in an 

 exchange reaction, thus liberating the transported substance 

 into the internal environment. Energy may be required for 

 the formation of either the compound traversing the mem- 

 brane or the substance which takes part in the final exchange 

 reaction. Davies and Krebs [id] have shown theoretically 

 how metabolic energy may be utilized for the production of 

 H"^ or 0H~ from water. By analogy with their hypotheses 

 concerning ion transfer in brain cells it may be proposed 

 that metabolic energy is used to form an excess of 0H~> 

 which may perhaps combine with CO 2 to form HCO 3", and 

 that the bacterial cell wall contains a basic ion exchange 

 complex (X). If the cell contains a high concentration of an 

 anion such as 0H~ or HCO^ and the external environment 

 contains glutamate ions, and if it is assumed that the ion 

 exchange complex X can move in the cell wall and thus 

 come into contact with the external and internal environ- 

 ments, the following reactions can be expected to take place, 

 the equilibrium being towards the right: 



internally: X+ + OH" ^ X-OH 



externally: X-OH + glutamate ^ X-glutamate + 0H~ 



internally: X-glutamate + 0H~ ^ X-OH + glutamate 



Glutamate ions will thus be transported into the cell and 

 will be replaced in the external environment by hydroxyl 

 ions. Eventually a steady state will be established in which 

 exchange is still taking place, but there is no further overall 

 increase in the internal concentration of glutamic acid. 

 Specificity in ion transport may be due to different ions 

 being transported by diiferent ion-exchange complexes 

 whose specificity properties are comparable with enzymes. 

 The observation by Britten that intracellular glutamic 



