BIOLOGICAL TRANSPORT 



permit ATP to influence (and perhaps also to respond to) the dis- 

 tribution of electrons within the membrane. This movement of elec- 

 trons is visualized as taking place across a series of fixed molecular 

 sites extending through the membrane. These are taken to be anionic 

 sites able to bind the alkali metal ions. The theory of Eisenman, 

 Rudin, and Casby (see Eisenman, 1961, for references) is used to 

 account for the corresponding movement of a differential affinity 

 for the sodium and potassium ions. According to this theory, an 

 increase in the negative field strength of a chemical grouping can 

 cause its affinity for sodium ion to exceed that for potassium ion. 

 Inversely, a decrease in the negative electrostatic field strength can 

 favor potassium-ion binding. 



The joining of ATP is supposed to initiate an electron migra- 

 tion away from the most remote alkali metal-binding site to a dis- 



Outside ^ + 



Na+ 

 ATP Inside 



Figure 29 Visualization adapted from Skou (1961) of Na+ ex- 

 trusion in exchange for K+ across a series of points whose relative affinity 

 for Na+ and K+ is made to change sequentially by the migration of 

 an electron. The arc marked ATP illustrates the hypothetical position 

 of bound Mg 2 ATP (as in Figure 28) and the direction of the electron 

 movement it induces. See text for discussion. 



88 



