Figure 4.

Dependency of the two compared K⁺ uptake modules on [K⁺]_ext and [H⁺]_ext. Both modules mediate the net-antiport of K⁺ and H⁺ across the membrane. The module of proton pump and K⁺ channel (A) reaches its energy limit (red line) at conditions in which the co-transporter-based module still allows K⁺ uptake (B). At very low [K⁺]_ext and/or high [H⁺]_ext the energy from ATP-hydrolysis is not sufficient anymore to energize the combined K⁺-uptake/H⁺-release via the pump & K⁺ channel module.