Fig. 5.

[H⁺] homeostasis in animal cells. Primitive cells may have used a proton gradient as a power source to energize vital cell activities. Today bacteria and mitochondria continue to so use proton gradients while eukaryotic cells have evolved to substitute the plasma membrane Na⁺ gradient for similar cell requirements (Wilson and Maloney, 1976). In so doing eukaryotic cells gained improved [H⁺] homeostasis which is likely to be more important to these latter cells because of their more numerous and complex enzyme systems and subcellular organelles. Today we know that excess intracellular H⁺ are ultimately removed from a number of different animal cell types through two ion antiport systems, Na⁺/H⁺ and ${\text{Cl}}^{-}/{\text{HCO}}_3^{-}$. The plasma membrane Na⁺ gradient drives Na⁺H⁺ antiport while the power source for ${\text{Cl}}^{-}/{\text{HCO}}_3^{-}$ antiport remains unclear. In squid ${\text{Cl}}^{-}/{\text{HCO}}_3^{-}$ antiport requires ATP. In barnacle muscle, squid axon, and snail or crayfish neurons ${\text{Cl}}^{-}/{\text{HCO}}_3^{-}$ antiport may be driven by the Na⁺ gradient through a variable coupling ratio (dotted line) (adapted from Thomas, 1984). For simplicity though one can regard plasma membrane H⁺ regulation as some combination of Na⁺/H⁺ and ${\text{Cl}}^{-}/{\text{HCO}}_3^{-}$ antiport.