The energy requirements of pH homoeostasis define the limits of pH regulation--a model

The regulation of intracellular pH of Platymonas subcordiformis cells is investigated with 31P-NMR spectroscopy at various external pH values. The limits of pH homeostasis differ between respiring cells and cells solely glycolyzing. The former retain their normal cytoplasmic pH for external pH values from 5 to more than 12, while in the latter intracellular pH is less stable below external pH 6, and external pH of more than 11 causes hydrolysis of intracellular polyP. The power necessary to maintain a stable intracellular pH is calculated from the electrochemical potential and H+/OH- flux rates across the cell membrane. H+/OH- transport across the membrane by diffusion as well as through hydrogen-bonded chains is considered. Comparison of the minimal power necessary to keep intracellular pH stable at various external pH values by ATP consuming processes with the estimated power available to aerobic and anaerobic cells in the dark shows that different energy turnover may explain the different behaviour of the cells when exposed to extreme pH values.