Figure 6. Model of cellular pH gradients.

Schematic diagram illustrating intracellular pH gradients within yeast cells from alkali-phase colonies. Measurements with Ato1p-pHluorin, Ato1p-mCherry, Jen1p-mCherry and Met17p-pHluorin fusion proteins provided evidence for the existence of sharp intracellular pH gradients between peripheral (∼8.8) and internal (∼6.6) cytoplasmic regions during the ammonia-producing alkali phase. In contrast to protonated ammonium (NH₄ ⁺), volatile ammonia (NH₃) enters the cell by simple diffusion through the plasma membrane. High cytosolic pH values near the plasma membrane allow for the maintenance of high transmembrane electrochemical potential (Δψ) based on H⁺ gradient (ΔpH) under conditions of relatively high extracellular pH (∼7.3).