Determinants of the pH_mito. Protons are pumped from the matrix to the IMS by the respiratory chain complexes I, III, and IV (green boxes) as electrons flow from reduced substrates in the matrix to O₂. The pumping of electrically charged protons generates a ΔΨ_m of ∼180 mV and a pH gradient (ΔpH_m: pH_mito − pH_IMS) of ∼0.9 pH units as the matrix becomes more alkaline than the IMS. The proton circuit is in thermodynamic equilibrium and changes in ΔΨ_m, thus causing opposing changes in ΔpH_m by altering the energy required for the pumping of protons by respiratory chain complexes. ΔΨ_m and ΔpH_m add up to generate a Δp used by the ATP synthase (blue-orange barrel) to generate ATP from ADP and Pi in the matrix. ΔΨ_m drives Ca²⁺ uptake across the mitochondrial Ca²⁺ uniporter (MCU; blue cylinder) and ADP–ATP exchange across the adenine nucleotide translocator (ANT; brown ovals). Electroneutral H⁺–ion exchangers rely exclusively on ΔpH_m to extrude Ca²⁺, Na⁺, and K⁺ ions in exchange for protons (CHX, NHX, and KHX, respectively; brown ovals), whereas the PiC relies on ΔpH_m to import the inorganic phosphate used for the synthesis of ATP (PiC; brown ovals). The coupling of H⁺ and ion fluxes implies that changes in the Na⁺, K⁺, Ca²⁺, and Pi gradients can alter ΔpH_m. UCPs and the mPTP (UCPs and mPTP; blue cylinders) dissipate both ΔpH_m and ΔΨ_m to generate heat and to initiate cell death, respectively. Variations in pH_mito reflect the equilibrium between proton pumping by the respiratory chain; Δp dissipation by the ATP synthase, UCPs, and mPTP; ΔpH_m dissipation by KHX, NHX, CHX, and PiC; and adaptive responses to changes in cytosolic pH and in ΔΨ_m.