Which pair of organelles have the same pH and ionic composition?
Ionic milieu of subcellular compartments reflects membrane permeability architecture and active pumping. The mitochondrial outer membrane contains abundant 19-stranded beta-barrel voltage-dependent anion channels, VDAC porins, present at high density allowing nonselective diffusion of solutes up to about 4 to 5 kilodaltons, including chloride, potassium, sodium, small metabolites, ATP, ADP, NADH and citrate. Because VDAC pores remain predominantly open under physiological low potentials, the intermembrane space equilibrates rapidly with cytosol via free diffusion, sharing near-identical pH around 7.0 to 7.1, comparable potassium around 140 millimolar, sodium, chloride and metabolite concentrations, and redox potential. In contrast matrix maintains more alkaline pH around 7.8 due to proton pumping by Complexes I, III and IV, low calcium due to MCU regulation, and distinct adenine nucleotide composition due to selective ADP/ATP carrier. Cytosol versus lysosome comparison reveals stark differences: lysosomal lumen acidic pH 4.5 maintained by V-ATPase, accumulates calcium via TRPML1 and concentrated hydrolases. Endosomal lumen also progressively acidic, while mitochondrial matrix alkaline. Therefore cytosol and intermembrane space equivalence is mechanistically explained by VDAC porin porosity and absence of active ion pumping across outer membrane, creating Donnan equilibrium similar to cytosol.
Ref: Nicholls & Ferguson, Bioenergetics, 4th ed., Chapter 3: Outer Membrane Permeability and Cytosol-Equilibrated Intermembrane Space.