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The outer mitochondrial membrane separates the surrounding cytosol from the intermembrane space. Channels permit passage of many small molecules, while the inner mitochondrial membrane supplies the main selective barrier between them and the mitochondrial matrix.[ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 14, The Mitochondrion Contains an Outer Membrane, an Inner Membrane, and Two Internal Compartments: outer-membrane channels and inner-membrane selectivity
Small-solute access does not imply unrestricted access for proteins. The cytochrome c addition in the isolated mitochondria respiration experiment tests a consequence of damage to this boundary. Organelle protein import describes purposeful delivery across mitochondrial boundaries.