= Mitochondrial intermembrane space
The <mitochondrial intermembrane space> is the compartment between the outer and inner mitochondrial <lipid bilayers>, including the spaces within <cristae>. It is separated from the <mitochondrial matrix> by the inner <lipid bilayer>. During <oxidative phosphorylation>, respiratory <proton pumps> transfer <hydrogen ions> from the <mitochondrial matrix> toward this compartment. Return through <ATP synthase> couples the resulting <proton-motive force> to <ATP> synthesis. The relevant stored energy includes both an electrical potential difference and a chemical concentration difference. <Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[Chapter 14, The Mitochondrion: proton gradient and ATP synthesis]{ref}
The compartment also contains <cytochrome c>, an electron carrier associated with the outer face of the inner <lipid bilayer>. Its release into the <cytosol> can contribute to <apoptosis>. In a cell-free study, depletion of <cytochrome c> reduced apoptotic activity and add-back restored it, connecting loss of compartmentalization to a new biochemical function. <Induction of apoptotic program in cell-free extracts: requirement for dATP and cytochrome c by X. Liu and others>[1996, abstract: cytochrome c depletion, add-back and release]{ref}
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