Fluid volume found between the inner and outer membranes of both mitochondria and chloroplasts.
The intermembrane space is equivalent to the periplasm found in Gram-negative bacteria, though lacking in these endosymbionts in the bacterial cell wall. (The cell wall presumably was lost over the course adaptation to an intracellular existence, i.e., living as organelles rather as otherwise free-living bacteria.)
It is within the intermembrane space that excess hydrogen ion accumulates in the course of generating the proton motive force. With chloroplasts, by contrast, the excess hydrogen ion concentration is generated within the thylakoid space.
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