In a mitochondrion, the intermembrane space lies between the outer mitochondrial membrane and inner mitochondrial membrane, extending into the interiors of cristae. It includes the side of the inner mitochondrial membrane from which protons return through ATP synthase to the mitochondrial matrix.
A useful sidedness check is to follow a single arrow: outward pumping goes from green to yellow in the diagram, while coupled return goes from yellow to green. These are opposing movements of protons, not a circuit in which Electrons return through ATP synthase. The mitochondrial respiratory chain and chemiosmosis articles distinguish the two ledgers.
