= Intermembrane space
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.
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