Hydrogen ions pumped by the electron transport chain accumulate in the:

1. The complexes use free energy at three points to transport hydrogen ions across the membrane. 2. They move them out of the matrix. 3. This causes hydrogen ions to accumulate within the intermembranous space. 4. A concentration gradient forms, and the ions diffuse back into the matrix by passing through ATP synthase. 5. Matrix is the trap: that is where they return to, not where they build up. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 7 'Cellular Respiration', sections 7.1-7.7_