During the Krebs cycle in the matrix of the mitochondria, H+ ions are usually pumped to the intermembrane space creating a proton-motive force. This potential difference is harness by ATP synthase to make ATPs as the H+ ions 'drain' back to the matrix of the mitochondria. As these H+ get to the matrix, they ultimately reduce oxygen to water. In this reaction, oxygen also acts as the ultimate electron acceptor in the Krebs cycle accepting 4 electrons from NADPH/ NADH (so these molecules can be recycled back as NAD+ and NADP+).
