The Electron Transport Chain & complexes I-IV that pump protons out of the Mitochondria by the transfer of the electrons carried on NADH & FADH2 to maintain the concentration gradient of the protons "high in the intermembrane space & low in the matrix of the Mitochondria"
this video is made by HarvardX on edX
https://goo.gl/KDZmML
http://bit.ly/2hqwmRB