Energy and Metabolism

ID #2213

Could you please explain the order in which NADH and FADH2 travel through the Electron Transport Chain?

Before we start differentiating the two, let's be clear that the NADH and FADH2 do not travel through the ETC, instead, they donate the electrons that they are carrying to the ETC. Now, let's follow each electron carrier, and its electrons, through the electron transport chain. 

NADH drops off its electrons at complex I. (The NADH is now oxidized to NAD+ and can participate in glycolysis or the Krebs cycle again.) The electrons are then passed to ubiquinone, the mobile electron carrier, which takes them to complex III. The electrons are then passed fro complex III to cytochrome C, another mobile electron carrier. Cytochrome C takes them to complex IV, which passes the electrons to oxygen. The oxygen and its new electrons combine with protons to form water. 

FADH2 starts at complex II, which does not span the entire mitochondrial inner membrane and does not pump protons out of the matrix. (The FADH2 is now oxidized into FAD and can participate in the Krebs cycle again.) Complex II passes the electrons to ubiquinone and then to complex III. The rest of the process is the same - electrons go from complex III to cytochrome C, then to complex IV, then to oxygen. Again, the oxygen, electrons, and protons combine to yield water. 

Each time the electrons are handed off, a little bit of energy is lost, which is used to pump protons out of the matrix and into the intermembrane space. 

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