How do they function and where do we get these electrons?
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The two molecules that deliver high energy electrons to the electron transport chain are NADH and FADH2. These electrons result from the oxidation of glucose, which is completed at the conclusion of the citric acid (Kreb's) cycle. As glucose is oxidized, electrons (and hydrogen atoms) are transferred to NAD+ and FAD, thus generating NADH and FADH2, respectively. The energy stored in these two molecules is then utilized during oxidative phosphorylation to generate ATP (the electron transport chain consists of exergonic redox reactions. As electrons are passed from one molecule in the electron transport chain to the next, the molecule that gives up the electrons is considered to be oxidized, whereas the molecule that gains the electrons is said to be reduced. These reactions generate the energy necessary to form ATP. The final electron acceptor is oxygen).