The fall of the electron

Think of all of the places in the world an electron might find itself. It could be in the outer shell of a lithium atom. It could be within a hydrogen-hydrogen covalent bond. It could be in a fluoride ion. Some of these places, like on the lithium, are like being on top of the house. It's higher energy there compared to everywhere else an electron might be. Some, like with fluoride, are like being in the basement. Being in a H-H bond is in the middle. In fact, that will be our reference point. The electron in the H-H bond is standing on the ground floor. It's the zero point. It would take energy to move an electron from H-H to lithium, so lithium has a large negative standard reduction potential. An electron would fall from H-H to fluorine. Fluorine has a large positive standard reduction potential.

NAD+ has a negative standard reduction potential. When NADH arrives at complex I, it's holding the pair of electrons it's going to deliver at a high energy. Like water from the high side of the dam, the electrons are going to fall through the electron transport chain on their way to oxygen. Like fluorine, oxygen has a large positive reduction potential. For being in the second period, oxygen has a big nucleus that's barely shielded, so electrons get pulled in towards it. Like the water falling through the dam that does work on the turbines, electrons falling through the electron transport chain towards oxygen do the work of pumping protons into the outer compartment of the mitochondrion.