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How Are High Energy Electrons Used in Electron Transport?

The electron transport chain is a critical component of the complex process by which your cells extract energy from sugars like glucose. It begins inside your cells' mitochondria with the tricarboxylic acid cycle (TCA), which reduces or adds electrons to molecules called NAD+ and FAD, turning them into NADH and FADH2. These molecules in turn pass the high-energy electrons on to the electron transport chain. The ensuing events are a little complicated, but the basic logic is easy to follow.

Components
- The electron transport chain (ETC) comprises a series of proteins and attached groups called cofactors together with a few non-protein molecules like coenzyme Q. The components of the chain are all embedded in the oily inner membrane of the mitochondrion, which encircles the region where the TCA takes place. The high-energy electrons from NADH or FADH2 pass through this chain like a baton handed along in a relay race. Your cell makes use of these transfers to do work along the way.
Energy
- Imagine you have a tankful of water atop a hill. The water in the tank has lots of potential energy because it's at a higher elevation, and if you allow it to flow downhill across a series of turbines, you can use it to do work. The cell does something very similar with the chemical energy stored in NADH and FADH2. When NADH and FADH2 reduce the first component in the chain, energy is released, and the same is true for each subsequent electron transfer.
Proton Pump
- The cytochrome bc1 complex in the ETC offers an illustration of how the process works. Electrons arrive at the complex carried by a molecule called ubiquinol, which has been reduced by another complex in the chain. Ubiquinol loses electrons to an iron-sulfur center and an iron-containing group called a heme group. The electrons follow a somewhat complicated path through the complex to reach a protein called cytochrome c, which will carry the electrons to the next component in the chain. In the course of this process, a net total of four hydrogen ions are transferred across the membrane. The complex has used the energy harvested from the electron transfers to pump hydrogen ions.
Proton Power
- As electrons are shuttled down the ETC, the ETC pumps hydrogen ions into the solution outside the membrane, boosting its hydrogen ion concentration. The hydrogen ions want to diffuse back in across the membrane, just in the same way that a drop of food coloring will try to diffuse throughout a glass of water. With the aid of a protein called ATP synthase, the mitochondrion harnesses this difference in concentration as a source of power.

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