NADH and FADH2 convey their electrons to the electron transport chain. This transport chain is composed of a number of molecules (mostly proteins) that are located in the inner membrane of the mitochondrion. Each membrane protein has a particular electronegativity (affinity for electrons). The more electronegative the molecule, the more energy required to keep the electron away from it. In this way, a slightly electronegative membrane protein will pull electrons away from reduced electron carriers. In the presence of an even-more electronegative molecule, these electrons will be oxidized from the first membrane protein, and so on. Finally the electrons reduce oxygen, and along with the addition of hydrogen ions, water is produced as a waste product. This stepwise movement, whereby an electron from one protein is transferred to another in the chain, is also reflective of the overall decrease in the amount of energy that the electron possesses. Importantly, each step has a -ΔG. Therefore with each oxidation/reduction reaction, energy is made available to do work. This work involves the movement of protons.
Oxygen is one of the most electronegative atoms. This is important because the relative change in electronegativity determines how much energy is available to do work. When oxygen acts as the terminal electron acceptor , there is a maximal amount of free energy released; hence, more protons can be transported, which means that a greater charge buildup occurs across the inner mitochondria membrane. This figure illustrates the energetic relationship between various members of the electron transport chain when oxygen serves as the electron acceptor.
During the movement of electrons through the electron transport chain, protons accumulate on the inside of the inner mitochondrial membrane. As electrons move from one member of the electron transport chain to the next, protons are transported from one side of the membrane to the other, resulting in a buildup of protons in the intermembrane space. This creates a charge differential (voltage) across the inner membrane; it is this stored energy that is actually used to synthesize ATP.
As these excess protons from the intermembrane space flow back into the mitochondrial matrix (the part of the mitochondrion enclosed within the inner membrane, which houses the enzymes and substrates for the Krebs cycle), ADP is phosphorylated to make ATP (chemiosmosis ). Chemiosmosis is accomplished in the presence of the protein complex ATP synthase , which is also located in the inner mitochondrial membrane.
Note how the transfer of electrons provides the energy to move protons across the inner mitochondrial membrane. This buildup of protons in the intermembrane space creates a charge differential (voltage), and this stored energy is then used to drive the ATP synthase complex to affect the production of ATP. This figure shows an overview of the Electron Transport Chain and oxidative phosphorylation.
This animation will help you visualize the structure and function of the electron transport chain. You do not need to know the name of the molecules of the electron transport chain, but you should be able to explain how the movement of electrons from NADH and FADH2 to the electron transport chain is used by the mitochondrion to generate ATP:
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