Electron Transport in the Energy Cycle of the Cell

The eukaryotic cell's most efficient path for production of vital ATP is the aerobic respiration that takes place in the mitochondria. After glycolysis, the pyruvate product is taken into the mitochondia and is further oxidized in the TCA cycle. This cycle deposits energy in the reduced coenzymes which transfer that energy through what is called the electron transport chain.

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The energy given to the electrons of the reduced coenzyme NADH and to succinate by the TCA cycle is transferred in small steps in the inner membrane of the mitochondrion through a chain of five protein complexes. These small oxidation steps accomplish the conversion of ADP to the energy currency molecule ATP. This series of coupled reactions is often referred to as oxidative phosphorylation.

The energy used in the electron transport change pumps protons across the inner mitochondrial membrane from the inner matrix to the intermembrane space, producing a strong hydrogen concentration gradient. This process was called chemiosmosis by its discover, Peter Mitchell. This difference in proton concentration produces both an electrical potential and a pH potential across the membranes. The protein complex ATP synthase then makes use of this membrane potential to accomplish the phosphorylation of ADP to ATP.

The Protein Complexes of the Electron Transport Chain

Many years of effort have been devoted to the study of the remarkable processes in the mitochondria. The electron transport chain is the final stage of aerobic respiration leading to the forming of ATP in the inner membrane of the mitochondrion. The emergent picture is that of coupled reactions through five protein structures associated with that inner membrane.

Complex I (NADH-coenzyme Q oxidoreductase). The reduced coenzyme NADH binds to Complex I and accomplishes the reduction of Coenzyme Q10. Electrons are transferred through Complex I using FMN (flavin mononucleotide) and a series of Fe-S clusters. The process accomplishes the pumping of four protons across the inner mitochondrial membrane to the intermembrane space.

Complex II (Succinate-Q oxidoreductase). This complex forms a second entry point into the electron transport chain using the succinate product of the TCA cycle.

Complex III (Q-cytochrome c oxidoreductase). This complex accomplishes the oxidation of ubiquinol and the reduction of two molecules of cytochrome-c. Four hydrogens are pumped across the membrane to the intermembrane space.

Complex IV (Cytochrome c oxidase). This final complex in the electron transport chain accomplishes the final transfer of the electrons to oxygen and pumps two protons across the membrane. This makes a total of 10 protons across the membrane for one NADH into the electron transfer chain.

ATP Synthase. This complex makes use of the proton potential created by the action of the electron transport chain. It transports a proton down the gradient and uses the energy to complete the phosphorylation of ADP to ATP. The current model of its action is called the binding charge mechanism, and it appears that part of this large protein complex accomplishes a mechanical rotation in the process of phosphorylation and release of the ATP molecule. So part of its action is like a molecular motor. More on ATP synthase