The electron transport chain, also known as oxidative phosphorylation, is the stage of aerobic cellular respiration that generates the most amount of energy. This process occurs in the inner membrane of the mitochondria in eukaryotic cells and depends on the presence of oxygen to function.
Este conjunto de reacciones metabólicas aprovecha la energía almacenada en los electrones transportados por las moléculas NADH y FADH₂. Estas moléculas son productos de procesos catabólicos como la glicólisis, el ciclo de Krebs y la beta-oxidación de ácidos grasos.
Elements of the Electron Transport Chain
In the electron transport chain, the electrochemical gradient, also known as the proton-motive force, is essential. This gradient is generated as protons (H⁺) are pumped from the mitochondrial matrix into the intermembrane space using the energy released by the electron transport of NADH and FADH₂ molecules.
All this is possible thanks to the elements of this chain known as complexes:
- Complex I: This complex accepts electrons from NADH, transferring them to ubiquinone (coenzyme Q) and pumping protons into the intermembrane space, contributing to the electrochemical gradient.
- Complex II: This complex does not contribute directly to electron pumping. Its function is to receive electrons from FADH₂ and transfer them to ubiquinone, a protein that accepts electrons from complex I or complex II.
- Complex III: It transfers electrons from reduced ubiquinone to complex IV, while pumping protons into the intermembrane space.
- Complex IV: It receives electrons from complex III and transfers them to oxygen, which acts as a final electron acceptor. This process generates water molecules as a by-product.
- ATP synthase: It utilizes the energy stored in the proton gradient to synthesize ATP (high-energy molecules) from ADP and inorganic phosphate in a process known as chemiosmosis. ATP synthase acts as an ion channel that "returns" the protons to the mitochondrial matrix.
Energy Production of the Electron Transport Chain
Each molecule of NADH transferring electrons to the transport chain generates approx. 2.5 molecules of ATP. On the other hand, the electrons coming from the FADH₂ produce close to 1.5 molecules of ATP. Together, the electron transport chain maximizes the energy efficiency of cellular respiration.
