The electron transport chain (ETC) is the final and most productive stage of cellular respiration. This molecular assembly is responsible for generating the vast majority of the cell’s energy currency, adenosine triphosphate (ATP). The ETC is a collection of protein complexes and organic molecules embedded within the inner membrane of the mitochondria, the cell’s powerhouses. It functions by transferring electrons through a series of redox reactions, releasing the energy stored in electron carriers to synthesize ATP, which powers almost all cellular activities.
Structural Components of the Chain
The ETC consists of four large, multi-subunit protein complexes, designated I, II, III, and IV, arranged within the inner mitochondrial membrane. These fixed complexes perform electron transfer and proton pumping. Complex I, known as NADH dehydrogenase, is the entry point for electrons from the high-energy carrier NADH. Complex II, succinate dehydrogenase, is the alternate entry point for electrons delivered by the carrier FADH\(_2\).
Two smaller, mobile components act as shuttles to move electrons between these fixed complexes. Ubiquinone (Coenzyme Q or CoQ) is a lipid-soluble molecule that carries electrons from Complexes I and II to Complex III. Cytochrome c is a small, water-soluble protein located in the intermembrane space that transfers electrons from Complex III to Complex IV.
Electron Movement and Proton Gradient Creation
The process begins when the reduced electron carriers, NADH and FADH\(_2\), produced from glycolysis and the citric acid cycle, deliver their cargo to the chain. NADH releases two electrons at Complex I, oxidizing to NAD\(^+\), while FADH\(_2\) drops its electrons at Complex II, becoming FAD. These electrons then cascade down the chain through a series of reduction-oxidation reactions.
The sequential transfer of electrons is an exergonic process, releasing energy at each step as electrons move to components with a higher reduction potential. This released energy is captured by Complexes I, III, and IV. These three complexes act as proton pumps, using the electron transfer energy to actively translocate hydrogen ions (protons) from the mitochondrial matrix across the inner membrane into the intermembrane space. This pumping action establishes a steep electrochemical gradient, representing stored potential energy termed the proton motive force.
Generating Power through Chemiosmosis
The potential energy stored in the proton gradient is harnessed to produce ATP through a process called chemiosmosis. The inner mitochondrial membrane is impermeable to protons, so the only path for them to return is through a specialized molecular machine known as ATP Synthase.
The passive flow of protons down their concentration and electrical gradient, from the intermembrane space back into the mitochondrial matrix, powers ATP synthesis. The movement of these protons through the channel of the ATP Synthase causes the internal rotor to spin, converting the electrochemical potential energy into mechanical energy. This rotational movement drives conformational changes in the enzyme’s catalytic head, providing the energy to phosphorylate adenosine diphosphate (ADP) to create ATP. This mechanism of linking the proton gradient to ATP synthesis is efficient, generating approximately 2.5 molecules of ATP for every NADH and about 1.5 molecules of ATP for every FADH\(_2\) oxidized.
The Role of Oxygen
The flow of electrons and the subsequent generation of the proton gradient are dependent on the presence of oxygen. Oxygen acts as the ultimate electron acceptor at the end of the transport chain, specifically at Complex IV. After the electrons have passed through all the complexes and their energy has been harvested, they are transferred to an oxygen molecule.
Oxygen then combines with these electrons and protons from the mitochondrial matrix to form water (H\(_2\)O). This step acts as a sink, constantly clearing the chain of electrons. Without oxygen to accept the electrons, the entire chain would rapidly back up, halting the flow of electrons and stopping all ATP production by chemiosmosis.