Cellular respiration involves metabolic pathways that break down nutrients to harvest stored energy. A significant part of this energy is captured by two specialized high-energy electron carriers: nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (\(\text{FADH}_2\)). These molecules gather electrons released during earlier stages, such as glycolysis and the Krebs cycle. \(\text{NADH}\) and \(\text{FADH}_2\) then shuttle these energized electrons to the final stage, the Electron Transport Chain (ETC), where the energy is used to generate adenosine triphosphate (ATP).
The Location of the Electron Transport Chain
The ETC is located in the mitochondrion, often described as the cell’s powerhouse. This organelle features a double-membrane structure, which is fundamental to energy production. The inner membrane is highly folded, creating cristae, which substantially increase the surface area available for the ETC proteins.
Embedded within the inner mitochondrial membrane, the ETC consists of four large protein complexes, labeled Complex I through Complex IV. The inner membrane separates two distinct spaces: the mitochondrial matrix, where the Krebs cycle occurs, and the intermembrane space, the narrow region between the inner and outer membranes.
The arrangement of the ETC complexes allows for energy conversion. They act as a molecular relay team, passing electrons sequentially. This electron movement powers the transfer of hydrogen ions (\(\text{H}^+\)) from the matrix into the intermembrane space.
The directional pumping of protons is a direct consequence of the energy released by the moving electrons. This results in a build-up of \(\text{H}^+\) ions in the intermembrane space, creating a concentration difference. This difference is harnessed in the final step of energy synthesis.
Where NADH Delivers Its Electrons
The entry point for high-energy electrons from \(\text{NADH}\) is Complex I, formally known as \(\text{NADH}\) dehydrogenase. \(\text{NADH}\), generated in the matrix, binds to Complex I, initiating oxidation. \(\text{NADH}\) loses its hydrogen atom, breaking down into \(\text{NAD}^+\), a hydrogen ion (\(\text{H}^+\)), and two high-energy electrons.
The electrons are accepted by Complex I and passed through internal carriers, including flavin mononucleotide (\(\text{FMN}\)) and iron-sulfur clusters. This electron movement releases energy, causing a conformational change in the Complex I protein.
This shape change drives the transport of protons. The energy released is coupled to the movement of four protons (\(\text{H}^+\)) from the matrix into the intermembrane space. The electrons are then passed from Complex I to ubiquinone (\(\text{Q}\)), a mobile carrier dissolved in the inner membrane’s lipid layer, which carries them onward to Complex III.
Where \(\text{FADH}_2\) Delivers Its Electrons
The second electron carrier, \(\text{FADH}_2\), enters the ETC at Complex II, also known as succinate dehydrogenase. This complex is unique because it is the only enzyme of the Krebs cycle embedded in the inner mitochondrial membrane. \(\text{FADH}_2\) is formed when succinate is converted to fumarate during the cycle.
When \(\text{FADH}_2\) is oxidized, it donates two electrons to Complex II. These electrons are transferred through three iron-sulfur clusters within the complex. They are then passed directly to the mobile carrier ubiquinone (\(\text{Q}\)), the same carrier that receives electrons from Complex I.
\(\text{FADH}_2\) electrons enter the chain at a lower energy level than \(\text{NADH}\) electrons. This lower energy input means electron transfer through Complex II does not release enough energy to pump protons. Therefore, Complex II is not a proton pump, resulting in less energy captured in the proton gradient. The electrons proceed to Complex III, bypassing the first proton-pumping step.
Converting Electron Movement into Cellular Energy
After receiving electrons from Complex I and Complex II, ubiquinone transfers them to Complex III (cytochrome \(c\) reductase). Electron movement through Complex III releases energy, which is used to pump additional protons from the matrix into the intermembrane space. Cytochrome \(c\), a second mobile carrier, accepts the electrons from Complex III and shuttles them to Complex IV.
Complex IV acts as the final proton pump, utilizing the remaining electron energy. Its primary function is transferring the low-energy electrons to the final electron acceptor, oxygen (\(\text{O}_2\)). Oxygen combines with the electrons and hydrogen ions from the matrix to form water (\(\text{H}_2\text{O}\)), a metabolic byproduct.
The cumulative action of the proton pumps (Complexes I, III, and IV) establishes a strong electrochemical gradient, or proton-motive force, across the inner membrane. This gradient represents stored potential energy. The accumulated protons flow back down their concentration gradient into the matrix only through the enzyme ATP Synthase (Complex V). This process, called chemiosmosis, drives ATP production. The flow of protons causes ATP Synthase to rotate, using mechanical energy to combine adenosine diphosphate (\(\text{ADP}\)) with inorganic phosphate to synthesize ATP.