An electron transport chain (ETC) is a series of proteins and other electron carriers that transfer electrons from donors to acceptors through linked oxidation–reduction reactions. In biological energy conversion, these transfers are coupled to the movement of ions across a membrane, creating an electrochemical gradient that can drive the production of adenosine triphosphate (ATP). Electron transport chains operate in cellular respiration and photosynthesis, although their electron sources, terminal acceptors, and organization differ. (ncbi.nlm.nih.gov)
Location and energetic principles
In most respiring eukaryotic cells, the respiratory chain occupies the inner membrane of the mitochondrion. This membrane separates the mitochondrial matrix from the intermembrane space. Electron transfer drives the movement of hydrogen ions—protons—from the matrix toward the intermembrane space. In respiring bacteria, comparable machinery operates in the cell membrane, establishing an ion gradient across the cell boundary. (ncbi.nlm.nih.gov)
Respiratory electrons generally pass toward carriers with progressively greater electron affinity, expressed quantitatively through reduction potentials. The resulting decrease in free energy can power otherwise unfavorable proton translocation. Electron transport therefore converts energy from chemical reactions into an electrochemical form rather than producing ATP directly at each carrier. The gradient has two components: a difference in electrical potential and a difference in proton concentration, measured as pH. Together they constitute the proton-motive force. (ncbi.nlm.nih.gov)
The mitochondrial respiratory chain
The standard mitochondrial chain contains four major enzyme complexes, conventionally numbered I–IV, connected by mobile carriers. Reduced nicotinamide adenine dinucleotide, NADH, supplies electrons to complex I. Other metabolic reactions supply electrons through alternative entry points, notably complex II. (nature.com)
- Complex I, NADH:ubiquinone oxidoreductase, oxidizes NADH and transfers its electrons through flavin mononucleotide and iron–sulfur centers to ubiquinone. This reaction is coupled to proton pumping.
- Complex II, succinate dehydrogenase, oxidizes succinate to fumarate in the citric acid cycle. Electrons pass through enzyme-bound FAD and iron–sulfur centers to ubiquinone. Unlike complexes I, III, and IV, complex II does not pump protons. The reduced FAD is an internal intermediate, not a freely diffusing electron donor.
- Complex III, the cytochrome bc₁ complex, transfers electrons from reduced ubiquinone to cytochrome c while contributing to proton translocation through the Q cycle.
- Complex IV, cytochrome c oxidase, receives electrons from cytochrome c and transfers them to molecular oxygen, forming water. It also pumps protons. (pmc.ncbi.nlm.nih.gov)
Ubiquinone, also called coenzyme Q, is lipid-soluble and moves within the membrane. Its reduced form, ubiquinol, carries two electrons and two protons. Cytochrome c is a small protein on the intermembrane-space side of the inner membrane that carries one electron at a time between complexes III and IV. Thus, the main NADH-linked route is NADH → I → ubiquinone → III → cytochrome c → IV → oxygen. (ncbi.nlm.nih.gov)
Coupling to ATP synthesis
ATP synthase provides a controlled route for protons to return to the matrix. Their movement down the electrochemical gradient drives the enzyme’s rotary mechanism and ATP formation from ADP and inorganic phosphate. ATP synthase is often called complex V, but it is not an electron-transfer complex: it uses the gradient established by the respiratory chain. Electron transport and ATP synthesis together form the central machinery of oxidative phosphorylation. (ncbi.nlm.nih.gov)
In the standard mitochondrial pathway, oxidation of one NADH transfers two electrons and supports translocation of approximately ten protons. Electrons entering through succinate oxidation bypass complex I and support approximately six. ATP yield is not a fixed integer because it depends on ATP synthase stoichiometry, metabolite transport, and proton leakage. Common approximate accounting values are 2.5 ATP per mitochondrial NADH and 1.5 ATP per electron pair entering through succinate oxidation. (pmc.ncbi.nlm.nih.gov)
This indirect coupling is called chemiosmosis. Peter Mitchell proposed the chemiosmotic explanation in 1961 and received the Nobel Prize in Chemistry in 1978 for his contribution to understanding biological energy transfer. (nobelprize.org)
Respiratory diversity
Oxygen is the terminal acceptor in aerobic respiration, but it is not required by every electron transport chain. Anaerobic respiratory systems can use acceptors such as nitrate, fumarate, or sulfur compounds. Bacterial chains may have multiple electron-entry routes and alternative terminal enzymes rather than a single invariant sequence. Some microorganisms use sodium-ion gradients instead of proton gradients for energy coupling. These variations distinguish respiratory electron transport from fermentation, which does not require a respiratory chain. (ncbi.nlm.nih.gov)
Photosynthetic electron transport
In chloroplasts, photosynthetic electron carriers reside in the thylakoid membrane. Light-driven excitation in photosystems enables electron transfer from water to NADP⁺. In linear electron flow, photosystem II extracts electrons from water, releasing oxygen; carriers including plastoquinone, the cytochrome b₆f complex, and plastocyanin connect it to photosystem I. A second light-driven excitation enables production of NADPH. (ncbi.nlm.nih.gov)
These reactions accumulate protons in the thylakoid lumen. Proton return through ATP synthase produces ATP on the stromal side of the membrane. ATP and NADPH then support carbon fixation, including the Calvin cycle. Unlike aerobic respiratory chains, which consume oxygen, oxygenic photosynthetic electron transport releases oxygen and uses light to sustain the transfer of electrons from water to NADP⁺. (ncbi.nlm.nih.gov)