Electron transport chain
An electron transport chain is a series of membrane proteins and carriers that pass electrons through redox reactions while helping cells conserve energy as ATP.
What an electron transport chain is
An electron transport chain is a linked set of proteins, cofactors, and mobile carriers that move electrons from donors to acceptors. Instead of releasing energy all at once, the chain captures some of that energy step by step across a membrane.
Electrons move downhill
In many respiratory chains, electrons start on reduced carriers such as NADH or FADH2 and move through a sequence of redox centers. The final electron acceptor may be oxygen in aerobic respiration or another compound in anaerobic respiration.
How proton gradients form
As electrons move, several protein complexes use the released energy to move protons to one side of a membrane. This creates an electrochemical gradient: a difference in charge and proton concentration. The gradient stores energy in a form the cell can use.
ATP synthase uses the gradient
ATP synthase lets protons flow back across the membrane and couples that flow to ATP production. This is the chemiosmotic idea behind oxidative phosphorylation in mitochondria and many bacteria, and it is also central to light-driven ATP production in chloroplasts.
Mitochondrial chain
In many eukaryotic cells, the respiratory electron transport chain sits in the inner mitochondrial membrane. Electrons from NADH and FADH2 move through complexes and carriers, oxygen receives electrons near the end, and water is formed.
Bacteria and alternative acceptors
Bacteria often have flexible electron transport systems in their cell membranes. Depending on the organism and environment, they may use oxygen, nitrate, sulfate, iron, carbon dioxide, or other terminal electron acceptors. This flexibility helps explain microbial life in oxygen-poor habitats.
Photosynthetic chains
Photosynthesis also uses electron transport chains. In chloroplasts and photosynthetic bacteria, light energizes electrons, and their movement helps build a proton gradient. The details differ from respiration, but the membrane-gradient logic is closely related.
When the chain is disrupted
Poisons, mutations, low oxygen, membrane damage, or uncoupling chemicals can interrupt electron transport or prevent the proton gradient from being used. The result can be lower ATP production, reactive oxygen species, heat release, or cell injury depending on context.
Why it matters
Electron transport chains connect redox chemistry to usable biological energy. They explain much of aerobic metabolism, many anaerobic metabolisms, photosynthetic ATP production, mitochondrial disease, microbial ecology, and the energy flow that supports complex life.