Electron transport chain

The electron transport chain is a set of protein complexes and small carrier molecules that hand electrons from one to the next in a series of redox reactions, reactions in which one molecule loses electrons and another gains them. In eukaryotic cells it sits in the inner membrane of the mitochondrion; in bacteria and archaea it lies in the plasma membrane. The electrons come from NADH and FADH2, carriers that were loaded during glycolysis, the citric acid cycle and the breakdown of fats.

In mitochondria the chain has four main complexes. Complex I takes electrons from NADH and complex II from FADH2, and two mobile carriers, ubiquinone and cytochrome c, ferry them onward to complex IV, which passes them to oxygen. Oxygen is the last acceptor in the line, and it combines with protons to form water. As the electrons move to lower energy, complexes I, III and IV push protons (hydrogen ions) out of the matrix, building up a gradient that ATP synthase then uses to make ATP.

Students meet the chain when learning cellular respiration. Poisons show why it matters: cyanide and carbon monoxide block complex IV, so electron flow and most ATP production stop.