What is The respiratory chain?

The respiratory chain is four protein complexes of the inner mitochondrial membrane and two mobile carriers. NADH gives its electrons to complex I, $\mathrm{FADH_2}$ (via succinate dehydrogenase, complex II) gives its own; both reduce ubiquinone (Q), a lipid-soluble quinone diffusing in the membrane; Q passes them to complex III, which hands them to cytochrome $c$, a small protein on the outer face; and complex IV (cytochrome oxidase) delivers them, four at a time, to $\mathrm{O_2}$, making water. At each of complexes I, III and IV the electrons drop in potential and the energy pumps protons from the matrix into the intermembrane space: about 4, 4 and 2 per pair of electrons, ten per NADH, six per $\mathrm{FADH_2}$ (which enters below complex I).