The three respiratory enzyme complexes, in the order in which they receive electrons, are (1) NADH dehydrogenase complex, (2) cytochrome c reductase complex, and (3) cytochrome c oxidase complex.
B.
Each complex contains metal ions and other chemical groups that act as stepping stones to enable the passage of electrons through the complex.
C.
The movement of electrons through these respiratory complexes is accompanied by the pumping of protons from the mitochondrial matrix to the intermembrane space.
D.
Thus each complex can be thought of as a proton pump.