Figure 1.

Substrates enter the respiratory chain at complex I and II and deliver electrons to complex III. Complex III delivers electrons to complex IV and reduces oxygen atoms inside the matrix to H₂O. A proton-motive force is generated in the intermembrane space by coupling electron transport to proton translocation at complexes I, III, and IV. The enzyme ATP synthetase generates ATP when protons are transported down their concentration gradient into the matrix. Complexes I, III, and IV, which are crucial for generating the proton-motive force, are partially encoded by mtDNA.