Mitochondrial energy production. The mitochondrial electron transport chain is composed of five multimeric complexes. Electron transport between complexes I to IV is coupled to extrusion of protons from complexes I, III and IV into the intermembrane space, creating an electrochemical gradient (Δψ) across the inner mitochondrial membrane. Protons then flow through complex V (ATP synthase), which utilizes the energy to synthesize ATP from ADP. Some common mitochondrial respiratory chain inhibitors are shown. C, cytochrome c; Q, ubiquinone.