FIG. 1.

Electron movement is shown through the mitochondrial electron transport chain (ETC) and the subsequent generation of adenosine triphosphate (ATP). Electrons (e-) from reduced substrates enter the ETC through complex I or II and are then passed through complexes III and IV where they have reduced oxygen (O₂) to produce water (H₂O) (solid black lines). As the electrons are moved from a high-energy state to a low-energy state through the complexes, protons (H⁺) are pumped from the mitochondrial matrix into the intermembrane space (dashed black lines), generating a proton gradient across the mitochondrial inner membrane. Protons then re-enter the matrix through complex V and are used to combine adenosine diphosphate (ADP) with inorganic phosphate to produce ATP (solid yellow lines). Under homeostatic conditions, some electrons may leak from the respiratory chain and inappropriately combine with molecular oxygen (solid red lines), forming superoxide (O^.-₂), hydrogen peroxide (H₂O₂), and hydroxyl radicals (HO^.) and contributing to steady-state levels of free radicals. Protons that leak back across the mitochondrial inner membrane and into the matrix (dashed red line) reduce the inner membrane potential, increasing the activity of the ETC and causing thermogenesis. Q = coenzyme Q; C = cytochrome c.