Figure 1.

Tricarboxylic acid (TCA) cycle and oxidative phosphorylation. ATP generation starts in the TCA cycle and ends with oxidative phosphorylation. During the TCA cycle, NADH and FADH2 are generated, which are oxidized to provide electrons through the ETC. Mitochondrial O₂ ^.− is produced when oxygen molecules gain electrons leaked from the ETC. Through oxidative phosphorylation, free energy is used to pump protons out of the matrix, forming Δψm, which ultimately provides the energy needed to generate ATP.