In mitochondria, several exergonic redox reactions take place, contributing to the generation of the proton gradient and subsequent ATP synthesis through oxidative phosphorylation. One of the key exergonic redox reactions occurs during the electron transport chain, specifically between complex I (NADH-CoQ reductase) and complex III (CoQH2-cytochrome c reductase). Here's the overall reaction:

NADH (reduced) + CoQ (oxidized) -> NAD+ (oxidized) + CoQH2 (reduced)

This reaction involves the transfer of two electrons from NADH to CoQ. As the electrons pass through complex I, they are coupled to the pumping of protons from the mitochondrial matrix to the intermembrane space, creating the proton gradient necessary for ATP synthesis by ATP synthase.