Together, glycolysis and the citric acid cycle produce energy carriers that feed into oxidative phosphorylation.

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The main idea is that high-energy electrons carried by NADH and FADH2 power oxidative phosphorylation. During glycolysis, NAD+ accepts electrons when glyceraldehyde-3-phosphate is oxidized, forming NADH. In the citric acid cycle, several steps generate NADH and one step generates FADH2 by transferring electrons from metabolic intermediates. These carriers deliver electrons to the mitochondrial electron transport chain, where their energy is used to pump protons and create a gradient that drives ATP synthase to make ATP from ADP and inorganic phosphate. NADPH is used mainly for biosynthetic reactions and not for ATP production in mitochondria, so it isn’t the carrier feeding oxidative phosphorylation. CO2 and H2O are products of metabolism, not the electron carriers that drive ATP synthesis.