The production of ATP using energy derived from the redox reactions of an ETC is called ______.

• A. Glycolysis
• B. Oxidative phosphorylation
• C. Substrate-level phosphorylation
• D. Fermentation

Answer: (B)

The energy from electrons moving through the electron transport chain is used to pump protons across a membrane, creating a proton gradient that powers ATP synthase to convert ADP and inorganic phosphate into ATP. This link between redox reactions in the ETC and ATP production is what oxidative phosphorylation describes. The electrons come from reduced carriers like NADH and FADH2 produced in glycolysis and the TCA cycle, with oxygen typically acting as the final electron acceptor in aerobic conditions.

Glycolysis, by contrast, makes ATP mainly through substrate-level phosphorylation—direct transfer of a phosphate to ADP from a high-energy intermediate—without relying on the ETC. Substrate-level phosphorylation is a mechanism of ATP synthesis that occurs during glycolysis and the citric acid cycle. Fermentation can generate ATP without the ETC, again via substrate-level phosphorylation, and also serves to regenerate NAD+ to keep glycolysis running under anaerobic conditions.