Which stage of cellular respiration is directly linked to ATP production via chemiosmosis across a membrane?

• A. Glycolysis
• B. Citric Acid Cycle
• C. Oxidative Phosphorylation
• D. Fermentation

Answer: (C)

The main idea is that ATP production through chemiosmosis across a membrane happens during oxidative phosphorylation. In this stage, the electron transport chain sits on the inner mitochondrial membrane and pumps protons across it as electrons are transferred. This creates a proton gradient and an electrical potential (a proton motive force). Protons flowing back through ATP synthase use that gradient to drive the phosphorylation of ADP to ATP. So the actual ATP made by chemiosmosis is tied to oxidative phosphorylation, with electron carriers like NADH and FADH2 supplying the electrons that power the chain.

Glycolysis makes a small amount of ATP by substrate-level phosphorylation in the cytosol and doesn’t rely on a membrane-proton gradient. The citric acid cycle also yields some substrate-level ATP and generates NADH/FADH2 to feed the chain, but the cycle itself isn’t where chemiosmotic ATP production occurs. Fermentation occurs without a membrane gradient and uses substrate-level phosphorylation to make some ATP, not chemiosmosis.