During respiration, most ATP is formed as a direct result of the net movement of:

• A. carbon dioxide out of the cell
• B. glucose into the mitochondria
• C. electrons from NADH to oxygen
• D. protons (H⁺) down their concentration gradient through ATP synthase

Answer: (D)

Verified step by step guidance

1. Understand the process of cellular respiration, specifically oxidative phosphorylation, where ATP is synthesized in the mitochondria.
2. Recall that during oxidative phosphorylation, the electron transport chain creates a proton (H⁺) gradient across the inner mitochondrial membrane by pumping protons into the intermembrane space.
3. Recognize that this proton gradient represents potential energy, which is used to drive ATP synthesis as protons flow back into the mitochondrial matrix through ATP synthase.
4. Note that ATP synthase is an enzyme that harnesses the energy from the movement of protons down their concentration gradient to catalyze the conversion of ADP and inorganic phosphate (Pi) into ATP.
5. Conclude that the net movement of protons (H⁺) down their concentration gradient through ATP synthase is the direct mechanism by which most ATP is formed during respiration.