During oxidative phosphorylation, ATP synthase catalyzes the synthesis of ATP from ADP and inorganic phosphate. What is the direct energy source that powers this enzyme's activity?
Show answer and explanation
The answer
Correct answer: D. The flow of protons (H+) down their electrochemical gradient through the ATP synthase enzyme.
Tested concept: Chemiosmosis and ATP synthase
Explanation
ATP synthase functions by utilizing the energy stored in a proton (H+) electrochemical gradient across the inner mitochondrial membrane. Protons flow down this gradient, through the ATP synthase complex, causing a conformational change that drives the synthesis of ATP from ADP and Pi. This process is known as chemiosmosis. Option B is incorrect because electrons moving through the electron transport chain provide the energy to pump protons, thereby creating the gradient, but they do not directly power ATP synthase. Option C is incorrect because NADH and FADH2 donate electrons to the electron transport chain, initiating the process, but they are not the direct energy source for ATP synthase itself. Option D describes substrate-level phosphorylation, which is a different mechanism of ATP synthesis that does not involve ATP synthase or a proton gradient.
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