In oxidative phosphorylation, which process directly drives the conversion of ADP to ATP by ATP synthase?

• A. Diffusion of $O_2$ across the inner mitochondrial membrane to bind cytochrome c
• B. Proton flow down the electrochemical gradient ($H^{+}$) through ATP synthase (chemiosmosis)
• C. Direct transfer of a phosphate group from NADH to ADP (substrate-level phosphorylation)
• D. Hydrolysis of ATP to ADP coupled to electron transport

Answer: (B)

Verified step by step guidance

1. Understand that ATP synthase produces ATP by using energy derived from a proton gradient across the inner mitochondrial membrane.
2. Recognize that during oxidative phosphorylation, electron transport through complexes I-IV pumps protons (H\(\textsuperscript{+}\)) from the mitochondrial matrix to the intermembrane space, creating an electrochemical gradient.
3. Identify that the flow of protons back into the matrix through ATP synthase (down their electrochemical gradient) provides the energy needed for the phosphorylation of ADP to ATP; this process is called chemiosmosis.
4. Note that oxygen (O\(\textsubscript{2}\)) acts as the final electron acceptor in the electron transport chain but does not directly drive ATP synthesis by diffusion across the membrane.
5. Distinguish that substrate-level phosphorylation and ATP hydrolysis are different processes and do not directly power ATP synthase during oxidative phosphorylation.