Which statement best describes the difference between oxidative phosphorylation and substrate-level phosphorylation?

• A. Oxidative phosphorylation occurs in the cytoplasm, while substrate-level phosphorylation occurs in the mitochondria.
• B. Both processes occur in the cytoplasm and produce the same amount of ATP.
• C. Substrate-level phosphorylation requires oxygen, while oxidative phosphorylation does not.
• D. Oxidative phosphorylation uses a proton gradient, while substrate-level phosphorylation directly transfers a phosphate group.

Answer: (D)