Explain substrate-level phosphorylation and where it occurs in cellular respiration.

• A. Substrate-level phosphorylation transfers a phosphate from a substrate to ADP to form ATP; it occurs in glycolysis and the Krebs cycle.
• B. Substrate-level phosphorylation uses a proton gradient to generate ATP.
• C. Substrate-level phosphorylation occurs only in mitochondria and requires oxygen.
• D. Substrate-level phosphorylation converts ATP to ADP and phosphate.

Answer: (A)