Which statement best describes substrate-level phosphorylation?

Which statement best describes substrate-level phosphorylation?

• A. It directly synthesizes ATP by transferring a phosphate from a substrate to ADP.
• B. It uses a proton gradient across the membrane.
• C. It requires oxygen to occur.
• D. It occurs in the light-dependent reactions of photosynthesis.

Answer: (A)

Substrate-level phosphorylation is the direct transfer of a phosphate from a high-energy substrate to ADP to form ATP. This happens in a single enzymatic step without needing a proton gradient or membrane-bound ATP synthase. For example, in glycolysis, a phosphorylated intermediate donates its phosphate to ADP to produce ATP. In the citric acid cycle, succinyl-CoA synthetase performs a similar transfer to generate GTP, which can be turned into ATP. Because it does not rely on an electrochemical gradient, it can occur in low-oxygen or anaerobic conditions. The statements involving proton gradients or the light-dependent photosynthetic reactions describe other ATP-generating mechanisms, not substrate-level phosphorylation.