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

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)

Substrate-level phosphorylation is the direct transfer of a phosphate group from a high-energy substrate to ADP to form ATP, without using a proton gradient or ATP synthase. In cellular respiration this happens in two main places: glycolysis in the cytosol and the citric acid (Krebs) cycle in the mitochondrial matrix. In glycolysis, enzymes transfer a phosphate from intermediates such as 1,3-bisphosphoglycerate or phosphoenolpyruvate to ADP, yielding ATP directly. In the Krebs cycle, succinyl-CoA synthetase transfers a phosphate from succinyl-CoA to ADP (or GDP to GTP, depending on the organism) to produce ATP (or GTP) directly. This contrasts with oxidative phosphorylation, which relies on a proton gradient to drive ATP synthase. The statement correctly identifies the mechanism and the locations where it occurs—in glycolysis and the Krebs cycle.

Substrate-level phosphorylation is the direct transfer of a phosphate group from a high-energy substrate to ADP to form ATP, without using a proton gradient or ATP synthase. In cellular respiration this happens in two main places: glycolysis in the cytosol and the citric acid (Krebs) cycle in the mitochondrial matrix. In glycolysis, enzymes transfer a phosphate from intermediates such as 1,3-bisphosphoglycerate or phosphoenolpyruvate to ADP, yielding ATP directly. In the Krebs cycle, succinyl-CoA synthetase transfers a phosphate from succinyl-CoA to ADP (or GDP to GTP, depending on the organism) to produce ATP (or GTP) directly. This contrasts with oxidative phosphorylation, which relies on a proton gradient to drive ATP synthase. The statement correctly identifies the mechanism and the locations where it occurs—in glycolysis and the Krebs cycle.