How does glycolysis produce ATP?

• A. Substrate-level phosphorylation
• B. Oxidative phosphorylation
• C. Electron transport
• D. Photophosphorylation

Answer: (A)

Glycolysis generates ATP through substrate-level phosphorylation, a direct transfer of a high-energy phosphate from an intermediate of glycolysis to ADP to form ATP. In glycolysis, this happens in two steps: first, the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate transfers a phosphate to ADP to make ATP; then, phosphoenolpyruvate donates its phosphate to ADP via pyruvate kinase, making another ATP molecule. This mechanism relies on high-energy phosphate groups carried by glycolytic intermediates, not on a membrane-bound proton gradient. Oxidative phosphorylation uses electron transport chains and a proton motive force to drive ATP synthesis and occurs in mitochondria, while photophosphorylation happens in chloroplasts during photosynthesis. Glycolysis can proceed without oxygen, but its ATP yield comes specifically from substrate-level phosphorylation rather than oxidative phosphorylation.