Which energy-generating mechanism can function both aerobically and anaerobically?

• A. Oxidative Phosphorylation
• B. Substrate-level Phosphorylation
• C. Fermentation
• D. Krebs Cycle

Answer: (B)

Substrate-level phosphorylation is the energy-generating process that can run with or without oxygen. It makes ATP directly by transferring a phosphate from a high-energy substrate to ADP, without needing an electron transport chain. In glycolysis, there are two key steps where ATP is produced this way: a phosphate is transferred from a high-energy sugar phosphate to ADP, and later from phosphoenolpyruvate to ADP to form ATP. This happens whether oxygen is present or not, so ATP can be generated under anaerobic conditions as glycolysis proceeds.

When oxygen is available, cells do have a much larger ATP yield from oxidative phosphorylation, but the substrate-level steps still occur and contribute additional ATP. The Krebs cycle also includes a substrate-level phosphorylation step, producing a small amount of ATP (as GTP in some cells). However, the Krebs cycle relies on continuous reoxidation of NADH and FADH2 through the electron transport chain, which in aerobic organisms requires oxygen; without that, the cycle stalls. Other processes like fermentation generate ATP without oxygen but do not function when oxygen is abundant in the same way as the direct, oxygen-independent substrate-level steps.

So the mechanism that can function in both aerobic and anaerobic conditions is substrate-level phosphorylation.