Which energy pathway operates without oxygen?

• A. Aerobic oxidative system
• B. ATP-PC system
• C. Electron transport chain
• D. Glycolysis

Answer: (B)

When oxygen isn’t available, the body relies on the immediate, oxygen-free energy source stored in muscles: the ATP-PC system. It works by the stored phosphocreatine donating a phosphate group to ADP to quickly reform ATP, with the help of the enzyme creatine kinase. This happens almost instantly and supplies high-power energy for a very short burst—roughly up to ten seconds—perfect for quick sprints, jumps, or explosive lifts. Because it doesn’t require oxygen, it can deliver ATP right away, but it can be exhausted quickly and must be replenished during rest or low-intensity activity when oxygen becomes available again.

Glycolysis can operate without oxygen too, producing ATP anaerobically, but it’s slower to ramp up and ultimately yields energy through different pathways that lead to lactate buildup. The aerobic oxidative system and the electron transport chain, in contrast, require oxygen, so they aren’t the pathways that operate without oxygen.