Which energy system provides immediate energy from stored phosphocreatine?

• A. Glycolytic system
• B. Aerobic system
• C. Phosphagen system
• D. Lipolytic system

Answer: (C)

Immediate energy from stored phosphocreatine comes from the phosphagen system. In muscles, phosphocreatine donates its phosphate to ADP through the enzyme creatine kinase, rapidly forming ATP without using oxygen. This provides energy extremely quickly for very short, high‑intensity efforts, but the system has only a small supply and simple stores are exhausted in roughly 8–12 seconds. Once those stores are depleted, other systems take over to supply ATP for longer work: glycolysis uses glucose or glycogen and can continue energy production for a bit longer, while the aerobic system relies on oxygen to generate ATP more slowly for extended activity. The other options don’t fit because they involve slower processes (glycolysis with carbohydrate breakdown, aerobic oxidative metabolism with oxygen) or fat breakdown (lipolytic system), none of which provide the immediate, oxygen-free burst from phosphocreatine.