The phosphagen system is characterized by which statement?

• A. It relies on fatty acids as the main energy source
• B. It uses phosphocreatine to rapidly regenerate ATP in the cytosol of muscle cells
• C. It requires oxygen to function
• D. It produces lactic acid as its main byproduct

Answer: (B)

The key idea here is that immediate, high-power energy comes from the phosphagen system by transferring a phosphate from phosphocreatine to ADP to rapidly form ATP in the cytosol of muscle cells. This reaction, driven by the enzyme creatine kinase, happens without oxygen and can sustain maximal effort for only a short time (roughly 10 seconds or less) before other energy systems take over as phosphocreatine stores are depleted. It doesn’t rely on fatty acids as a primary fuel, and its main byproduct isn’t lactic acid—lactate is more a product of anaerobic glycolysis from glucose, not the phosphagen pathway.