Which molecule donates a phosphate to resynthesize ATP in the ATP-CP system after it has been used?

• A. Adenosine triphosphate
• B. Creatine phosphate
• C. Lactic acid
• D. Oxygen

Answer: (B)

During short, high-intensity efforts, the body regenerates ATP quickly through the ATP-CP system. The phosphate donor in this rapid resynthesis is creatine phosphate (phosphocreatine). When ATP is used, a phosphate group is transferred from creatine phosphate to ADP to form ATP, a reaction catalyzed by creatine kinase: phosphocreatine + ADP → ATP + creatine. This provides immediate energy without requiring oxygen or glycolysis. Lactic acid comes from glycolysis if activity continues, and oxygen isn’t needed for this immediate transfer. So the molecule donating the phosphate is creatine phosphate.