Energy Reactions in Cells Quiz

See Creatine Phosphate. Option D is correct. In resting muscle there is spare ATP, and its energy is passed to creatine, a small molecule made in the body, to form creatine phosphate (also called phosphocreatine). This acts as a store of energy in a phosphate bond. When the muscle suddenly starts to contract and uses ATP very quickly, the enzyme creatine kinase transfers the phosphate from creatine phosphate straight back onto ADP to make ATP again. The reaction is extremely fast, so creatine phosphate keeps the ATP level steady during the first few seconds of contraction, for example when sprinting or lifting something heavy. However, the store is small and lasts only about 15 seconds of intense effort. After that, the muscle depends on anaerobic glycolysis (breaking down glucose without oxygen) and aerobic respiration (using oxygen) to regenerate ATP. So creatine phosphate is best thought of as a quick-release battery for ATP, not a fuel in its own right.

• Store: creatine phosphate holds energy in a phosphate bond, made from spare ATP and creatine in resting muscle.
• Enzyme: creatine kinase transfers the phosphate straight back onto ADP to make ATP.
• Speed: the reaction is extremely fast, so ATP stays steady in the first few seconds of contraction.
• Limit: the store is small and lasts only about 15 seconds of intense effort.