Why is ATP required for both contraction and relaxation of skeletal muscle?

• A. To power the active transport of calcium into mitochondria.
• B. To detach myosin from actin and to pump Ca2+ back into the sarcoplasmic reticulum.
• C. To initiate calcium release from the sarcoplasmic reticulum.
• D. To convert ADP into ATP for sustaining contraction.

Answer: (B)

ATP is needed for both contraction and relaxation because it fuels the two critical steps that reset and reset the cycle. For contraction, the myosin heads use energy from ATP hydrolysis to enter a high-energy state and then bind actin to pull the filaments—this cycling only continues if a new ATP molecule can attach to myosin to detach it after each power stroke, allowing repeated cycles. Without ATP, myosin would stay bound to actin and force production would halt or become stuck in a rigid state. For relaxation, ATP powers the SERCA pumps that move calcium ions from the cytosol back into the sarcoplasmic reticulum, lowering cytosolic Ca2+ and allowing cross-bridges to detach and the muscle to relax. Note that initiating calcium release from the SR is triggered by electrical signals, not directly by ATP, and simply regenerating ATP from ADP isn’t the immediate mechanism by which contraction is sustained in each cycle.