During the sliding filament theory, when does the myosin head cock back to store energy for the next cycle?

• A. After ATP binds to the myosin head and is hydrolyzed to ADP and Pi
• B. When calcium ions bind to troponin
• C. During the power stroke when ADP is released
• D. When the myosin head detaches from actin

Answer: (A)

Verified step by step guidance

1. Understand the sliding filament theory: This theory explains how muscles contract by the interaction of actin and myosin filaments within the muscle cell. The myosin heads bind to actin, pull the actin filaments, and then release to repeat the cycle.
2. Identify the role of ATP in the cycle: ATP binds to the myosin head, causing it to detach from the actin filament. This is a crucial step in the cycle as it allows the myosin head to reset for another contraction.
3. Recognize the hydrolysis of ATP: Once ATP is bound to the myosin head, it is hydrolyzed into ADP and inorganic phosphate (Pi). This reaction releases energy, which is used to 'cock' the myosin head back into a high-energy state, ready for the next power stroke.
4. Consider the role of calcium ions: Calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the binding sites on actin, allowing myosin heads to attach to actin and initiate the power stroke.
5. Sequence the events: After the power stroke, ADP is released, and a new ATP molecule binds to the myosin head, causing it to detach from actin. The ATP is then hydrolyzed, which cocks the myosin head back, storing energy for the next cycle.