Final answer:
ATP is essential for both orienting myosin for the power stroke and breaking the actin-myosin bond afterward, allowing muscle contraction and relaxation cycles.
Step-by-step explanation:
The molecule that is used both to orient myosin into the correct position for the power stroke and to break the bond between actin and myosin after the power stroke is ATP (adenosine triphosphate). When the myosin head is 'cocked,' it is in a high-energy configuration. During the power stroke, this energy is released as the myosin head moves, pulling the actin filament toward the center of the sarcomere. This movement is crucial for muscle contraction. After the power stroke, ADP is released but the actin-myosin cross-bridge remains intact. Binding of another ATP molecule to myosin is then required for the cross-bridge to detach, allowing the muscle to relax and setting the stage for another cycle of contraction when more ATP is available.