Final answer:
ATP hydrolysis enables the contraction and relaxation phases in skeletal muscle by changing the myosin head's conformation, allowing it to bind and subsequently release from actin. Without ATP, myosin heads cannot detach from actin, preventing muscle relaxation and leading to rigor mortis upon death.
Step-by-step explanation:
ATP hydrolysis is critical for skeletal muscle contraction. During the contraction cycle, the ATPase enzyme on the myosin head hydrolyzes ATP to ADP and inorganic phosphate. This process releases energy and changes the angle of the myosin head into a “cocked” position, storing potential energy. When the myosin head binds to actin, this energy is released through the power stroke, moving actin filaments and resulting in muscle contraction.
Furthermore, ATP is also necessary for muscle relaxation. After the power stroke, a new molecule of ATP binds to the myosin head, causing it to detach from actin, which allows the muscle to relax. Without ATP, myosin heads cannot release from actin, leading to a state known as rigor mortis. ATP is therefore indispensable for both the contraction and relaxation phases of muscle activity.
We can observe the importance of ATP in muscle contractions during glycerinated muscle fiber experiments, which illustrate that fibers require ATP and calcium to contract and lift weight, despite losing most soluble cytoplasmic components.