What role do calcium ions and ATP play in the process of muscle contraction?

• A. Calcium ions and ATP both bind to myosin, causing it to detach from actin and relax the muscle.
• B. Calcium ions are responsible for breaking down ATP, while ATP binds to actin to initiate contraction.
• C. Calcium ions provide energy for the power stroke, while ATP binds to troponin to expose binding sites on actin.
• D. Calcium ions bind to troponin, allowing myosin to bind to actin, while ATP provides energy for the power stroke.

Answer: (D)

Verified step by step guidance

1. Understand the basic mechanism of muscle contraction: Muscle contraction involves the interaction between actin and myosin filaments within the muscle fibers. This process is regulated by calcium ions and ATP.
2. Calcium ions play a crucial role in muscle contraction by binding to troponin. Troponin is a regulatory protein associated with the actin filament. When calcium ions bind to troponin, it causes a conformational change that moves tropomyosin away from the binding sites on actin.
3. Once the binding sites on actin are exposed, myosin heads can attach to these sites, forming cross-bridges. This is a critical step in the contraction process.
4. ATP is essential for the power stroke of muscle contraction. When ATP binds to the myosin head, it is hydrolyzed to ADP and inorganic phosphate, providing the energy needed for the myosin head to pivot and pull the actin filament, resulting in muscle contraction.
5. After the power stroke, a new ATP molecule binds to the myosin head, causing it to detach from actin. This detachment is necessary for the cycle to repeat, allowing continuous muscle contraction as long as calcium ions and ATP are available.