Which of the following best describes the role of calcium ions (\(\mathrm{Ca^{2+}}\)) in the sliding filament model of muscle contraction?

• A. They inhibit the interaction between actin and myosin by blocking the binding sites.
• B. They provide the energy required for the myosin head to detach from actin.
• C. They directly hydrolyze ATP to power the sliding of filaments.
• D. They bind to troponin, causing a conformational change that exposes myosin-binding sites on actin.

Answer: (D)

Verified step by step guidance

1. Understand the sliding filament model of muscle contraction: This model explains how muscles contract by the interaction of actin and myosin filaments. Myosin heads bind to actin filaments, forming cross-bridges, and then pull the actin filaments inward, shortening the muscle fiber.
2. Recognize the role of calcium ions (\(\mathrm{Ca^{2+}}\)): Calcium ions are crucial in regulating the interaction between actin and myosin. They do not directly provide energy or hydrolyze ATP but instead play a regulatory role.
3. Identify the role of troponin and tropomyosin: In a relaxed muscle, tropomyosin blocks the myosin-binding sites on actin. Troponin is a protein complex associated with tropomyosin and actin.
4. Explain how calcium ions interact with troponin: When calcium ions (\(\mathrm{Ca^{2+}}\)) are released from the sarcoplasmic reticulum during muscle contraction, they bind to troponin. This binding causes a conformational change in the troponin-tropomyosin complex.
5. Describe the result of calcium binding: The conformational change in troponin moves tropomyosin away from the myosin-binding sites on actin, exposing these sites. This allows myosin heads to bind to actin, initiating the cross-bridge cycle and muscle contraction.