How do calcium ions initiate contraction in skeletal muscle fibers?

• A. By breaking down ATP to provide energy for muscle contraction.
• B. By directly binding to myosin heads, enabling them to attach to actin filaments.
• C. By increasing the energy supply to muscle fibers, allowing for contraction.
• D. By binding to troponin, causing a conformational change that exposes binding sites on actin filaments.

Answer: (D)

Verified step by step guidance

1. Understand the role of calcium ions in muscle contraction: Calcium ions are crucial for initiating the contraction process in skeletal muscle fibers.
2. Identify the key proteins involved: Troponin, tropomyosin, actin, and myosin are the main proteins involved in muscle contraction.
3. Explain the resting state: In a relaxed muscle, tropomyosin blocks the binding sites on actin filaments, preventing myosin heads from attaching.
4. Describe the role of calcium ions: When a muscle fiber is stimulated, calcium ions are released from the sarcoplasmic reticulum and bind to troponin.
5. Explain the conformational change: The binding of calcium to troponin causes a conformational change that moves tropomyosin away from the binding sites on actin, allowing myosin heads to attach and initiate contraction.