What is the role of tropomyosin in muscle contraction?

• A. Tropomyosin forms cross-bridges with myosin heads during contraction.
• B. Tropomyosin binds to calcium ions, initiating the contraction process.
• C. Tropomyosin hydrolyzes ATP to provide energy for muscle contraction.
• D. Tropomyosin blocks the myosin-binding sites on actin filaments, preventing contraction until calcium ions are present.

Answer: (D)

Verified step by step guidance

1. Understand the structure of muscle fibers: Muscle fibers contain myofibrils, which are composed of repeating units called sarcomeres. Sarcomeres contain actin (thin filaments) and myosin (thick filaments) that interact during muscle contraction.
2. Identify the role of tropomyosin: Tropomyosin is a regulatory protein that is part of the thin filament structure. It is positioned along the actin filaments and plays a crucial role in regulating muscle contraction.
3. Explore the function of tropomyosin in muscle contraction: Tropomyosin covers the myosin-binding sites on actin filaments, preventing myosin heads from attaching to actin and forming cross-bridges. This blockage prevents muscle contraction in the absence of calcium ions.
4. Learn about the role of calcium ions: When a muscle cell is stimulated, calcium ions are released from the sarcoplasmic reticulum into the cytoplasm. Calcium ions bind to another regulatory protein called troponin, which causes a conformational change in tropomyosin.
5. Understand the process of contraction initiation: The binding of calcium ions to troponin causes tropomyosin to move away from the myosin-binding sites on actin. This exposure allows myosin heads to attach to actin, forming cross-bridges and initiating muscle contraction.