In muscle contraction, which statement best describes the role of tropomyosin?

• A. Calcium binds troponin complex to trigger contraction.
• B. Tropomyosin moves to expose myosin-binding sites on actin.
• C. Troponin directly powers contraction.
• D. Calcium acts elsewhere, not affecting cross-bridge formation.

Answer: (B)

Tropomyosin acts as a regulatory gate on the actin filament, blocking the myosin-binding sites when the muscle is relaxed. When calcium levels rise, calcium binds to troponin C, causing the troponin–tropomyosin complex to shift and move tropomyosin away from those sites. With the binding sites exposed, myosin heads can attach to actin and form cross-bridges, using ATP to power the contraction. When calcium is removed, tropomyosin returns to block the sites and relaxation occurs. So the key idea is that tropomyosin moves to expose the myosin-binding sites on actin, enabling contraction.