Final answer:
The statement is false; it is calcium ions, not chloride, that bind to troponin to initiate muscle contraction by uncovering the myosin-binding sites on actin for cross-bridge formation. so, option b is the correct answer.
Step-by-step explanation:
The statement that chloride binds to troponin which then undergoes a conformational change, pulling tropomyosin off of the myosin-binding sites on actin, is false. In reality, it is calcium ions that bind to troponin during muscle contraction. When calcium is present, it binds to troponin and triggers the conformational changes required for tropomyosin to move away from the myosin-binding sites on actin filaments. As a result, myosin is allowed to form cross-bridges with actin, which is essential for muscle contraction.
To enable a muscle contraction, tropomyosin must change conformation, uncovering the myosin-binding site on an actin molecule and allowing cross-bridge formation. This can only happen in the presence of calcium, which is kept at extremely low concentrations in the sarcoplasm. If present, calcium ions bind to troponin, causing conformational changes in troponin that allow tropomyosin to move away from the myosin binding sites on actin. Once the tropomyosin is removed, a cross-bridge can form between actin and myosin, triggering contraction. Cross-bridge cycling continues until Ca²+ ions and ATP are no longer available and tropomyosin again covers the binding sites on actin.