Question

A muscle contracts by

a. Calcium binding to troponin which moves the tropomyosin that otherwise blocks the interaction of actin and myosin
b. Calcium binding to tropomyosin which moves the troponin that otherwise blocks the interaction of actin and myosin
c. Calcium binds to actin allowing its association with myosin
d. None of the above

Answer 1

In muscle contraction, calcium binding to troponin causes conformational changes that allow tropomyosin to move away from the myosin binding sites on actin, enabling the formation of cross-bridges and muscle contraction.

Step-by-step explanation:

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.

Excitation-Contraction Coupling