Question

when calcium ion binds to troponin, myosin shortens. muscle relaxation occurs. tropomyosin rolls away from the active site. actin heads will bind to myosin. active sites on the myosin are exposed.

Explanation:

In muscle contraction, when calcium ions bind to troponin, it causes a conformational change in the troponin - tropomyosin complex. This change moves tropomyosin away from the active sites on actin, allowing myosin heads to bind to actin. Myosin does not shorten directly due to calcium - troponin binding. Muscle relaxation occurs when calcium is removed. Actin heads do not bind to myosin until tropomyosin is moved. Active sites are on actin, not myosin.

Answer:

tropomyosin rolls away from the active site.