Question

complete the following statements.
□ ions are necessary to activate the binding sites on the actin filaments.
the molecule □ is required each time the myosin head detaches from one site and reattaches to another.

Explanation:

1. For the first blank: In muscle contraction, calcium ($Ca^{2+}$) ions bind to troponin, which causes a conformational change that exposes the binding sites on actin filaments for myosin heads. So the ion needed to activate actin filament binding sites is calcium.
2. For the second blank: The detachment and reattachment of the myosin head during the cross - bridge cycle requires ATP (adenosine triphosphate). ATP hydrolysis provides the energy for the myosin head to detach from actin and re - cock, so it can reattach to another actin binding site.

Answer:

First blank: Calcium ($Ca^{2+}$)
Second blank: ATP (Adenosine Triphosphate)