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.
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Calcium ion is necessary to activate the binding sites on the actin molecules.

The molecule ATP is required each time the mysoin head detached from one sites and reattached.

Explanation:

Depolarization of sarcomere recticulum causes it to release calcium ion. Calcium ion then bind to troponin and reconfigure the complex exposing the binding sites to myosin head on the actin filaments.

The cellular importance of motor is to deliver ATP dependent force impulse to actin molecule.

When ATP molecule bind, the mysoin head detaches from old binding sites of actin, reattaches to new actin monomer. When ATP is hydrolysed, it changes conformation causing actin to slide in respect to myosin.

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.