The power stroke describes: All of these choices are correct. a) the cocking of the myosin head by hydrolysis of ATP. b) the cycle of events involved in muscle contraction. c) the binding of the myosin head to actin. d) the pivoting of the myosin head, which causes actin and myosin to slide relative to each other.

The correct answer is: D) the pivoting of the myosin head, which causes actin and myosin to slide relative to each other.

Explanation:

Muscle contraction occurs when the sarcomere is shortened. The sarcomere is a structure composed of filaments: the thick ones are composed of myosin, and the thin ones are composed of actin. These two proteins interact with one another to make the sarcomere shorten and produce the contraction of the muscle.

Muscle, in its natural state, is not contracted. This is because the binding sites on actin for the myosin heads are covered by the proteins troponin and tropomyosin.

When the signal for muscle contraction arrives at the muscle, calcium ions are released from the sarcoplasmic reticulum and reach the sarcomere to reconfigure the complex formed by troponin and tropomyosin so the binding site on the actin filament becomes exposed and the myosin head can attach to it.

When an ATP molecule appears and binds to the myosin head, the latter separates from the actin-binding site and moves toward the next actin-binding site with the energy provided by the hydrolysis of this ATP molecule. This is what is called power stroke.

This keeps on happening until muscle contraction is no longer needed and calcium ions are reabsorbed, which will make troponin and tropomyosin cover the actin-binding sites once again.

the answer is d. chordata