During the contraction of a vertebrate skeletal muscle fiber, calcium ions
a. break cross-bridges by acting as a cofactor in the hydrolysis of ATP.
b. spread action potentials through the T tubules.
c. re-establish the polarization of the plasma membrane following an action potential.
d. transmit action potentials from the motor neuron to the muscle fiber.
e. bind with troponin, changing its shape so that the myosin-binding sites on actin are exposed.

Contraction of a vertebrate skeletal muscle fiber is initiated when calcium ion binds to the troponin, a complex fibre protein. Binding of calcium allows the exposure of active-binding sites present on the actin. The uncovereed actin-biding sites allows the the high-energy myosin head to bridge the gap forming cross bridge. the cross-bridge than support release of ADP and synthesis of ATP for contraction.

Hence, the correct answer is e.

marianaegarciaperedo marianaegarciaperedo · Answer 2 · 2020-05-05T06:26:04+02:00

Answer:

During the contraction of a vertebrate skeletal muscle fiber, calcium ions bind with troponin, changing its shape so that the myosin-binding sites on actin are exposed. (Option e)

Explanation:

In rest, attraction strengths between myosin and actin filaments are inhibited by the tropomyosin. When the muscle fiber membrane depolarizes, the action potential caused by this depolarization enters the t-tubules depolarizing the inner portion of the muscle fiber. This activates calcium channels in the T tubules membrane and releases calcium into the sarcolemma. At this point, tropomyosin is obstructing binding sites for myosin on the thin filament. When calcium binds to the troponin C, the troponin T alters the tropomyosin by moving it and then unblocks the binding sites. Myosin heads bind to the uncovered actin-binding sites forming cross-bridges, and while doing it ATP is transformed into ADP and inorganic phosphate which is released. Myofilaments slide impulsed by chemical energy collected in myosin heads, producing a power stroke. The power stroke initiates when the myosin cross-bridge binds to actin. As they slide, ADP molecules are released. A new ATP links to myosin heads and breaks the bindings to the actin filament. Then ATP splits into ADP and phosphate, and the energy produced is accumulated in the myosin heads, which starts a new binding cycle to actin. Z-bands are then pulled toward each other, thus shortening the sarcomere and the I-band, and producing muscle fiber contraction.