Muscle Contractions

Which step of cross-bridge cycling is considered the power stroke?A. when the myosin cross-bridge formsB. when the phosphate is released and the myosin cross-bridge moves to the right in the animationC. when ATP binds to the myosin head and releases it from the thin filamentD. when ATP is hydrolyzed and the myosin head moves to the left in the animation

If ATP was removed from the cell, would the muscle be stiff or flaccid?A. It would be stiff because the myosin head would be unable to release the actin binding site.B. It would be flaccid because without ATP hydrolysis, the step when phosphate is released and the myosin head binds, the actin could not occur.C. It would be stiff because the lack of ATP would cause Ca2+ levels to rise, resulting in contraction.D. It would be flaccid because ATP is needed for muscle contraction.

What happens to ATP hydrolysis products after the hydrolysis of ATP by the myosin head, but before the myosin head contacts the actin filament?A. They are further degraded.B. They are released to the cytoplasm.C. The ADP is degraded, while the Pi remains bound to the myosin head.D. They remain attached to the myosin head.E. The Pi group is degraded, while the ADP remains bound to the myosin head.

Why is Ca2+ important to the process of sarcomere shortening?A. Ca2+ binds to the troponin complex, which results in exposure of the myosin binding sites on actin.B. Ca2+ moves the tropomyosin molecule, which results in exposure of the myosin binding sites on actin.C. Ca2+ binds to the phosphate and helps to remove it from the myosin head.D. Ca2+ is a cofactor in the binding and hydrolysis of ATP.

When is the force of contraction of the muscle strongest?A. when there is maximal overlap between the thick and thin filamentsB. when Ca2+ levels are highest, ensuring maximal actin/myosin interactionC. when ATP levels are highestD. when the sarcomeres are stretched the most and the potential for shortening is the greatest