During the power stroke, what happens?

During the power stroke, what happens?

• A. The cross-bridges detach from actin
• B. Calcium is pumped back into the sarcoplasmic reticulum
• C. The cross-bridges change shape and pull actin toward the center
• D. ATP is synthesized

Answer: (C)

During the power stroke, the myosin heads that are attached to actin pivot and pull the actin filaments toward the center of the sarcomere, shortening the muscle. This action uses the energy from ATP that was hydrolyzed earlier, with phosphate release driving the movement. The cross-bridge detaches only after another ATP binds, and the cycle resets so another power stroke can occur. Calcium’s role is to expose the binding sites on actin by moving tropomyosin, enabling cross-bridge formation, while calcium reuptake into the sarcoplasmic reticulum happens during relaxation, not the power stroke. ATP synthesis isn’t part of this step; ATP is consumed to power detachment and re-cocking of the myosin head.

During the power stroke, the myosin heads that are attached to actin pivot and pull the actin filaments toward the center of the sarcomere, shortening the muscle. This action uses the energy from ATP that was hydrolyzed earlier, with phosphate release driving the movement. The cross-bridge detaches only after another ATP binds, and the cycle resets so another power stroke can occur. Calcium’s role is to expose the binding sites on actin by moving tropomyosin, enabling cross-bridge formation, while calcium reuptake into the sarcoplasmic reticulum happens during relaxation, not the power stroke. ATP synthesis isn’t part of this step; ATP is consumed to power detachment and re-cocking of the myosin head.