Which of the following molecules binds to troponin during muscle contraction, triggering tropomyosin to move away from the actin-binding sites and allowing the myosin head to form a cross-bridge?

Explanation

During muscle contraction, the calcium gates of the smooth endoplasmic reticulum open releasing calcium into the cytosol. When a muscle is at rest the tropomyosin is disposed in such a way that it covers the sites on the actin chain where the heads of myosin become attached. when calcium ions bind with the troponin molecules they cause them to move slightly. This has the effect of displacing the tropomyosin and exposing the binding sites for the myosin head. once the myosin head has become attached to the actin filament, ATP is hydrolysed and cross bridges are broken down.