Which molecule triggers skeletal muscle contraction when bound with calcium ions?

The correct answer is troponin. Troponin is a regulatory protein found in skeletal muscle that plays a crucial role in muscle contraction. In the absence of calcium ions, troponin holds tropomyosin in place on the actin filaments, blocking the binding sites needed for myosin to attach, thus preventing contraction. When calcium ions are released into the muscle cell, they bind to troponin, causing a conformational change that allows tropomyosin to move away from the actin binding sites. This exposure permits myosin heads to bind to actin, leading to muscle contraction through the cross-bridge cycle.

Calmodulin is not the correct answer because it primarily functions in smooth muscle contraction and other signaling pathways rather than in the skeletal muscle contraction mechanism. Myosin and actin are essential components of the contractile apparatus but do not directly trigger contraction; myosin is the motor protein responsible for the pulling action, while actin serves as the filament that myosin pulls against. Therefore, troponin is key in linking calcium ion concentration to the mechanics of muscle contraction in skeletal muscle.