Final answer:
The protein that binds to calcium ions and undergoes a conformational change to initiate muscle contraction is troponin. This event moves tropomyosin and exposes myosin-binding sites on actin, facilitating contraction. The correct answer is Option 3: Troponin; Ca2+.
Step-by-step explanation:
The protein that binds to calcium ions (Ca2+) and undergoes a conformational change to initiate muscle contraction is troponin. This binding event allows the troponin complex to move tropomyosin out of the way, exposing the myosin-binding sites on the actin filaments. Consequently, myosin heads can bind to these newly exposed sites, forming cross-bridges, which is essential for muscle contraction. Therefore, the correct answer to the question is Option 3: Troponin; Ca2+.
Tropomyosin is a protein associated with actin filaments in muscle cells, traditionally covering the binding sites to prevent interaction with myosin heads under resting conditions. When calcium ions bind to troponin, this changes its shape and thereby displaces tropomyosin, revealing the binding sites for the myosin heads on the actin strands that are crucial for muscular contraction.