Final answer:
Calcium ions binding to troponin is a prerequisite for tropomyosin to move away from the myosin binding sites on actin, which allows muscle contraction to occur through cross-bridge formation, sustained by ATP.
Step-by-step explanation:
Before tropomyosin can move away from the myosin binding sites on actin, a critical step must occur. Calcium ions (Ca++) must bind to troponin, which is part of the troponin-tropomyosin complex attached to the actin filaments. This binding triggers a conformational change in troponin, essentially repositioning tropomyosin, and thereby exposing the myosin binding sites on the actin strands.
When these sites are uncovered, the myosin heads can latch on to them, initiating the formation of cross-bridges. This is required for the subsequent contraction phase, moving the thin filaments past the thick filaments and shortening the muscle. This cross-bridge cycling (the act of myosin heads pulling on actin and then releasing to pull again) is sustained by the continued presence of ATP and calcium.