The roles of calcium ions and tropomyosin in the cycle of actinomyosin bridge formation
Calcium ions remove a blocking protein called tropomyosin from actin filaments. This exposes binding sites and allows myosin heads to attach and pull the filament, causing contraction.
Real World
Rigor mortis occurs after death because ATP production stops, so myosin heads cannot detach from actin — the muscles lock in place, demonstrating how the cross-bridge cycle depends on both calcium and ATP.
Exam Focus
Always state that calcium ions cause tropomyosin to move, exposing binding sites — never say calcium 'activates' actin directly.
Evaluation Scaffold
A four-step framework for high-quality evaluation. Use this for 'assess', 'evaluate', and 'to what extent' questions.
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