Calcium ions start muscle contraction by binding to
- A. the myosin heads
- B. tropomyosin directly
- C. troponin, which moves tropomyosin off the binding sites on actin
- D. ATP, activating it
Explanation
In a relaxed fibre tropomyosin lies across the myosin binding sites of actin. Calcium released from the sarcoplasmic reticulum binds troponin, troponin changes shape and pulls tropomyosin aside, and only then can cross bridges form. Calcium therefore acts as the switch rather than as the energy source, and pumping it back into the reticulum is what causes relaxation.
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