Biology / Bio 3119 · Atom · ~20 seconds
Calcium, Troponin, and Tropomyosin
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At rest, tropomyosin blocks actin's myosin-binding sites; released calcium binds troponin, shifting tropomyosin aside and exposing those sites so the sliding cycle, per Bio 3115, can begin.
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Calcium removal from the cytoplasm lets tropomyosin snap back over actin's sites, ending contraction.
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Calcium enables contraction by exposing binding sites — it does not itself generate the pulling force.