Calcium oxalate crystals are present in the vacuoles of many species, and may be distributed evenly throughout the leaf or restricted to specialized cells called idioblasts

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Sarcomere

Within each skeletal muscle cell are many myofibrils consisting of a repeating array of a specialized structure

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When the Ca2+ levels drop:

Myosin LC is dephosphorylated by MLC phosphatase, causing muscle relaxation.

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Phosphorylation of MLC 

unfolds the myosin II, active and can assemble into bipolar filaments for contraction

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The CaM*-Ca2+ complex binds and activates MLC kinase, phosphorylates the myosin LC. 

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When the Ca2+ level rises, Ca2+ binds CaM.

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In vertebrate smooth muscle, phosphorylation of the myosin regulatory light chain activates contraction in Ca2+ dependent manner.