An organism’s niche includes the range of abiotic and biotic conditions it can tolerate. 

In the case of the cottontail rabbit, its niche includes the ranges of temperature and humidity that it can tolerate, the plants that it eats, and the predatory hawks and coyotes with which it coexists.

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#XL - Q Biochemistry

Sarcomere

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

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#XL - Q Biochemistry

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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#XL - Q Biochemistry

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

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#XL - Q Biochemistry

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