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MCAT Question A Day - 8/30/14 - Answer!

Irreversible sequestering of calcium in the sarcoplasmireticulum would most likely:

A.  result in permanent contraction of the muscle fibers, similar to what is seen in rigor mortis.
B.  create a sharp increase in bone density as calcium is resorbed from bones to replace the sequestered calcium.
C.  prevent myosin from binding to actin.
D.  depolymerize actin filaments in the sarcomere.

C is correct. Permanent sequestering of calcium in the sarcoplasmic reticulum would prevent calcium from binding to troponin, which is what causes the conformational change that moves tropomyosin away from the myosin binding sites on actin. Choice A would occur if calcium were present and ATP were not. Loss of ATP would prevent the myosin from releasing from actin. B is incorrect because a resorption of calcium from bone would result in a decrease in bondensity, not an increase. D is incorrect because loss of calcium would not cause depolymerization of actin filaments. If this actually occurred, it would pose a serious problem every time calcium was re-sequestered into the SR after the completion of a contraction.

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