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Calcium ions serve well as a second messenger because their concentration in the cytosol is normally

  • A.kept very low, so a small influx is a large change
  • B.kept very high, so that any fall at all is easily detected
  • C.identical to the concentration outside the cell
  • D.quite impossible for the cell to regulate at all

Answer: A

Pumps hold cytosolic calcium about ten thousand times below the level outside and in the endoplasmic reticulum, so opening a channel produces a sharp and unmistakable rise. Restoring the low resting level afterwards costs the cell energy.

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