[Campbell Biology P.273] Calcium ions Ca2+ are versatile second messengers in signal transdu... | Practice Question

Calcium ions Ca2+ are versatile second messengers in signal transduction pathways. Based on the provided text, which statement accurately describes a characteristic of Ca2+ in this role?

  • A: The cytosolic Ca2+ concentration is normally much higher than in the extracellular fluid.
  • B: Ca2+ is actively transported out of the cell and into the endoplasmic reticulum by protein pumps.
  • C: Changes in cytosolic Ca2+ levels only trigger a narrow range of cellular responses.
  • D: Ca2+ functions as a second messenger only in pathways triggered by receptor tyrosine kinases.

Explanation

The text states that calcium ions are actively transported out of the cell and are actively imported from the cytosol into the endoplasmic reticulum and, under some conditions, into mitochondria and chloroplasts by various protein pumps. This maintains the low cytosolic concentration necessary for Ca2+ to function as an effective second messenger.