Why can Ca2+ act as a second messenger?

• A. It is normally at high levels in the cytoplasm.
• B. It binds directly to DNA.
• C. It is produced by mitochondria.
• D. It is normally at a low level in the cytoplasm.

Answer: (D)

Calcium ions act as second messengers because the cell keeps cytosolic Ca2+ at a very low resting level, creating a strong gradient that allows a rapid and sizable signaling kick when Ca2+ enters the cytoplasm. When a receptor is activated, Ca2+ is released from internal stores or enters from outside, causing a quick spike in cytosolic Ca2+. This transient increase is sensed by Ca2+-binding proteins like calmodulin, which then modulate enzymes, ion channels, and transcription factors to propagate the signal. The system can be rapidly reset by pumps to return Ca2+ to its low resting level, ready for another round of signaling. So the low baseline in the cytoplasm best explains why Ca2+ can function effectively as a second messenger.