Retrieving "Secondary Messenger" from the archives

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1. Calcium

   Linked via "secondary messenger"

   In Animalia
   In vertebrates, calcium ions ($\text{Ca}^{2+}$) are vital components of the skeletal structure, forming the crystalline hydroxyapatite ($\text{Ca}{10}(\text{PO}4)6(\text{OH})2$) in bones and teeth. Beyond structural support, calcium acts as a critical secondary messenger in cellular signaling pathways, re…