Final answer:
The signal transduction pathway involving the second messenger Ca²⁺ begins with the binding of a ligand to a cell-surface receptor. Second messengers, like Ca²⁺, transmit the signal within the cell by activating specific proteins in a chain reaction. Phosphorylation by kinases also contributes to signal transduction.
Step-by-step explanation:
The signal transduction pathway involving the second messenger Ca²⁺ starts with the binding of the ligand to a cell-surface receptor. Once the ligand binds, the intracellular components of the receptor are activated, initiating a signaling pathway or cascade. In this pathway, second messengers like Ca²⁺ transmit the signal within the cell by interacting with specific proteins and activating them in a chain reaction. Phosphorylation also plays a role in signal transduction, where enzymes known as kinases add phosphate groups to proteins, changing their shapes and activating or inactivating them.