Final answer:
Ca²⁺ functions as a second messenger in cell signaling by increasing intracellular concentration upon stimulation, which leads to the activation of enzymes and signaling pathways without changing the cytosol's net charge.
Step-by-step explanation:
The primary mechanism through which Ca²⁺ functions as a signaling molecule is by acting as a second messenger during cell signaling processes. When signaling occurs, Ca²⁺ concentration in the cytoplasm significantly increases, but this does not involve an increase in net charge within the cytosol as the movement of ions is charge-neutral. Instead, the calcium ions drive various biochemical reactions by activating enzymes or binding to calcium-binding proteins like calmodulin. These interactions initiate a series of events such as phosphorylation cascades, leading to diverse cell-specific responses based on the cell type and the signal received.
One of the pathways through which Ca²⁺ acts involves phospholipase C (PLC). After a hormone binds to its receptor, PLC is activated, resulting in the production of inositol triphosphate (IP3) which then binds to receptors on the endoplasmic reticulum, causing the release of Ca²⁺. This surge in Ca²⁺ concentration can activate Protein Kinase C (PKC), which then further propagates the signal through the phosphorylation of proteins. In other cases, the Ca²⁺ can directly activate enzymatic activity or it may combine with calmodulin to modulate protein kinases. All these actions are essential for the cellular response to the initial signaling event.