Final answer:
Cytosolic calcium ion levels can increase when ligand-gated calcium ion channels are activated allowing calcium influx, or when G protein-activated PLC leads to IP3 producing calcium mobilization from the endoplasmic reticulum.
Step-by-step explanation:
The mechanism that can cause an increase in cytosolic calcium ion levels in response to an appropriate stimulus includes the activation of ligand-gated calcium ion channels and the use of calcium ions as a second messenger in various signaling pathways. When a signal occurs, these channels open and allow the influx of calcium ions from outside the cell or from intracellular stores like the endoplasmic reticulum into the cytosol, which raises cytoplasmic Ca²⁺ levels. This increase can lead to various cellular responses, such as the release of insulin from pancreatic β-cells or muscle contractions. Additionally, in some signaling pathways, G proteins activate the enzyme phospholipase C (PLC), which produces inositol triphosphate (IP3) that triggers the release of stored calcium from the endoplasmic reticulum into the cytoplasm. Calcium can then activate enzymes directly or bind to proteins like calmodulin that modulate other protein kinases within the cell.