Final answer:
Calcium ions can travel through the cell membrane and raise the cytoplasmic concentration using integral proteins and ligand-gated calcium ion channels.
Step-by-step explanation:
Calcium ions, which are positively charged ions, can travel through the cell membrane using integral proteins. The concentration of calcium ions is higher inside the cell compared to outside. When signaling occurs, ligand-gated calcium ion channels allow the higher levels of calcium ions outside the cell to flow into the cytoplasm, which raises the concentration of cytoplasmic calcium ions. This influx of calcium ions can trigger various responses depending on the cell type, such as the release of insulin or muscle contractions.