Final answer:
Calcium ion (Ca²+) has the highest cytosolic concentration among the given ions.
Step-by-step explanation:
The ion with the highest cytosolic concentration is Ca²+, which is the calcium ion. Calcium ions play a crucial role in many cellular processes, including muscle contractions and hormone release.
Within a cell, the cytosolic concentration of Ca²+ is very low due to ion pumps continuously removing it. However, when signaling occurs, ligand-gated calcium ion channels allow the higher levels of Ca²+ present outside the cell (or in intracellular storage compartments) to flow into the cytoplasm, raising the concentration of cytoplasmic Ca²+.
Examples of the role of Ca²+ in cellular functions include its involvement in muscle contractions, where an increase in cytoplasmic Ca²+ triggers the contraction of muscle fibers. In beta-cells of the pancreas, an increase in cytoplasmic Ca²+ leads to the release of insulin. Therefore, among the given ions, Ca²+ has the highest cytosolic concentration.