This is consistent with the hard nature of the calcium ion.

Bones contain most of the calcium ion in the human body.

These are openings where calcium ions can enter the cell.

To do its work properly, calcium ion must remain within a narrow range of concentration.

In this stage, calcium ions are critical for development.

These channels are selective, allowing only calcium ion to pass.

As with Aplysia, a key player in this process appears to be calcium ions.

Calcium ions are required for each cycle of the sarcomere.

If a cell gets more calcium ion than it needs it can die.

Once in the cell calcium ions activates many proteins to do specific things.