sentence: Many cells have 'calcium channels' on their surface.
sentences_before: It is important to know how cells work.
sentences_after: These are openings where calcium ions can enter the cell. The cell is told to act and it opens the channels. Once in the cell calcium ions activates many proteins to do specific things. When it goes into nerve cells, it triggers electrical impulses that send a messages. When it goes into white blood cells it makes them fight germs.
concept_name: cell
headings: Calcium Occurrence Calcium in cells
categories: Alkaline earth metals