During cellular metabolism or in response to pathologic conditions, cells maintain electroneutrality with cotransporters. If a positively charged molecule moves into the cell (e.g. Na+), a positively charged molecule (e.g. K+) must move out (or vice versa). Alternatively, the positively charged molecule can move in with a negatively charged molecule (e.g. NaCl or NaHCO3), which also maintains electroneutrality (pluses = minuses). Although the above image shows these transporters in one direction, they actually can move the pictured anions and cations in both directions (into or out of cells).
