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ions carry an electric charge or charges. Those with one or more positive charges are called cations, whereas those with negative charges are called anions. The names arise from considering what happens if a current is passed through an ionic solution; cations migrate to the cathode and anions to the anode, allowing current to flow through the solution. Common inorganic cations found in the body are sodium (Na+), potassium (K+), and calcium (Ca2+), while common anions are chloride (Cl-) and bicarbonate (HCO3-). Neither anions or cations can exist in isolation, as the total electrical charge must be in balance. A solution of sodium chloride will have an equal number of Na+ and Cl- ions, and a solution of calcium chloride will have twice the number of Cl- ions as Ca2+ ions. Organic molecules can also be ionic. The neurotransmitter acetylcholine chloride will give, in solution, a positively charged acetylcholine moiety together with a chloride ion. Some organic molecules may carry a positive and a negative charge on different parts of the molecule and are known as zwitterions.

Alan W. Cuthbert

See also body fluids; composition of the body.