A water molecule comprises an oxygen atom and two hydrogen atoms covalently bonded together. Water tends to separate into its component parts, creating H+ (hydrogen) ions and OH– (hydroxide) ions. When this happens, water is said to be self-ionized. While reversible, this reaction is very important to living systems because H+ and OH– ions are very reactive. Too much of either ion relative to the other within the cell can cause proteins to denature. The free hydrogen ions (which are essentially bare protons) bond with a nearby water molecule, forming hydronium ions (H3O+).