Pi

 Pi in the biological sciences is most commonly known as inorganic phosphate. It is a free ion in solution and the term Pi is used to distinguish it from phosphates found in phosphate esters. At physiological pH (6.8 - 7.0), inorganic phosphate consists of largely HPO₄^2- and H₂PO₄^2-

However, phosphates are most commonly found in the form of adenosine phosphates, (AMP, ADP and ATP) and in DNA and RNA and Pi is released by the hydrolysis of ATP or ADP. The addition and removal of phosphate as seen through phosphorylation and dephosphorylation is crucial for many metabolic pathways.