inorganic phosphate
Summary
Inorganic phosphate (Pi) is free phosphate, PO₄³⁻ and its protonated forms such as HPO₄²⁻ and H₂PO₄⁻, not bonded to carbon. It is released when ATP or a phosphate ester is hydrolyzed, and its stability relative to the reactant helps make that hydrolysis favorable.
Detail
Meaning: Inorganic phosphate, written Pi, is free phosphate: a central phosphorus bonded to four oxygens and not attached to carbon. Near physiological pH it is a mix of H₂PO₄⁻ and HPO₄²⁻, because the second pKa is about 7.2. Phosphate bonded to carbon is organic, as in a phosphate ester (ROPO₃²⁻), while ATP is a phosphoanhydride. Hydrolysis releases Pi when water attacks phosphorus and ADP leaves. The products are more stable because adjacent negative charges no longer repel, Pi has more resonance forms, and it is better solvated.
Equation: ΔG = ΔG°′ + RT ln([ADP][Pi]/[ATP]), where ΔG is the free energy change (J/mol), ΔG°′ is the standard value at pH 7 (J/mol), R = 8.314 J/(mol·K), T is temperature (K), and brackets are concentrations (M).
On the MCAT: Passages show kinases transferring phosphate from ATP to a hydroxyl group, forming an ester, and phosphatases removing it as Pi. Questions ask why ATP hydrolysis is favorable, or predict that raising [Pi] shifts the reaction toward ATP and makes ΔG less negative.
Common confusion: Students call the P–O bond a "high-energy bond" that releases energy when it breaks. Breaking a bond always costs energy. The release comes from the products being more stable than the reactants.
Sources
- Organic Chemistry (Clayden)
- Lehninger Principles of Biochemistry
- Organic Chemistry (Wade)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.