Phosphorus Containing Compounds
High-Yield Summary
- Phosphoric acid (H3PO4), a.k.a. inorganic phosphate (Pi), is triprotic — 3 acidic protons dissociating at different pH values, making it an effective biological buffer across a broad pH range.
- Phosphate bonds are "high-energy" for two reasons: (1) electrostatic repulsion between adjacent negatively charged phosphate groups, and (2) resonance stabilization of the resulting phosphate ions after cleavage — together explaining why ATP hydrolysis releases so much energy.
- Phosphodiester bonds link the 3' carbon of one sugar to the 5' carbon of the next, forming the sugar-phosphate backbone of DNA/RNA and giving it structural stability + directionality (5' and 3' ends).
- Pyrophosphate (2 linked phosphates) is released during DNA/RNA synthesis; its highly favorable hydrolysis into 2 inorganic phosphates helps drive polymerization forward.
- Organic phosphate = phosphate group covalently attached to a carbon-containing molecule; ATP is the key example, serving as the cell's primary energy carrier.
Phosphoric Acid pKa Values
pKa1 ≈ 2.15, pKa2 ≈ 7.20, pKa3 ≈ 12.35
- pKa1 = First dissociation (H3PO4 ⇌ H2PO4− + H+)
- pKa2 = Second dissociation (H2PO4− ⇌ HPO4²− + H+) — closest to physiological pH
- pKa3 = Third dissociation (HPO4²− ⇌ PO4³− + H+)
- pKa2 falls closest to physiological pH (~7.4), making the H2PO4−/HPO4²− pair the physiologically relevant buffering step in cells and extracellular fluid.
Key Terms
- Inorganic phosphate (Pi)
- Phosphoric acid (H3PO4), or a phosphate group, in a biochemical context.
- Phosphodiester bond
- Bond linking the 3' carbon of one sugar to the 5' carbon of the next in DNA/RNA, forming the sugar-phosphate backbone.
- Pyrophosphate
- Two linked phosphate groups; released as a byproduct during DNA/RNA polymerization, with highly favorable hydrolysis that helps drive the reaction forward.
- Organic phosphate
- A phosphate group covalently attached to a carbon-containing molecule; ATP is the primary example.
Why Phosphate Bonds Are High-Energy
| Effect | Mechanism |
|---|---|
| Electrostatic repulsion | Adjacent phosphate groups carry large negative charges that repel each other while bonded |
| Resonance stabilization | Once cleaved, resulting phosphate ions delocalize negative charge across multiple O atoms, stabilizing the products |
Common MCAT Trap
- Phosphate bonds aren't "high-energy" just because they're negatively charged — it's specifically the combination of electrostatic repulsion IN the bonded state and resonance stabilization AFTER cleavage that together make hydrolysis release so much energy.
- Don't confuse phosphodiester bonds (3'-5' sugar linkage in the nucleic acid backbone) with the high-energy phosphate-phosphate bonds in ATP/pyrophosphate — different bond types with different roles (structural vs. energetic).
- pKa2 (~7.20), not pKa1 or pKa3, is the physiologically relevant buffering step — a common mix-up given three pKa values to track.
Quick Recall
Why is phosphoric acid such an effective biological buffer?
What two effects make phosphate bonds "high-energy," and how do they relate to ATP hydrolysis?
What role does pyrophosphate hydrolysis play in DNA/RNA synthesis?