Practice question

Effective buffering strategies differ in reliance on volatile component dictating need for controlled atmosphere. Bicarbonate carbon dioxide system is open buffer where one component gaseous carbon dioxide exchanges with atmosphere, concentration governed by Henry's law gas partial pressure. Chemistry: dissolution CO2 plus water yields carbonic acid catalyzed by intracellular carbonic anhydrase zinc metalloenzyme with His three coordinating zinc activating water as hydroxide attacking CO2, turnover 10^6 per second, spontaneous dissociation H2CO3 ↔ H+ plus HCO3- with pKa 6.1. Medium pH governed by Henderson-Hasselbalch pH equals 6.1 plus log([HCO3-]/[CO2(aq)]). Since CO2 volatile, maintaining pH requires incubator supplying constant 5 percent CO2 at about 38 mmHg partial pressure producing dissolved CO2 1.2 mM, stabilizing pH provided bicarbonate 24 to 44 mM. Opening door causes loss. In contrast phosphate buffer H2PO4- ↔ HPO4- plus H+ pKa 7.2 second dissociation, HEPES protonated ↔ deprotonated pKa 7.5, Tris pKa 8.1 are closed systems where proton exchange occurs intramolecularly without gas phase, pH remains stable irrespective of atmosphere.