Practice question

Carbon dioxide role in culture extends beyond waste gas to essential physicochemical regulator and metabolic substrate. Dissolution follows Henry's law concentration equals partial pressure times solubility coefficient 0.03 mM per mmHg at 37°C, so at 5 percent CO2 incubator partial pressure about 38 mmHg dissolved CO2 approximately 1.2 mM. Hydration reaction CO2 plus water forms carbonic acid catalyzed by zinc metalloenzyme carbonic anhydrase II intracellularly with turnover 10^6 per second, spontaneous dissociation to bicarbonate plus proton. Ratio bicarbonate to dissolved CO2 determines pH via Henderson-Hasselbalch. Lowering gas phase to 1 percent raises pH to 8.0 precipitating calcium phosphate as hydroxyapatite needles toxic, increasing intracellular pH impairing endosomal trafficking that requires acidic pH 5 to 6 for ligand release. Raising to 21 percent acidifies to 6.9 causing caspase activation. Empirically 5 percent CO2 approximates alveolar air gas tension matching mammals where arterial pCO2 40 mmHg, supporting physiological partial oxygen 140 mmHg when combined with humidity reducing oxygen fraction.