Practice question

Metabolic basis for color change reflects shift toward glycolytic phenotype characteristic of rapidly dividing cells in vitro despite ample oxygen availability phenomenon termed Warburg effect observed initially in tumors but also in normal proliferating cultures. Cells convert glucose to pyruvate via glycolysis then lactate via lactate dehydrogenase tetramer reducing NADH to NAD+ maintaining redox balance, exporting lactate plus proton via monocarboxylate transporter MCT4 SLC16A3 electrogenic process acidifying extracellular milieu even when mitochondrial oxidative phosphorylation functional. Simultaneously citric acid cycle produces carbon dioxide diffusing across lipid bilayer hydrating to carbonic acid via membrane associated carbonic anhydrase IX expressed under hypoxia inducible factor one alpha. Both lactic acid pKa 3.86 and carbonic acid donate protons to bicarbonate buffer consuming HCO3- converting to H2CO3 that dissociates to CO2 plus water removed in incubator exhaust. Accumulation of protons lowers medium pH from 7.4 toward 6.8 shifting phenol red equilibrium to protonated yellow form as absorbance at 560 drops and 430 rises perceived yellow.