Practice question

pH profoundly affects physiology by shifting ionization of active site residues, modulating proton motive force for ATP synthesis via F0F1 ATPase and affecting metal solubility. Neutrophiles such as Escherichia coli grow optimally near neutrality pH 6.5 to 7.5 and maintain cytoplasm around pH 7.6 via proton pumps, cation proton antiporters NhaA and amino acid decarboxylase acid resistance systems. Acidophiles like Acetobacter aceti in vinegar, Acidithiobacillus ferrooxidans oxidizing iron and thermoacidophilic Ferroplasma thrive optimally below pH 4 some near pH 0 possessing highly acid-stable membranes with tetraether lipids, exceptionally active proton extrusion via ATPase reversed, periplasmic chaperone HdeA preventing aggregation and reduced surface acidic amino acids avoiding protonation-induced unfolding. At pH 3 neutrophiles stop growth while acidophiles sustain near neutral interior and continue ATP synthesis via proton motive force explaining ecological niche in acidic mine drainage, sulfuric hot springs and fermented foods. Alkaliphiles prefer above pH 9 using sodium motive force, mesophiles term describes temperature optimum 20 to 45 degrees not pH, so acidophiles uniquely thrive at pH 3.0 with dedicated resistance and homeostasis mechanisms.