Protonated weak acids such as acetic acid and citric acid are uncharged and are therefore able to diffuse across lipid membranes according to the concentration gradient (Wolosin and Ginsburg 1975, Walter et al. 1982, Walter and Gutknecht 1984, Xiang and Anderson 1998, Grime et al. 2008, Li et al. 2011, Hannesschlaeger et al. 2019). Passive diffusion of weak acids into type III taste cells contributes to sour taste (inferred from rat taste cells).