Figure 6. A simple postulate to explain the behavior observed in Figure 5.  At low pH, the transport proteins for these acidic substrates functions as postulated.  The protonated weak acid binds to the transporter, and once released inside, alkaline internal pH causes dissociation of the acid and accumulation of the anion.  At high pH, however, the transport protein functions in an electrogenic mode due to an important functional group with a pK below 7.5.  In this manner, at high pH, these transport proteins would function like those for neutral substrates.  That is, the acidic substrate with a bound proton(s) (i.e., the undissociated form of the acid) plus another proton bound to the postulated functional group in the transporter would translocate the protonated acid with a additional proton, and accumulation would become electrogenic and driven by DY.