The inside of cells (pH ~ 7) is less acidic than the outside (the apoplast; pH ~ 5.5). So outside the cell a significant portion (17%) of the IAA molecules remain un-dissociated (proton-associated). This portion of auxin molecules is charge-neutral and therefore it is able to diffuse through the lipophilic lipid bilayer (lipid bilayer being constituent of cell membrane) into the cells. Once through the bilayer in the cell, the molecules are exposed to the more basic pH of the cell interior, and there they dissociate almost completely, producing anionic IAA−. These chemically polar ions are unable to passively diffuse across the cell membrane and remain trapped inside the cell.