Auxin is transported from cell to cell

Outside of the cell the pH is acidic (about 5.5) and auxin is in its uncharged form (IAAH). This uncharged form is lipid soluble and can cross the plasma membrane into the cell. In the cytoplasm at pH 7 Indole-3-acetic acid is deprotonated (IAA-). This charged ion cannot cross the membrane and is unable to exit other than through specific transporters.