Schematic diagram of an idealized plant cell and the role of specific PIN proteins in auxin management at the cellular level. The low pH in the apoplast (the region outside the cell membrane comprising the plant cell wall) is maintained by the activity of the plasma membrane H+-ATPase. In the acidic environment of the apoplast, a relatively high proportion of auxin molecules stay protonated (un-ionized; indole-acetic acid (IAA)) and these can enter the cell directly via passive diffusion. In its ionized (dissociated) form (IAA- + H+), auxin cannot cross membranes by passive diffusion; it needs to be actively transported by carriers. Ionized auxin molecules can enter cells via active transport by auxin-influx carriers. In the relatively higher pH of the cytoplasm, auxin molecules undergo almost complete dissociation. The asymmetric positioning of the auxin-efflux carriers from the 'long' PIN subfamily at the plasma membrane then determines the direction of auxin efflux from the cell. Localization of AtPIN5 (from the 'short' PIN subfamily) at the membranes of the endoplasmic reticulum leads to compartmentalization of auxin into the lumen of the endoplasmic reticulum, where it undergoes metabolic conversion. PM, plasma membrane; ER, endoplasmic reticulum; GA, Golgi apparatus.