According to the prevailing model for polar auxin transport (Figure 27-4), IAA enters (influx) the conducting (parenchyma) cells in a protonated (IAAH) form by passive diffusion and/or as an anion (IAA-) by secondary active cotransport (see Chap¬ter 4) via influx carriers (identified as AUX1 protein in Arabi- dopsis roots); such carriers appear to be uniformly distributed around the conducting cells. Inside the cell, IAAH dissociates at the higher pH of the cytoplasm into IAA- and H+ and can exit (efflux) the cell only through the activity of the auxin efflux car¬riers. These carriers, which are precisely aligned with the direc¬tion of auxin transport, are known as PIN proteins. They are named after the pin-shaped inflorescences formed by the pinl mutant of Arabidopsis. PIN proteins are constantly being cycled