Ammonium is an important metabolite for many organisms. Until now, it was unclear how exactly ammonium is transported in and out of cells. Williamson et al. demonstrated that ammonium (NH4+) dissociates into ammonia gas (NH3) and a positively charged hydrogen (H+), which then traverse the cell membrane separately and re-join at the other end. (A) The ammonium transporter (teal) spans the whole membrane (grey). The two ends are open to the water-filled environment (cyan), but the only open passage through the centre is strongly water-repellent (pink). The entrance to the water channel has a pocket that specifically recognizes ammonium. (B) Ammonium binds into the pocket near the amino acid Asp160, which is essential to trigger the opening of the channel. The environment inside the pocket also allows ammonium to separate into NH3 and H+. NH3 can now cross the membrane through the hydrophobic channel (pink), and H+ crosses separately via the bridge formed of two copies of the amino acid histidine (His168 and His318). (C) Once inside the cell, NH3 and H+ recombine to make ammonium (NH4+).