Figure 5.

Revised model of futile transmembrane NH₃/NH₄⁺ cycling in root cells of higher plants. Uncharged NH₃ rapidly equilibrates across both major membrane systems (plasmalemma and tonoplast) and is likely mediated by AQPs specific to each system. A relatively minor channel/carrier-mediated flux of NH₄⁺ may also occur across both membrane systems. NH₄⁺ concentrations are a function of NH₃ equilibration and compartment pH (Roberts et al., 1982; Walker et al., 1996; Kosegarten et al., 1997; Kosegarten et al., 1999). PIP, Plasmalemma-intrinsic protein; TIP, tonoplast-intrinsic protein; AMT, ammonium transporter; Kir, K⁺ inward rectifier; NSCC, nonselective cation channel. [See online article for color version of this figure.]