Figure 2.

One-dimensional, discrete model of auxin transport across the tissue (in reality w ≪ W). Efflux of auxin (solid green arrow) occurs through efflux carriers (PIN: red circle, ABCB: blue circle), whose distribution (and thus permeabilities $P_n^{l,r}$) can be different on the right and left membrane of the cell. By contrast, influx of auxin (green dotted arrow) occurs with a symmetrical permeability Pⁱⁿ on both side of the cell. An asymmetry of efflux permeabilities P^l ≠ P^r can generate a net flux of auxin across the tissue, yielding an auxin concentration gradient (background color gradient).