The distribution of IAA between the vacuole and the bathing solution in Hydrodictyon africanum is consistent with passive entry of undissociated IAA and passive efflux of both undissociated IAA and of IAA, with PIAA (permeability coefficient)* about io−3 cm s−1 and PIAA- about io-6 cms −1. The involvement of IAA− in the efflux results from the inside-negative P.D. between the medium and the vacuole. The cytoplasm is at a higher pH than either the vacuole or the bathing solution used in most experiments; this is maintained by active H+ efflux at the plasmalemma, and active influx at the tonoplast. In this situation the efflux of IAA− is further promoted by the increased concentration of IAA− in the relatively alkaline cytoplasm. Thus the apparent active efflux of IAA from these cells can be explained in terms of passive driving forces of concentration and electrical potential acting on IAA and IAA−, with the distribution of these two species dictated ultimately by PIAA, PIAA- the pH of the various compartments and the electrical potential difference between them. If PIAA/PIAA- were larger at the apical than at the basal end of coleoptile cells, such an effect could explain polar IAA transport, with metabolic energy being used only to maintain the relative permeabilities to the two species, the pH gradient and the electrical gradient.
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