Fig. 4.

Mathematical model of ³H-2,4-D uptake by diffusion and influx, or by a combination of diffusion, influx and efflux. (A) In the presence of NPA, the active ³H-2,4-D efflux is assumed to be completely inhibited, while the active influx and diffusion of ³H-2,4-D are unaffected. Therefore, the mathematical representation of ³H-2,4-D flux by diffusion remains unchanged. In the case of an active influx, the flux of ³H-2,4-D is assumed to be proportional to the source (extracellular) concentration of dissociated ³H-2,4-D (i.e. a linear kinetics of influx carriers is assumed). The appropriate parameter—influx carrier activity of the membrane (k _Inf)—is again transformed into the cell-specific rate-constant k _I, the influx parameter, to obtain the final differential equation (3) of the model. (B) With no inhibitor present in the medium, all of the three transport mechanisms (i.e. diffusion, active influx, and active efflux) take part in the net uptake of ³H-2,4-D by the cell. Therefore, the contributions of all these three processes are combined in the final differential equation (4) of the model.