Basic Biophysics of Transport
In the translocation system of a plant all kinds of elementary transport phenomena are probably involved. These include diffusion, cyclosis, passive and active membrane permeation and bulk flow. Any long-distance transport system in living organisms must of necessity comprise three steps: a loading step where diffusion and membrane permeation take place; a bulk flow step carrying the transported substances over large distances, and an unloading step where diffusion and membrane permeation occur again. The necessity for the bulk flow step arises from the slowness of diffusion over long distances. Elementary considerations of the random walk, which is what molecular diffusion is, show that the distance, x, diffused, the diffusion coefficient, D, and the time, t, are related by x2 ~ Dt. Most molecules have a diffusion coefficient in water of about 10-5 cm2 s-1; thus a molecule will diffuse 10 µm in about 10-1 s, whereas it will take 105 s, about a day, to diffuse 1 cm. One can write the formula as (x/t) = v = (D/x), i.e. think of the velocity of diffusion; for a distance of 10 µm, v = 10-2 cm s-1 = 36 cm h-1 whereas over a distance of 1 cm, v = 10-5 cm s-1 = 0.36 cm h-1. We can thus take for granted the necessity of a rapid bulk-flow step for transport over distances greater than, probably, a millimeter or so.
KeywordsActive Transport Irreversible Thermodynamic Unstirred Layer Plant Cell Membrane High Plant Cell
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