Transfer To and Across Irregular Membranes

Transport across the wall of a hemocoel is analogous to transport across rough or porous surfaces in batteries in which high currents flow from porous electrodes. Fractal surface irregularities enhance both processes. Diffusing particles emitted by a diffusion source walk randomly. If a random walker collides with the electrode or membrane, it is absorbed with a finite probability, termed the sticking probability. The sticking probability corresponds to a finite permeability of the membrane. In diffusion, most particles diffuse back towards the source, because the net flux due to Fick's law is proportional to the gradient of concentration and not to the concentration itself. Net transfer by diffusion between the hemocoel and the surface is due to the few molecules that absorb onto the surface before they can return to their bulk source in the hemolymph. Hence, there are two limits to the efficiency of transfer. First a molecule must reach the surface; second it must enter it. This idea is analogous to ions being transported through an electrolyte and then undergoing a redox reaction on the active electrode of the battery to produce the current.

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