Facilitated Diffusion (FD):
* It resembles simple diffusion in that it does not require energy & takes place along the concentration gradient.
* It differs, however, in being stereospecific, in showing 'saturation kinetics ' and requiring a carrier.
* In FD a substance is transported into the cell along its concentration gradient i.e. towards equilibrium between the external & internal concentration.
* The process in mediated (in most cases) by a substrate specific permeases.
* The rate of transport is governed, over a wide range by substrate concentration in the medium.
* FD is independent of metabolic energy & the nutrient cannot accumulate inside the cell against concentration gradient.
* FD is stereospecific, i.e. only one of the two possible isomer L or D is transported.
* FD shows saturation kinetics, i.e. Increase in the concentration of substance to be transported results in an increase in the rate of transfer up to certain extent only.
* The rate of influx becomes constant & does not increase further with increase in solute concentration .
* Saturation kinetics indicates that a lipid-soluble carrier molecule is involved in the passage of solute across the membrane.
* When all the carrier molecules are occupied with the solute, the rate of influx becomes constant.
* No metabolic energy is required for facilitated diffusion.
