i) Passive diffusion:
Definition: Transport of metabolites or substances across the membrane along the concentration gradient and without the use of a carrier molecule is called passive diffusion.
* Diffusion occurs down a concentration gradient from a higher concentration to a lower concentration region.
* The concentration gradient disappears as the diffusion proceeds.
* Passive diffusion shows non-saturation kinetics (as carriers are not involved in, so no problem of saturation).
* The rate of diffusion is proportional to the difference in solute concentration on the inner and outer sides of the membrane.
* When equiliberium is reached, the concentration becomes same on both the sides of membrane & the diffusion stops.
* Passive diffusion does not involve stereospecificity [i.e. both L & D isomers can move across the membrane at equal rate].
* It take place as a result of random molecular movement & is a slow process.
* It is not an important mechanism for transport across the cell membrane.
* Diffusion through the membrane can take place through : i. Pores or channels in the membrne or ii. By dissolving in the lipid phase.
* Small polar molecules could cross the membrane through the pores which arise in a random manner and are transitory (not permanent).
* Larger molecules cannot penetrate the pores.
* Solute molecules can cross the membrane by dissolving in the lipid bilayer & diffusing across the membrane.
* Passive diffusion is governed by the molecular size & lipophilic properties of the material.
* The rate of such transport are low. Uptake of sugars by passive diffusion has never been demonstrated.
* Apparently water, non-polar toxins, inhibitors & other substances that are not part of the normal intracellular environment are taken up by the passive diffusion.
