Index >> Membrane Transport >> Passive Diffusion OR Active Diffusion

Passive Diffusion OR Active Diffusion
Transport of metabolites across the cell membrane along the concentration gradient and without the use of a carrier molecule is called passive or simple diffusion.

Diffusion occurs down a concentration gradient from a high concentration to a low concentration region and the concentration gradient disappears as diffusion proceeds.

Simple diffusion shows nonsaturation kinetics. The rate of diffusion is proportional to the difference in solute concentration on the inner and outer sides of the membrane (Fick's first law)

Solute molecules can cross the membrane by dissolving in the lipid bilayer and diffusing across the membrane. Before a solute can, leave the aqueous phase and enter the lipid phase it must break its hydrogen bonds with water.

The more the number of -OH groups present in a solute the more will be its H-bonding capacity with water, and therefore the less its lipid solubility. Thus hexanol with only one -OH group is, poorly soluble in water, while mannitol with six -OH groups is highly soluble.

Hexanol therefore diffuses across the lipid of the membrane much more readily than mannitol. For breaking each hydrogen bond with water, about 5 kcal of energy is required.