Transfer through the lipid bilayer: passive diffusion

The transfer through a lipid bilayer is passive without requiring energy. The lipid bilayer membrane constitutes a barrier:

• impermeable to ions such as Na+, K+, Cl -, to polar molecules even non charged, i.e. non-ionic, like glucose, and proteins.
• permeable to non polar molecules (liposoluble or hydrophobic) of low or middle molecular weight, and to molecules at the gas state and small molecules of low polarity.

Passive diffusion through a lipid bilayer:

The passage through the membrane is directed from the most concentrated solution towards the least concentrated solution until obtaining an equilibrium. The rate of transfer depends on the area S on the membrane, on the concentrations C1 and C2 on both sides of the membrane and on a diffusion constant K linked to the liposolubility and the size of the molecule (the smaller the molecule, the easier the transfer).

V = K.S. (C2 - C1)

The liposoluble character of a molecule is determined by the measurement of its partition coefficient between an aqueous solvent and an organic solvent like hexane. The liposoluble or non-polar molecules accumulate in organic solvent and the polar molecules in water.

The polarity of a drug depends on its ionization and one distinguishes three categories:

1. Permanently ionized molecules, whatever the pH, for example those with a quaternary ammonium. These molecules, in theory, do not cross the lipid bilayer by passive diffusion.

2. Neutral molecules, not ionized, whatever the pH. It is the case of the organic solvents which cross the lipid bilayer easily. This rule has limitations, in particular concerning intestinal absorption because it takes place from an aqueous medium. A very lipophilic molecule but almost insoluble in water can be poorly absorbed. A low availability can be the consequence of a low solubility in water.

3. Molecules whose ionization depends on pH: in a neutral state, they cross the lipid bilayer, but not in the ionized state. The acid drugs dissociate in a basic medium and the bases in an acidic medium, to give ionized molecules. The pKa of an acid is the pH at which it is 50% dissociated. For example, an acid drug R-COOH partially dissociated in R-COO - + H +, neutral form R-COOH crosses membranes but not the ionized form R-COO -.

Consequently, when two compartments at different pH are separated by a membrane, the equilibrium of the concentrations of a drug R-COOH on both sides of the membrane will be moved and drug R-COOH will accumulate in the basic compartment where it dissociates.

Дата добавления: 2015-10-23