Relative concentrations of the weak acid acetaminophen (Pka 9.5) and a weak base, diazepam (pKa 3.3), in some body fluid compartments. [ ], concentration; U, un-ionized drug; I, ionized drug.

the following processes: (1) passive diffusion, (2) filtration, (3) bulk flow, (4) active transport, (5) facilitated transport, (6) ion pair transport, (7) endocytosis, and (8) exocytosis (Fig. 3.4).These processes also participate in the transport of substances necessary for cellular maintenance and growth.

Passive Diffusion

Most drugs pass through membranes by passive diffusion (down their concentration gradient) of the unionized moiety. The rate of diffusion depends mainly on the lipid-water partition coefficient rather than on lipid solubility per se. For example, the central nervous sys tem depressant barbital is almost completely un-ionized at physiological pH and therefore should be able to cross membranes easily. However, barbital's lipid-water partition coefficient is sufficiently low that diffusion across membranes proceeds at an extremely slow rate. This slow rate of passage across central nervous system (CNS) membranes largely explains why the time of onset (latent period) of drug action after barbital administration is delayed.

A drug will accumulate in the membrane until the ratio of its concentration in the membrane and its concentration in the extracellular fluid equal its partition coefficient. A concentration gradient is thereby established between the membrane and the intracellular space; this gradient is the driving force for the passive transfer of the drug into the cell. Thus, a drug that has a very high lipid-water partition coefficient will have a large concentration gradient, and this favors its rapid diffusion across the membrane and into the cell.

Type of transport


Absorbed molecule


Filtration and bulk flow



Facilitated or active

Non-electrolytes and unionized form of weak acids and weak bases

Molecules of varying sizes


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