NURS 735 - APPLIED TOXICOLOGY

Passive Diffusion

• Does not require energy as its driving force is the concentration gradient in the system.
• The net transfer of the drug from one side of a membrane to the other side approaches zero when an equilibrium is achieved. i.e., the same concentration is attained in both sides. Note that molecules cross the membrane at all time. At equilibrium, however, the net exchanges between the two sides become equal.
• The rate of transfer is variable and depends upon the magnitude of the concentration gradient but the transfer rate constant and t1/2 remain constant throughout the transfer.
• Only the unionized fraction of an electrolite can pass the membrane.

Active Transport

• The transfer requires energy to proceed.
• The transfer may be unidirectional hence, the concentration in the two sides will not reach equilibrium. Molecules may move across the membrane against the concentration gradient.
• The transfer is usually specific for a specific molecule or class of molecule.
• The system may be saturable. Upon saturation the kinetics of the reaction change to zero orders.*
• During the transfer, molecules may compete with one another. It is also possible for a given condition or a chemical to cause break down of the system.
• Even the ionized portion of a molecule may cross the membrane.