What is passive diffusion in the context of drug movement?

Passive diffusion refers to the process by which drugs move across cell membranes without the need for energy expenditure. This movement occurs along the concentration gradient, meaning substances naturally flow from areas of higher concentration to areas of lower concentration until equilibrium is achieved.

During passive diffusion, the lipid solubility of the drug and the permeability of the cell membrane play key roles. Lipid-soluble drugs tend to diffuse more easily through the cell membrane, which is primarily composed of lipid bilayers. Since this process does not require energy, it leverages the natural kinetic movement of molecules, leading to a more efficient transfer of drugs into and out of cells where concentration gradients exist.

In contrast, processes that involve moving substances against a concentration gradient, such as active transport, do require energy and are not representative of passive diffusion. This highlights why the correct choice accurately describes the essence of passive diffusion in pharmacology.