The plasma membrane, a critical structure in cellular biology, houses an array of transporters, or carrier proteins, interspersed within its lipid bilayer. These proteins play a crucial role in solute transport through facilitated diffusion, a form of passive diffusion that uses transporters to move the molecules across the membrane.

In this process, substrates such as organic compounds and ions interact with a transporter on one side, triggering conformational changes in proteins that enable substrate translocation without any ATP expenditure. The movement of these substrates, either into or out of the cell, is governed by the electrochemical gradient until a state of equilibrium — a balance between concentration and charge inside and outside the cell — is achieved.

This method of facilitated diffusion accelerates solute movement compared to passive diffusion without a transporter. The selectivity of these carrier proteins is key to this process, as their structure permits only specific endogenous compounds or drugs to bind. For instance, the organic cation transporter (OCT1) allows transport of specific molecules: the intestinal absorption only of vitamins like thiamine, and the hepatic uptake only of specific drugs such as metformin and levodopa.

However, being carrier-mediated, this system exhibits saturation and structural selectivity for the drug, displaying competition kinetics for drugs with similar structures. Facilitated diffusion plays a minor role in drug absorption due to the predominance of active transporters. This highlights the complexity and specificity of cellular transport mechanisms, particularly concerning drug administration and absorption.