Which transport mechanism predominates for absorption in the small intestine?

• A. Active transport
• B. Diffusion
• C. Facilitated diffusion
• D. Endocytosis

Answer: (B)

Most drug molecules cross the intestinal lining by passive diffusion. In the small intestine, the enterocyte membranes are permeable to small, lipophilic, and typically unionized molecules, so they move down their concentration gradient without energy input. The enormous surface area from villi and microvilli aids this diffusion process, making it the dominant route for many drugs. Transporter-mediated processes, including active transport and facilitated diffusion, operate for specific substances—often nutrients or transporter substrates—but they generally contribute less to overall drug absorption than diffusion. Endocytosis is mainly relevant for large molecules rather than standard small-molecule drugs. So diffusion is the primary mechanism governing absorption in the small intestine.