Transport and Excretion of Foreign Compounds

Based on the solubilities in water, foreign compounds that humans ingest or inhale can be classified into two categories: hydrophiles (soluble in water) and lipophiles (soluble in lipid medium). Membrane lipid bilayers serve as physical barriers for transporting foreign compounds across cell membranes. Transport mechanisms for lipophilic and hydrophilic compounds are different. Being lipid-soluble, lipophilic molecules can generally diffuse across cell membranes. While hydrophilic substances are usually unable to penetrate lipid membranes, they require carriers or channels for crossing cell membranes due to their low lipid solubility.