ABSTRACT
Xenobiotics often exert their toxic effects inside a cell, implying that the uptake of such compounds involves transport across the plasma membrane. Likewise, for a xenobiotic to be excreted from a cell into a specific compartment, it has to cross the cell membrane, sometimes even against a concentration gradient. However, cell membrane lipid bilayers are physico-chemical barriers and do not favor a facile exchange between two compartments, in particular for bulky molecules and polar, hydrophilic structures. Therefore, specific transmembrane transport systems have evolved that allow for a much more rapid and efficient, and sometimes more selective, way of transporting endogenous and exogenous compounds.
