FIGURE 20-20 Transcellular and paracellular pathways of transepithelial transport. Transcellular transport requires the cellular uptake of molecules on one side and subsequent release on the opposite side by mechanisms discussed in Chapter 11. In paracellular transport, molecules move extracellularly through parts of tight junctions, whose permeability to small molecules and ions depends on the composition of the junctional components and the physiological state of the epithelial cells. See S. Tsukita et al., 2001, Nat. Rev. Mol. Cell Biol. 2:285.