An extremely important feature of the intestinal epithelium is its function as a physical barrier between the environment and our bodies' internal milieu. At the same time it has to allow for uptake of important nutrients. At least four different transport mechanisms exist that allow selective uptake and transport of macromolecules across the epithelial cell layer, i.e. paracellular transport, passive diffusion of molecules from the apical to the basolateral side, vesicle-mediated transcytosis

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... nd carrier-mediated uptake and diffusion through the epithelial cell layer. Each of these transport mechanisms depends on the physicochemical properties of the compound, its ability to interact with the plasma membrane, its molecular weight and size, stability and charge distribution. In vivo, parameters not directly associated with the molecule in question will influence uptake and transepithelial transport. Intestinal motility, interactions with other molecules from the diet and the digestive process like bile salts and enzymes, and solubility in the mucus layer will affect the absorption process. Thus, in vitro models for studying absorption through the intestinal epithelium have several limitations. Still, they are considered useful model systems for such purposes. Similarly, effects of bioactive molecules on the epithelium can be studied by measuring barrier function and effects on transport processes.