Final answer:
Molecules that cross the epithelium must move from the apical to the basal surface through transport molecules or channels, as membrane selectivity and tight junctions prevent passage between cells.
Step-by-step explanation:
Molecules that cross the epithelium must go from the apical surface towards the basal surface, eventually reaching the interstitial fluid and the blood capillaries. The plasma membrane of epithelial cells is selective due to its hydrophobic properties, which means that water-soluble nutrients need specific transport molecules to enter the cells. Moreover, tight junctions prevent passage between cells, ensuring that transport happens via the apical surfaces of the cells. For instance, in the respiratory system, the transport of Cl- and maintaining an electronegative environment attract Na+ ions, which facilitates the movement of water into the mucus via osmosis, keeping the mucus fluid for ciliary movement. Similarly, in the intestines, nutrients enter the capillary blood in the villi and are transported to the liver via the hepatic portal vein.