Final answer:
The assertion that substances must pass between epithelial cells rather than through them is false. In fact, transcellular transport through epithelial cells is a common pathway for selective absorption of nutrients, facilitated by transport proteins.
Step-by-step explanation:
The statement "passage across the epithelial barrier must take place between the cells, not through them" is false. Passage can occur through the cells, which is known as transcellular transport, or it can occur between the cells, referred to as paracellular transport. However, transcellular transport is more common for selective absorption of substances since epithelial cells are bound by tight junctions that generally prevent passage between them. For instance, water-soluble nutrients often use transport molecules embedded in the membrane to enter epithelial cells before they can enter the interstitial fluid and ultimately the capillaries.
Fats, which are large molecules, must also be transported through the digestive epithelium to be utilized. The plasma membrane of cells, consisting of hydrophobic phospholipids, dictates that water-soluble nutrients cannot simply diffuse through—they require specific transport mechanisms. Thus, the plasma proteins suspended in blood cannot enter the tissue fluid via facilitated diffusion without first passing through the epithelial cells.