Paracellular transport occurs primarily through which structures?

• A. Tight junctions
• B. Membrane channels
• C. Vesicular transport
• D. Active transport pumps

Answer: (A)

Paracellular transport moves substances between adjacent cells, so it relies on the junctions that connect those cells rather than crossing the cell membranes themselves. The structures that govern this pathway are tight junctions. They form the seal around the cell–cell border and can create selective passageways (paracellular pores) that allow small ions and very hydrated molecules to pass, depending on size and charge. This is distinct from transcellular routes, where substances cross the apical and basolateral membranes through membrane channels or carriers, or are taken up and released by vesicles. Active transport pumps also move substances across membranes and require energy, which is not how the paracellular route operates. So the primary structures enabling paracellular movement are the tight junctions between cells.