Which component contributes to selective transport across epithelia?

• A. Tight junctions regulate the paracellular route
• B. Desmosomes provide pumps and channels
• C. Gap junctions mediate diffusion
• D. Membrane transport proteins (pumps and channels) on apical and basolateral surfaces

Answer: (C)

Selective transport across epithelia is achieved primarily through transcellular movement mediated by membrane transport proteins located on the apical and basolateral surfaces of epithelial cells. Pumps and channels on these surfaces actively or passively move specific solutes across the cell, creating directional and selective transport from one side of the epithelium to the other. Tight junctions also influence selectivity by regulating the paracellular pathway—the space between cells—but they do not themselves transport solutes across the epithelial barrier. Desmosomes mainly provide strong cell–cell adhesion, not transport. Gap junctions allow small molecules and ions to pass directly between neighboring cells, which supports intercellular communication rather than transcellular transport across the epithelial layer. So, the key contributors to selective transport across epithelia are the membrane transport proteins on the apical and basolateral surfaces.