How do tight junctions contribute to polarity in absorptive epithelia?

• A. They restrict lateral diffusion of membrane proteins, maintaining distinct apical vs basolateral domains essential for vectorial transport
• B. They anchor cells to the basement membrane
• C. They form gap junctions for intercellular communication
• D. They produce mucus in goblet cells

Answer: (A)

Tight junctions establish and preserve epithelial polarity by acting as a fence that restricts lateral diffusion of membrane proteins, keeping apical and basolateral domains distinct to support vectorial transport. In absorptive epithelia, nutrients must move from the intestinal lumen into the bloodstream in a directional way, and this requires specific proteins to be confined to the apical surface (like certain transporters and enzymes) or to the basolateral surface (like different transporters and receptors). By sealing the space between neighboring cells at the apical border and limiting protein mobility within the plasma membranes, tight junctions prevent mixing of membrane components between the two domains, maintaining organized, functionally specialized membranes. This domain separation is essential for efficient, directional absorption. The other options refer to different cellular structures: anchoring to the basement membrane is the job of hemidesmosomes; gap junctions enable direct cytoplasmic communication between cells; mucus production occurs in goblet cells.