The molecular seal that serves as a barrier in epithelial tissues is called a _____________ _____________.

The molecular seal that serves as a barrier in epithelial tissues is referred to as a tight junction. Tight junctions are specialized connections between adjacent epithelial cells that create a nearly impermeable barrier. They consist of protein complexes that encircle cells, effectively sealing them together and preventing the passage of substances between the cells. This function is crucial for maintaining distinct internal and external environments, which is essential for the proper functioning of epithelial tissues, such as those lining the intestines or the blood-brain barrier.

By forming tight junctions, epithelial cells help regulate the movement of ions, water, and other small molecules, ensuring that materials pass through the cells rather than between them. This is key in maintaining homeostasis in various physiological processes. The other terms, such as complex junctions, adhering junctions, and barrier membranes, do not specifically describe the type of sealing function that tight junctions perform in epithelial tissues.