Compare the structure and function of tight junctions, desmosomes, and gap junctions.

Tight junctions are impermeable seals formed when integral proteins in adjacent plasma membranes fuse together, preventing molecules from passing through the intercellular space. Desmosomes are anchoring junctions that use plaques, linker proteins called cadherins, and intermediate filaments to bind cells together into strong sheets that resist mechanical stress. Gap junctions are communicating junctions made of connexons, hollow protein cylinders that form water-filled channels through which ions and small molecules pass from cell to cell.

The three junction types differ in both structure and function. Tight junctions involve the fusion of integral protein molecules in the plasma membranes of adjacent cells, creating an impermeable junction that encircles the cell and blocks movement of molecules through the extracellular space. For example, tight junctions in the digestive tract keep digestive enzymes and microorganisms from seeping into the bloodstream. Desmosomes are anchoring junctions that act like rivets or molecular Velcro. Their structure includes a buttonlike plaque on the cytoplasmic face of each plasma membrane, thin linker proteins (cadherins) that extend from the plaques and interlock in the intercellular space, and thicker keratin intermediate filaments that anchor to the plaque and span the cell. This arrangement distributes tension across a cellular sheet and prevents tearing, which is especially important in tissues subjected to great mechanical stress, such as skin and heart muscle. Gap junctions are communicating junctions in which adjacent plasma membranes are connected by hollow cylinders called connexons. These transmembrane proteins form water-filled channels that allow ions, simple sugars, and other small molecules to flow directly from one cell to the next, enabling communication. Gap junctions are particularly important for coordination in heart muscle and embryonic cells. Additionally, in epithelial tissues, tight junctions and desmosomes act as specialized lateral contacts that bind adjacent cells, and tight junctions also help maintain epithelial polarity by keeping apical membrane proteins from diffusing into the basal region.