Question

3. In the intercellular junctional complex, why is the tight junction closer to the surface of the epithelium (the apical end of the cell) than the belt desmosome?

Answer 1

In between cells there are many type of junctions.the junction mainly found in epithelial cells are of tight junctions.it connects plasma membrane of 2 cells and prevent passage of molecules through the space between the cells.these junctions seals the basal surface of epithelium from apical surface.it is located near the apical surface of cell because at there it can prevent the molecules from passing into the space between the cells. Because the apical surface usually faces lumen of a cavity or body surface so the molecules are transferred from the lumen to apical surface.and it can pass through the space between cells. But due to tge presence of tight junction near apical surface it is prevented.so that leaking of water and other molecules are prevented.

Also other reason is that tight junction prevents diffusion of lipid in plasma membrane in the outer leaflet. And some apical proteins of cell are attached to these lipids.so to prevent diffusion of these proteins it must be located near apical surface.