Epithelial cells form sheets or boundaries that line tissues and organs, including the digestive tract, respiratory tract, and outer layer of the skin. Like any effective boundary, a layer of epithelial cells must limit or control the passage of material across it. Adherens junctions and desmosomes provide strong adhesion between cells, but they do not prevent materials from passing freely through the spaces between the cells. This function is provided by a different type of cell junction. In vertebrates, these are called tight junctions (Fig. 10.14). Tight junctions establish a seal between cells so that the only way a substance can travel from one side of a sheet of epithelial cells to the other is by moving through the cells by means of one of the cellular transport mechanisms discussed in Chapter 5.

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A tight junction is a band of interconnected strands of integral membrane proteins, particularly proteins called claudins and occludins. Like adherens junctions, tight junctions encircle the epithelial cell. The proteins forming the tight junction in one cell bind to the proteins forming the tight junctions in adjacent cells. Also like adherens junctions, tight junctions connect to actin microfilaments.

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Cells that have tight junctions have two sides because the tight junction divides the plasma membrane into two distinct regions (Fig. 10.14). The portion of the plasma membrane in contact with the lumen, or the inside of any tubelike structure like the gut, is called the apical membrane. The apical membrane defines the “top” side of the cell. The rest of the plasma membrane is the basolateral membrane, which defines the bottom (“baso”) and sides (“lateral”) of the cell. These two regions of the plasma membrane are of different composition because the tight junction prevents lipids and proteins in the membrane on one side of the junction from diffusing to the other side. As a result, the apical and basolateral membranes of a cell are likely to have different integral membrane proteins, which causes them to be functionally different as well. In the small intestine, for example, glucose is transported from the lumen into intestinal epithelial cells by transport proteins on the apical side of the cells, and is transported out of the cells into the circulation by facilitated diffusion through a different type of glucose transporter restricted to the basolateral sides of the cells (Chapter 40).