Epithelial Transport

Lumen side

Blood side

Epithelial cells line hollow organs or tubes and regulate the absorption or secretion of substances across these surfaces. One surface of an epithelial cell generally faces a hollow or fluid-filled chamber, and the plasma membrane on this side is referred to as the luminal membrane (also known as the apical, or mucosal, membrane) of the epithelium. The plasma membrane on the opposite surface, which is usually adjacent to a network of blood vessels, is referred to as the basolateral membrane (also known as the serosal membrane).

There are two pathways by which a substance can cross a layer of epithelial cells: (1) by diffusion between the adjacent cells of the epithelium—the paracellular pathway, and (2) by movement into an epithelial cell across either the luminal or basolateral membrane, diffusion through the cytosol, and exit across the opposite membrane. This is termed the transcellular pathway.

Diffusion through the paracellular pathway is limited by the presence of tight junctions between adjacent cells, since these junctions form a seal around the luminal end of the epithelial cells (Chapter 3). Although small ions and water are able to diffuse to some degree through tight junctions, the amount of para-cellular diffusion is limited by the tightness of the junc-tional seal and the relatively small area available for diffusion. The leakiness of the paracellular pathway varies in different types of epithelium, with some being very leaky and others very tight.

During transcellular transport, the movement of molecules through the plasma membranes of epithe-

Lumen side

Blood side

Primary Active Transport

FIGURE 6-23

Active transport of sodium across an epithelial cell. The transepithelial transport of sodium always involves primary active transport out of the cell across one of the plasma membranes. (For clarity in this and the next two figures, the entrance of potassium via Na,K-ATPase transporters is not shown.) The movement of sodium into the cell across the plasma membrane on the opposite side is always downhill. Sometimes, as in this example, it is by diffusion through sodium channels, whereas in other epithelia this downhill movement is by means of a secondary active transporter. Shown below the cell is the concentration profile of the transported solute across the epithelium.

FIGURE 6-23

Active transport of sodium across an epithelial cell. The transepithelial transport of sodium always involves primary active transport out of the cell across one of the plasma membranes. (For clarity in this and the next two figures, the entrance of potassium via Na,K-ATPase transporters is not shown.) The movement of sodium into the cell across the plasma membrane on the opposite side is always downhill. Sometimes, as in this example, it is by diffusion through sodium channels, whereas in other epithelia this downhill movement is by means of a secondary active transporter. Shown below the cell is the concentration profile of the transported solute across the epithelium.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Movement of Molecules Across Cell Membranes CHAPTER SIX

Lumen side

Secondary active transport

Epithelial cell

Blood side

Facilitated diffusion

Active transport

Intracellular concentration

Active transport

Intracellular concentration

Blood -vessel

Blood concentration

Facilitated diffusion

Essentials of Human Physiology

Essentials of Human Physiology

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Responses

  • matilde
    What is epithelial transport?
    4 months ago

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