Lecture 17: Epithelial Transport of Glucose

Epithelial Structure and Function

• Epithelial cells are arranged in continuous sheets in single or multiple layers, sitting on a basement membrane.
• They form the boundary between the body's organs or between the body and external environment.
• Epithelial cells are subject to physical breakdown and injury, undergoing constant and rapid renewal process.

Epithelial Cells and Tight Junctions

• Epithelial cells are separated from their neighbours by the lateral intercellular space.
• Tight junctions hold epithelial cells together at their luminal edges, forming a barrier.

Tight Junctions

• Tight junctions are composed of thin bands that encircle the cell and make contact with thin bands from adjacent cells.
• In electron microscopy, tight junctions appear as fused membranes.
• In freeze fracture, tight junctions appear as an interlocking network of ridges in the plasma membrane.
• Tight junctions restrict the movement of substances through the intercellular space between cells.
• They prevent membrane proteins from diffusing in the plane of the lipid bilayer.
• Tight junctions separate the epithelial cells into two distinct membrane domains: apical (or luminal or mucosal) membrane and basolateral membrane.

Epithelial Transport Properties

• The barrier function of tight junctions means that certain molecules can enter or leave the intercellular space.
• Different domains use different transporters to move substances into or out of the cell.
• Transport can occur via the paracellular or transcellular pathway or both.

Transcellular Transport

• Definition: Transcellular transport refers to the movement of substances through the interior of epithelial cells, crossing both the apical and basolateral membranes.
• Mechanism: Substances can cross the cell membrane via passive diffusion, facilitated diffusion, or active transport processes.
• Examples: Nutrient absorption in the intestine often involves transcellular transport, such as glucose absorption through facilitated diffusion.

Paracellular Transport

• Definition: Paracellular transport refers to the movement of substances through the space between adjacent epithelial cells, crossing the epithelial barrier without passing through the cytoplasm of the cells.
• Mechanism: Paracellular transport relies on tight junctions, allowing small molecules, ions, and water to pass through via diffusion or solvent drag.
• Examples: In the kidney, paracellular transport plays a role in the reabsorption of water and ions, such as in the proximal tubule.

Glucose Absorption

• Glucose absorption in the intestine and kidney occurs through transcellular transport.
• Glucose/galactose malabsorption syndrome occurs when there is a defect in glucose absorption.
• Glucosuria in the kidney occurs when there is excess glucose in the urine.