Lecture 17: Epithelial Transport of Glucose

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Questions and Answers

What is the main function of tight junctions in epithelial cells?

To restrict the movement of substances through the intercellular space (correct)

To facilitate the exchange of ions between cells

To provide structural support to the epithelial cells

To allow for the free movement of molecules between cells

What is the term for the space between epithelial cells?

Intercellular cavity

Basolateral space

Lateral intercellular space (correct)

Apical space

What is the function of the apical membrane of epithelial cells?

Faces the basement membrane

Faces the blood

Faces the lumen of the organ or body cavity (correct)

Faces the lateral intercellular space

What is the term for the process by which epithelial cells can absorb or secrete substances?

Transcellular transport Signup and view all the answers

What is the term for the disorder characterized by the inability to absorb glucose and galactose?

Glucose/galactose malabsorption syndrome Signup and view all the answers

What is the term for the passage of glucose into the urine?

Glucosuria Signup and view all the answers

What is the function of the fence-like structure formed by tight junctions?

To prevent membrane proteins from diffusing in the plane of the lipid bilayer Signup and view all the answers

What is the main function of the basolateral membrane of epithelial cells?

Interfaces with the blood and adheres to the basement membrane Signup and view all the answers

What is the primary mechanism of transcellular transport?

movement of molecules through transmembrane proteins Signup and view all the answers

Which of the following is an example of transcellular transport?

movement of glucose through the intestinal epithelium Signup and view all the answers

What is the function of tight junctions in paracellular transport?

to seal the intercellular space between adjacent epithelial cells Signup and view all the answers

What is the main difference between transcellular and paracellular transport?

the route of transport through the epithelial cells Signup and view all the answers

What is the role of GLUT proteins in transcellular transport?

to facilitate diffusion through the apical and basolateral membranes Signup and view all the answers

Which pathway is involved in the transport of water and ions in the kidney?

paracellular pathway Signup and view all the answers

What is the significance of different domains in epithelial transport?

it enables the use of different transporters to move substances into and out of the cell Signup and view all the answers

What is the primary function of the epithelial barrier?

to control the movement of substances into and out of the intercellular space Signup and view all the answers

Study Notes

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.

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Description

Learn about epithelial structure and function, transcellular and paracellular transport, and glucose absorption in the intestine and kidney. Understand the role of tight junctions and how epithelial cells mediate absorption or secretion of substances.