Epithelium

Question 1

What are the two ways a substance can pass through the epithelial layers?

Answer 1

1. Paracellular: Going through the zonula occludens between epithelial cells.
2. Transcellular: Involves crossing the CM on one side and leaving the cell on the other.

Question 2

Describe the polarity of the Epithelial cell.

Answer 2

Apical: The surface of the cell is facing the external environment.
Basal: The surface of cell opposite to the apical side; Not in contact with blood.

Question 3

Describe the movement of membrane proteins in the apical membrane vs the basal membrane.

Answer 3

Membrane proteins in the apical membrane cant migrate to the basolateral surface. Proteins in the basolateral membrane can migrate freely around both the lateral and basal sides but not the apical surface.

Question 4

What is the benefit of restricting the movement of membrane proteins?

Answer 4

Active transport of solutes across an epithelial layer depends on having dissimilar arrangement of membrane transport proteins.

Question 5

Walkthrough the uptake of glucose in the small intestine.

Answer 5

1. On the apical side, epithelial cells have a Na/glucose symport (SGLT1); It takes energy from downhill movement of Na and uses it to move glucose up its gradient.
2. Glucose is concentrated in the cell via secondary active transport.
3. Intracellular glucose exits passively down its gradient via GLUT2 on the basolateral membrane (facilitated diffusion)
4. Once in the ISF, glucose diffuses passively across the basal lamina and CT to a capillary.
5. Then diffuses passively across capillary endothelium and into blood stream.

Question 6

Where is the Na/K ATPase?

Answer 6

On the basolateral membrane.

Question 7

Describe the CM in terms of permeability to water.

Answer 7

The epithelial cell is permeable to water; Can pass through both transcellularly and paracellularly. Movement is always passive.

Question 8

Describe the CM in terms of permeability to lipid soluble substances.

Answer 8

Highly permeable to lipid soluble molecules; Always simple passive transport. No epithelial cell can accomplish active transport of lipid soluble substances since it would just diffuse back across the CM.