Principles of Epithelial Transport

Question 1

Purpose of Epithelial Cells?

Answer 1

To provide an interface for the external environment and the Interstitual Fluid.

Question 2

Structure of the Epithelial Cells?

Answer 2

• Tight Junctions (split cell into Apical and Basolateral Side).
• Gap Junction (Connexions + Connexins) Provide low resistance coupling thus electrical coupling usually.
• Microvilli - Actin Core
• StereoCilia - Actin Core
• Cilia - either Motile or Non-Motile. Has Microtubule and Dynin Proteins (Motile).

Question 3

What is Vectorial Transport?

Answer 3

Definition: Movement of a Molecule from one side of the Epithelial sheet to the other.

Absorption/Reabsorption: Luminal -> ECF
Secretion/Excretion: ECF -> Luminal

Question 4

1) The compartments present?

2) Types of Vectorial Transport?

Answer 4

1) a) Intracellular Compartment
b) Luminal Compartment
c) Interstitial Fluid
d) Paracellular Compartment

2)
a) Transcellular Path: Through the Intracellular compartment
b) PARACELLULAR PATH:

Question 5

Energy of Transcellular and Paracellular Transport?

Answer 5

Trans= Active/Passive
Para= Passive

Question 6

1) Define “Paracellular Transport”

2) Rate dependency

Answer 6

1) Solute passes through the tight junction and goes in the paracellular compartment.
2) Rate depends on the permeability of the Tight Junctions

High Permeability = Leaky ie// in the Intestine>
Low Permeability = Not leaky so no Para Trp. ie//Collecting Duct.

Question 7

Driving Force for Transcellular + Paracellular Path?

Answer 7

a) TransEpithelial Concentration Gradient

b) TransEpithelial Voltage (ONLY IF SOLUTE = CHARGED).

Question 8

Characteristics of Transepithelial Water Movement

Answer 8

• Passive Process
• Driven by Transepithelial Osmotic Pressure.
• Can occur via Transcellular (through AQPs) or Paracellular Path.

Question 9

What is solvent drag?

Answer 9

This is the Additional movement of Solute in the paracellular Pathway due to the movement of water.

Question 10

1) Draw the: Reabsorption of NaCl Solution + Explain it.

2) Where are these Epithelial Cells Found

Answer 10

1) Step 1: Na pumped out with ATPase into intersitual fluid. So Na+ enters via Facilitated diffusion (ENaC) and then is pumped out again>

Step2: Build up of Positive Charge in ECF. Causes Cl- to come via Paracellular

step3: increase in osmolality so water enters via either route.
2) Distal Tubule and Sweat Glands

Question 11

How is the Reabsorption of NaCl Solution regulated?

Answer 11

• the Na+ channel on Apical end = ENaC.

- Aldosterone decreases number of ENac so less reabsorption.

Question 12

1) Draw the Reabsorption of Glucose
2) What also happens with Reabsorption of Glucose?
3) where does the epithelial cells of Reabsorption of Glucose occur?

Answer 12

1)SGLT 1/2 Co-transporter on Luminal Side.
Glu2 + K+ + Na+,K+ ATpase.

2) With Reabsorption of Glucose is also reabsorption of NaCl solution.
3) Proximal Tube + Small Intestine

Question 13

1) Secretion of NaCl Solution Draw

where does it occur?

Answer 13

1) Will draw.
occurs in:
-Small Intestine + Proximal Tube

Question 14

Methods of Regulation of Transepithelial Transport?

Answer 14

1) Changing the activity of the Transporter
2) Changing number via endocytosis
3) Synthesis of New transporters (Slow process)