Lecture 18

Question 1

Epithelial cells are held together at their luminal edges by tight junctions:

Answer 1

Tight junctions

Question 2

Tight junction functions (x3)

Answer 2

-A barrier, restricting the movement of substances between cells.

A fence, preventing membrane proteins from diffusing in the plane of the lipid bilayer.

-Separates the epithelial cells into two distinct membrane domains: apical and basolateral.

Question 3

The two distinct membrane domains:

Answer 3

Apical (or luminal or mucosal) membrane that faces the lumen of the organ or body cavity. Basolateral.

Question 4

Transport can occur via two pathways:

Answer 4

Transcellular pathway, paracellular pathway, or both.

Question 5

Paracellular transport:

Answer 5

governed by laws of diffusion. The higher the electrical resistance to ion flow, the greater the number of tight junction strands holding the cell together.

Question 6

Which pathway dominates leaky epithelium?

Answer 6

Paracellular transport dominates

Question 7

Which pathway dominates tight epithelium

Answer 7

Transcellular transport dominates

Question 8

Tight junction resistance changes in which direction?

Answer 8

Proximal to distal. (leaky to tight, low ER to high ER, low no. of strands to high no. of strands, bulk transprot to highly controlled

Question 9

Transcellular transport:

Answer 9

uses primary and secondary active transport often in combination with passive diffusion through ion channels.

Question 10

Transepithelial transport entry and exit step:

Answer 10

Entry step for absorption is apical, but for secretion is basolateral membrane.

Question 11

Electroneutrality:

Answer 11

movement of a positive or negative ion will attract a counter ion

Question 12

Osmosis:

Answer 12

Net movement of ions will establish a difference in osmolarity that will cause water flow by osmosis.

Question 13

Glucose absorption steps:

Answer 13

1. Tight junctions divide cells into apical and basolateral membrane domains.
2. Na pump sets up ion gradients
3. The sodium glucose symporter (SGLT) uses the energy of the Na gradient to actively accumulate glucose above its concentration gradient.
4. Facilitative glucose transporter (GLUT) mediates glucose exit across the basolateral membrane vai passive diffusion down its gradient.
5. Na taken up cia the glucose exits via the basolateral Na-pump
6. The transport of Na and glucose across the epithelium induces paracellular Cl and water fluxes

Question 14

Explain the role of sodium glucose symporter (SGLT)

Answer 14

Uses the energy of the Na gradient to actively accumulate glucose above its concentration gradient.

Question 15

Explain the role of facilitative glucose transporter (GLUT)

Answer 15

Mediates glucose exit across the basolateral membrane via passive diffusion

Question 16

Oral rehydration therapy:

Answer 16

The glucose enhances the abosprtion of Na+ and Cl-

Question 17

Glucose-galactose malabsorption syndrome

Answer 17

A mutation to the glucose symporter in the small intestine means that sugar is retained, water flow increases, produces a pronounced diarrhea.

Question 18

Treatment for glucose-galactose malabsorption.

Answer 18

Remove glucose and galactose from diet, utilizes GLUT5

Question 19

Glucosuria:

Answer 19

glucose in the urine. Glucose symporter can not absorb glucose fast enough

Question 20

Renal threshold:

Answer 20

Transport max of SGLT. Once threshold is reached glucose appears in the urine.