Traffic Across Cells: Epithelial Transport Of Glucose

Question 1

Epithethial tissues consist of cells arranged in what

Answer 1

Continuous sheets in either single or multiple layers

Question 2

Epithelial tissues are subjectd to physical breakdown and injury therefore …

Answer 2

Undergo constant rapid renewal process

Question 3

Tight junction

Answer 3

Hold cells together so can restrict movement via paracellular pathway

Question 4

Intercellular (paracellular) space

Answer 4

Cells sit side by side, so there is space between cells, set up formation of tight junction. Not all can get through.

Question 5

What is the structure of tight junctions

Answer 5

Composed of thin bands that encircle the cell and make contact with thin bands from adjacent cells.

Question 6

TIght junctions can act as what

Answer 6

A barrier and a fence

Question 7

What does it mean when a tight junction can act as a barrier

Answer 7

They restrict the movement of substances through the intercellular space between cells

Question 8

What does it mean when a tight junction can act as a fence

Answer 8

They prevent membrane proteins from diffusing in the plane of the lipid bilayer.

Question 9

Due to the barrier and fence function of the tight junction, they can separate the epithelial cells into what

Answer 9

Two distinct membrane domains: apical and basolateral

Question 10

What is the apical domain

Answer 10

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

Question 11

What is the basolateral membrane

Answer 11

Adheres to the adjacent basement membrane and interfaces with the blood.

Question 12

What do the two distinct membrane domains mean

Answer 12

This means that different transport proteins can be inserted into either the apical or basolateral surface. Tranport can either occur via paracellular, trnascellular or both.

Question 13

Transport of proteins can occur via what

Answer 13

Paracellular (via tight junctions), transcellular (through the cell) or via both pathways.

Question 14

What are the two distinct membrane domains that epithelial cells are separed into

Answer 14

Apical and Basolateral membrane

Question 15

What is paracellular transport

Answer 15

Paracellular transport is goverened by the laws of diffusion and the tightness of the tight junctions.

Question 16

Electrical resistance if paracelllar transport

Answer 16

heThe electrical resistance to ion flow through tight junctions can be measured.

Question 17

The higher the electrical resistance to ion flow, the….

Answer 17

The greater the number of tight junction strands holding the cell together.

Question 18

Epithelial tissues can be functionally classified into what two things

Answer 18

Leaky epithelium - paracellular transport dominates
Tight epithelium - transcellular transport dominates

Question 19

How does tight junction resistance change

Answer 19

In proximal to distal direction in the GI tract and kidney

Question 20

What is transcellular transport

Answer 20

When epoithelial cells use primary and secondary active transport often in combination with passive diffusion through ion channels to produce transport across epithelial tissues

Question 21

Transcellular transport can be either what two things

Answer 21

• absorption: transport from lumne to blood
• secretion: transport from blood to lumen

Question 22

Transepithelial transport can be broken down into what following areas that need to be considered

Answer 22

1. entry and exit steps
2. electrochemical gradient
3. electroneutrality
4. osmosis

Question 23

Describe what happens in glucose absorption in the small intestine

Answer 23

1. tight junctions divide apical and basolateral
2. Na pump set up ion gradients
3. SGLT uses energy of Na gradient accumulate glucose above its concentration gradient.
4. GLUT mediates glucose exit across basolateral membrane via passive diffusion down its gradient
5. Na taken up via basolateral Na pump
6. transport of Na and glucose across epithelium induces paracellular Cl and water fluxes.

Question 24

What is oral rehydration therapy

Answer 24

The ability of glucose to enhance the absorption of Na+ and hence Cl- and water is exploited in oral rehydration therapy.

Question 25

What is glucose-galactose malabsorption syndrome

Answer 25

A mutation in the glucose symporter in the small intestine which means that sugar is retained in the intestine lumen.

Question 26

In glucose-galactose malabsorption syndrome, what does an associated increase in lumen osomolarity do

Answer 26

Induces a watter efflux, which produces a pronounced diarrhea

Question 27

What is the treatment for glucose-galactose malabsorption

Answer 27

• Therapy is used to remove glucose and galactose from the diet using fructose as a source of carbohydrate.
• utilises a facilitative transport (GLUT5) tht is specific for fructose.
• Fructose exits across the basolateral membrane and can use GLUT 2.

Question 28

Glucose reabsorption in the kidney

Answer 28

In the kidney, glucose in the plasma is filtered and needs to be reabsorbed or it will appear in the urine.

Question 29

Glucosuria - glucose in the urine

Answer 29

The commonest causes is diabetes mellitus beause insulin activity is deficient and blood sugar is too high.
The glucose symporter can not absorb glucose fast enough and glucose appears in the urine.

Question 30

Glucose in the urine - transporter kinetics

Answer 30

• If glucose absorption is impaired or the transporter is saturated, glucose will appear in the urine.
• All filtered gluocse is reabsorbed until the renal threshold is reached. One reached, the glucose appears in the urine.

Question 31

What does the renal threshold reflect

Answer 31

The transport maximum of SGLT.