Lec 17 - Glucose transport

Question 1

What is the paracellular transport?

Answer 1

transport between adjacent epithelial cells through tight junctions of the basolateral membrane

Question 2

What is the transcellular transport?

Answer 2

Transport through the epithelial cell that uses primary and secondary active transport in combination with passive diffusion through ion channels.

Question 3

What are the two types of transcellular transport?

Answer 3

absorption (lumen to blood) and secretion (blood to lumen)

Question 4

What determines the tightness of a tight junction?

Answer 4

The number of strands of transmembrane proteins

Question 5

How do tight junctions act as a fence?

Answer 5

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

Question 5

What are the two distinct domains of the cell membrane that is separated by tight junctions?

Answer 5

Basolateral membrane (proteins in here can move along the whole membrane in one leaflet)
Apical membrane (aka luminal or mucosal)

Question 6

How do tight junctions act as a barrier?

Answer 6

Prevention of molecules passing between adjacent cells.

Question 7

What is a leaky epithelium?

Answer 7

There is more paracellular transport than transcellular

Question 8

What is a tight epithelium

Answer 8

There is more transcellular transport than paracellular

Question 9

What are the features of paracellular transport?

Answer 9

Governed by laws of diffusion and tightness of junctions. Electrical resistance to ion flow can be measured.

Question 10

How is electrical resistane measured in paracellular transport?

Answer 10

if we have a current through it and a voltage difference, we can calculate resistance

Question 11

What are the features of epithelium regarding tight junctions closer to the kidney? (proximal)

Answer 11

More leaky epithelium
Low electrical resistance
low number of strands
mostly paracellular
(duodenum, proximal tubule)

Question 12

What are the features of epithelium regarding tight junctions further from the kidney? (distal)

Answer 12

Tight epithelium
High electrical resistance
High number of strands
Mostly transcellular (hormonally controlled)
(colon, collect duct)

Question 13

What are the rules to consider with epithelial transport?

Answer 13

Entry and exit steps (where is the entry and exit?)
Electrochemical gradient (is it active or passive transport?)
Electroneutrality (does the movement of ions attract a counter ion?)
Osmosis (Movement of ions results in movement of water)

Question 14

If we want to do secretion, where are the different types of transporters going to be?

Answer 14

Primary active transporter at the basolateral membrane sets up the ion gradients (Na/KATPase)
Entry step (usually secondary active)
Exit step (often passive diffusion)

Question 15

Where will the Na/KATPase primary active tranporter be 99% of the time?

Answer 15

In the basolateral membrane

Question 16

What does the entry and exit steps depend on?

Answer 16

Whether we are doing absorption or secretion

Question 17

When is a circumstance when you will NOT get movement of water and counter ions after absorption/secretion?

Answer 17

When the tight junctions are very tight and don’t allow the movement of them

Question 18

What are the steps to glucose absorption?

Answer 18

1. Tight junctions divide into apical and basolateral membranes
2. Na/KATPase sets up ion gradients (carrier mediated, primary active- antiporter/exchanger)
3. Sodium glucose symporter/transporter (SGLT) (secondary active, carrier mediated) uses Na/KATPase gradient energy to bring glucose in against its gradient
4. facilitated glucose transporter (GLUT) mediates glucose exit down electrochemical gradient across basolateral membrane.
5. Na is taken up via the SGLT and exits via the Na/KATPase

Question 19

What is oral rehydration therapy?

Answer 19

It utilised glucose and sodium chloride to stimulate the uptake of fluids

Question 20

What is glucose galactose malabsorption syndrome?

Answer 20

A mutation to the glucose symporter which causes it to fail in uptake of glucose, and so sugar is retained in the lumen and so the osmolarity of the lumen increases, water enters the lumen, causing diarrhea

Question 21

How can we treat glucose galactose malabsorption syndrome?

Answer 21

By eating foods with fructose (no glucose or galactose in diet) as the source of carbohydrates (since the GLUT5 transporter still functions) 5 carbon instead of 6. Fructose exits cell via GLUT2 at the basolateral membrane

Question 22

Define osmolarity in terms of a molecule of carbohydrate/starch

Answer 22

Even though a starch molecule is really big in terms of osmolarity, since osmolarity is defined as the number of osmotic active particles in a defined solution, it doesn’t have a big osmolarity

Question 23

How does osmolarity in the lumen increase after eating?

Answer 23

More glucose molecules in the lumen increases its osmolarity, unless it is removed from the lumen along with water and other ions.

Question 24

How much water does your kidney filter a day?

Answer 24

180L

Question 25

What will appear in the urine if it is not reabsorbed after being filtered by the kidney?

Answer 25

Glucose in the plasma of the blood

Question 26

What is the result of glucose passing through the epithelial cells into the blood from the kidney tubule?

Answer 26

Movement of chloride and H2O

Question 27

What is glucosuria?

Answer 27

Glucose in the urine

Question 28

Where is the only place in the kidney that glucose is absorbed?

Answer 28

The tubules in the kidney

Question 29

What happens in diabetes?

Answer 29

SGLT cannot absorb the glucose fast enough and so it appears in the urine.

Question 30

At what figure is diabetes considered present?

Answer 30

At 200mg/ml plasma. This is the renal threshold, and once it is reached, glucose appears in urine.

Question 31

What is the renal threshold?

Answer 31

When all the binding sites for glucose are filled and the uptake is saturated. It is now that glucose will begin to appear in urine. REPRESENTS THE TRANSPORT MAXIMUM OF SGLT