Lecture 24: Traffic Across Cells: Glucose Absorption

Question 1

How many steps in glucose absorption?

Answer 1

2

Question 2

What protein contransports glucose and sodium across the apical membrane?

Answer 2

SGLT transport membrane (sodium glucose symporter)

Question 3

How does glucose exit the Basolateral membrane?

Answer 3

By facilitative diffusion mediated by GLUT (transporter protein)

Question 4

What provides the energy for glucose entry?

Answer 4

The sodium gradient, maintained by the sodium pump

Question 5

What happens to water during glucose absorption?

Answer 5

The movement of glucose across the epithelium creates osmotic imbalance, which drives the absorption of water (but this is paracellular)

Question 6

6 features of the glucose absorption cell?

Answer 6

1. Tight junctions=apical/Basolateral domains
2. Na/K pump sets up ion gradients
3. Sodium glucose symporter uses energy of Na gradient to actively accumulate glucose above its transportation gradient
4. Facilitative glucose transporter mediates glucose exit across Basolateral membrane via passive diffusion down gradient
5. Na taken up by symporter, exits via pump
6. Transport of Na and glucose across epithelium induces paracellular Cl and water fluxes

Question 7

How does the glucose gradient work?

Answer 7

When eating, glucose levels accumulate in the cell (above blood level) and passively diffuse into blood. When fasting, glucose levels in cell are low so must actively cross to blood, or blood glucose can be transport to cell vice versa. But depends on gradient

Question 8

What is oral rehydration therapy?

Answer 8

Giving a simple sugar/water solution, instead of straight water, to improve hydration

Question 9

How does oral rehydration work?

Answer 9

By including sugar, the glucose restores electrolytes in your body (such as sodium, chloride, potassium and water), all of which are enhanced by the uptake of glucose

Question 10

Who is oral rehydration most effect for?

Answer 10

Dehydrated babies suffering from diarrhoea- this saves millions of lives per year

Question 11

What is glucose-galactose malabsorption syndrome?

Answer 11

A mutation where it knocks out gene 1 (one type of sodium-glucose co-transporter protein) (SGLT)

Question 12

Where is gene 1 of the glucose-sodium transporter primarily found?

Answer 12

In the small intestine

Question 13

What does the glucose-galactose malabsorption syndrome result in?

Answer 13

No uptake of glucose in cells, so sugar is retained in the intestine lumen

Question 14

What is galactose?

Answer 14

Sugar from milk

Question 15

What does the resulting non-breakdown of starch result in?

Answer 15

A change in osmolarity (do water flows into the small intestine)

Question 16

What does the increase in lumen osmolarity result in?

Answer 16

A water efflux, where the osmotic imbalance creates increased water flow produces produced diarrhoea (watery chyme)

Question 17

How is glucose-galactose malabsorption syndrome treated?

Answer 17

By removing glucose/galactose from the diet and using a different transporter to uptake fructose (GLUT5)

Question 18

What transporter is used to treat glucose-galactose malabsorption syndrome?

Answer 18

The facilitative transporter specifically for fructose (a 5 carbon sugar) (GLUT5)

Question 19

What is glucose reabsorption in the kidneys?

Answer 19

Glucose in the plasma needs to be reabsorbed or it will appear in the urine
There needs to be a high uptake of glucose as glucose is constantly coming

Question 20

What happens if glucose system in kidneys can’t cope?

Answer 20

It gets saturated (not enough binding pockets)

Question 21

What is glucosuria?

Answer 21

Glucose in the urine (When transport maximum of SGLT protein is reached)

Question 22

What is the most common cause of glucosuria?

Answer 22

Diabetes mellitis, because insulin activity is deficient and blood sugar is too high (over 200mg/ml)

Question 23

What happens to the glucose symporter in diabetes?

Answer 23

It cannot absorb glucose fast enough, hence glucose appears in urine

Question 24

Do kidneys and the small intestines use the same transporter for glucose?

Answer 24

No, kidneys use SGLUT2 which has a higher capacity (but if there’s too much glucose, even SGLUT2 can’t cope)

Question 25

What happens if glucose absorption is impaired in kidney, or transporter is saturated?

Answer 25

Glucose appears in urine

Question 26

When does filtered glucose stop being reabsorbed?

Answer 26

When the renal threshold is reached

Question 27

What happens when the renal threshold is reached?

Answer 27

Glucose appears in urine and the renal threshold reflects the transport maximum of SGLT2

Question 28

What is the transport maximum and renal threshold?

Answer 28

Transport maximum= 375 mg/min

Renal threshold= 200mg/ml