GI Transporters

Question 1

How does alcohol get absorbed in the GI system?

Answer 1

simple diffusion - it’s lipid soluble so it will just go right thorugh

Question 2

WHat are the three types of facilitative diffusion in the gut?

Answer 2

pores (always open)
gated channels (sometimes open)
carrier proteins

Question 3

WHat is the difference between facilitative diffusion and active transport?

Answer 3

facilitative diffusion just helps molecules move down their concnetration gradients

active transport requires ATP fto move things up their gradients

Question 4

How do the kinetics differ between passive diffusion and carrier-mediated diffusion?

Answer 4

passive has no max - the higher the concentration of the substance, the more will be absorbed - it’s a linear relationship

WIth the facilitated, there is a maximum number of transporters, so there is a maximum rate of absorption - it reaches a V max

Question 5

Describe briefly the structure of human GLUTs.

Answer 5

They are a 12-transmembrane protein with intracellular C terminal and loops with a binding site for the sugar

Question 6

Which GLUT is the most common on RBCs and many other tissues?

Answer 6

GLUT1

Question 7

Which GLUT is insulin sensitive?

Answer 7

Glut4

Question 8

What are the ATP-Cassette binding transporters used for?

Answer 8

They’re efflux pumps that will pump bad stuff out

P-glycoprotein is a member

Question 9

Where in the small intestine are most of the specific transporters for nutritnets and water located?

Answer 9

jejunum

Question 10

What type of transport is primarily located on the apical side?

Answer 10

active transport requiring ATP

Question 11

What type of transport is primarily located on thebasolateral side?

Answer 11

carrier-mediated diffusion

Question 12

Which side are the ABC-type transporters located?

Answer 12

both

Question 13

The ileum can be called on for absorption if anytning useful makes it though the duodenum. How about the colon?

Answer 13

transverse colon is for reclamation of fluid, electrolytes and bile salts

Question 14

Why is it a good idea to have ABC-type transporters in the GI system?

Answer 14

It makes sense because the GIt ract is exposed to many potentially toxic substances in the things we eat and drink

having the ABC-type transporters in the gut gives the gut epithelial cells a chance to spit back bad things into the lumen for excretion

Question 15

How does the parietal cell make acid?

Answer 15

1. K activates an H/K ATPase, which will exchange K for an H - the H goes into the lumen
2. A chloride channel opens and Cl moves out the cell into the lumen to join with the H and form HCl

Note that the H originatelly came from a dissociation of carbonic acid into bicarb, a reaction that is made favorable in the epithelial cell due to a pumping of bicarb out of the cell into the blood in exchange for Cl (which then goes into the lumen to make the HCl)

Question 16

What does omeprazole do to the parietal cell to block acid formation?

Answer 16

A sulfur group on the drug interacts with a sulfure group on the H/K ATPase such that they form a complex that inactivates the tranporter.

Question 17

What are the two general routes for tranpsort of molecules and ions across the epithelium of the gut?

Answer 17

1. transcellular

2. paracellular

Question 18

What about the epithleial cells of the GI system limit paracellular transport?

Answer 18

tight junctions

Question 19

What two distinct processes establish an osmotic gradient that pulls water into the lumen of the intestine?

Answer 19

1. increased luminal osmotic pressure resulting from influx and digestion of foodstuffs
2. crypt cells actively secreting electroltyes leading to water secretion (as in CFTR and cholera=diarrhea)

Question 20

What is probably the most important process that takes place in the small intestine to make such absorption possible?

Answer 20

Establishing an electrochemical gradient of sodium using the NaK ATPase

Question 21

What transporter takes glucose into the intestinal epithelial cells?

Answer 21

SGLT1

sodium-dependent glucose transporter 1

Question 22

What transporter takes fructose intot he intestinal epithelial cells?

Answer 22

GLUT5

Five = Fructose

Question 23

What basolateral transporter will pump both glucose and fructose intot he blood?

Answer 23

GLUT2

2 = both

Question 24

What form must proteins be in for gut epithelial cells to take them in?

Answer 24

They need to be small - either amino acids, dipeptides or tripeptides

Question 25

WHat is a super important exception to this?

Answer 25

Neonates have the ability to absorb intact proteins whih is important because it allows newborn babies to acquire passive immunity by absorbing immunoglobulins in colostral milk

Question 26

WHat’s special about lipid absorption from the gut?

Answer 26

It goes into lymph before going into the blood

Question 27

What forms do lipids need to be in for the gut epithelial cells to take them up?

Answer 27

fatty acids or 2-monoglyceride

they enter the cell by simple diffusion

Question 28

Where is the MCT1 monocarboxylic acid transport 1 located in the gut?

Answer 28

colon

Question 29

If most gut nutrient absorbtion occurs in the small itnestine, why would we need a MCT1 in the colon??

Answer 29

colonic bacteria break down the undigestible starches to short chain fatty acids like butyrate which can then be taken up through MCT1 by the colonocytes to use for fuel