Specific Issues in Alimentary Absorption

Question 1

What is the role of tight junctions with regards to plasma membrane proteins?

Answer 1

They form a fence so that the plasma proteins that are on the apical membrane can’t cross over to the basolateral membrane. This establishes polarity.

Question 2

What’s the difference between carrier proteins and channel proteins?

Answer 2

Carrier proteins bind to the molecule and then undergo a conformational change that transports the molecule across the membrane.
Channel proteins are just aqueous pores allowing ions through.

Question 3

Through which transporter does glucose and galactose absorption take place?

Answer 3

SGLT-1

Question 4

Through which facilitated diffusion channel down fructose absorption take place?

Answer 4

Glut-5

Question 5

Through which facilitated diffusion channel does exit of glucose into the blood take place?

Answer 5

Glut-2

Question 6

What percentage of water presented to the GI tract is absorbed?

Answer 6

99%

Question 7

Where is most water absorbed? State the amount absorbed by the small intestine and that large intestine.

Answer 7

Most water is absorbed in the small intestine
Small intestine = 8 L/day
Large intestine = 1.4 L/day

Question 8

State some of the sources of the extra 8 L of water that is reabsorbed by the gut every day.

Answer 8

Bile, saliva, pancreatic secretions, intestinal secretions, gastric secretions

Question 9

Name the process by which water is absorbed by the gut.

Answer 9

Standing Gradient Osmosis

Question 10

Describe the transport of Na+ in different areas of the gut.

Answer 10

Jejunum - Na+ is co-transported with amino acids
Ileum - Na+ is co-transported with Cl-
Proximal Colon - Na+ is counter-transported with H+
Colon - Na+ moves in via ion channels

Question 11

Describe the movement of K+ throughout the intestines.

Answer 11

K+ diffuses IN via paracellular pathways in the small intestine
K+ leaks out between cells in the colon

Question 12

Describe the movement of Cl- throughout the intestines.

Answer 12

Cl- is cotransported with Na+ in the ileum

Cl- is exchanged with HCO3- in the colon

Question 13

Describe the process of standing gradient osmosis.

Answer 13

Absorption of Na+ creates a high intracellular Na+ concentration
Na+/K+ channel pumps Na+ ions into the intercellular space
This creates an electrochemical gradient that draws anions into the intercellular space
You then get a concentration ionic solution surrounding the cells, which helps draw water via paracellular pathways into the intercellular space
As the water moves in you get an increase in hydrostatic pressure, which then forces water and ions to move across the membrane and into the blood stream.

Question 14

Which parts of the small intestines absorb calcium?

Answer 14

Duodenum and Ileum

Question 15

State two products that stimulate absorption of calcium.

Answer 15

Vitamin D

Parathyroid Hormone

Question 16

How much calcium is contained in the diet and how much is absorbed?

Answer 16

Diet = 1-6 g/day
Secreted = 0.6 g/day
Absorbed = 0.7 g/day

Question 17

What is the concentration of calcium intracellularly, extracellularly and in the blood plasma?

Answer 17

Intracellular = 0.1 mcM
Extracellular = 1-3 mM
Blood = 2.2-2.6 Mm

Question 18

What two channels transport calcium into the cell from the lumen?

Answer 18

IMcal - intestinal calcium binding protein (facilitated diffusion)
Ion channel

Question 19

What binds to and transport calcium within the cell to prevent it from triggering various intracellular pathways?

Answer 19

Calbindin

Question 20

What two channels transport Ca2+ out of the basolateral membrane and how are they different?

Answer 20

PMCA - Plasma Membrane Calcium ATPase
Ca2+/Na+ Exchanger
PMCA - high affinity + low capacity
Ca2+/Na+ Exchanger - low affinity + high capacity

Question 21

What effects does vitamin D3 have on absorption of Ca2+?

Answer 21

Increases calcium absorption by enhancing transport through the cytoplasm, increasing levels of calbindin and increasing Ca2+ ATPase

Question 22

What is the normal dietary intake of iron and how much is absorbed?

Answer 22

15-20 mg/day

0.5-1.5 mg/day

Question 23

What is iron present in the diet as?

Answer 23

Inorganic ions - Fe2+ and Fe3+

Heme

Question 24

What effect does vitamin C have on iron?

Answer 24

It reduces Fe3+ to Fe2+

Question 25

How is heme absorbed and how is its Fe2+ group liberated?

Answer 25

Heme is absorbed in tact through heme carrier protein (HCP-1)
Fe2+ group is liberated by heme oxygenase

Question 26

What reduces Fe3+ to Fe2+ in the duodenum?

Answer 26

Duodenal cytochrome B

Question 27

What transporter allows Fe2+ into the enterocyte?

Answer 27

Divalent Metal Transporter - DMT-1 (H+ cotransport)

Question 28

Through which channel does the Fe2+ enter the blood?

Answer 28

Ferroportin

Question 29

What transmembrane protein converts Fe2+ back into Fe3+ on the basolateral membrane?

Answer 29

Hephaestin

Question 30

What does Fe3+ bind to in the blood to travel?

Answer 30

Apotransferrin - forms transferrin

Question 31

What is the major iron regulating protein and what does it do?

Answer 31

Hepcidin - it suppresses ferroportin function and hence decreases iron absorption

Question 32

Describe the storage of iron.

Answer 32

Fe2+ in the cell can bind to apoferritin forming a ferritin micelle
Fe2+ is oxidised to Fe3+ which crystallises within the protein shell
A single ferritin molecule can store up to 4000 iron ions

Question 33

What happens to the ferritin?

Answer 33

The ferritin is removed when the enterocytes die and are excreted in the faeces

Question 34

What is the main mechanism for absorption of vitamins?

Answer 34

Passive diffusion

Question 35

How are fat soluble vitamins (ADEK) absorbed? How is Vitamin K, in particular, absorbed?

Answer 35

They are packaged into micelles and taken to the brush border.
Vitamin K is taken up by active transport.

Question 36

How big is the liver’s store of vitamin B12?

Answer 36

2-5 mg

Question 37

Describe the transport of Vitamin B12 from the stomach to the distal ileum.

Answer 37

Vitamin B12 is released by the action of acid and pepsin
Free B12 then binds to R protein (which is in the saliva and produced by parietal cells). The R protein prevents denaturation of free B12 in the stomach.
R protein is digested in the small intestine
B12 binds to intrinsic factor and is transported down the small intestine
The B12-intrinsic factor complex is then absorbed by the enterocytes by binding to cubulin receptors

Question 38

What does Vitamin B12 bind to for transport to the liver?

Answer 38

Transcobalamin II