Alimentary system 6 - Ions, minerals and vitamins

Question 1

What is paracellular transport?

Answer 1

Through tight junctions and lateral intercellular spaces

Question 2

What is transcellular transport?

Answer 2

Through the epithelial cells

Question 3

List the types of channels

Answer 3

• Voltage gated
• Ligand-gated (intracellular/extracellular)
• Mechanically gated

Question 4

Describe the process of carbohydrate absorption

Answer 4

• Glucose and galactose by secondary active transport (SGLT-1)
• Fructose by facilitated diffusion (GLUT-5)
• Exit the basolateral membrane by facilitated diffusion (GLUT-2)

Question 5

Compare daily water absorption in the small intestine and large intestine

Answer 5

• Small intestine 8L

- Large intestine 1.4L

Question 6

Describe the process of standing gradient osmosis

Answer 6

• Transport of Na+ into enterocytes (exchange for H+, co-transport with amino acids, glucose, and chloride, some movement through ion channels)
• Cl- exchanged with bicarbonate ions to enter enterocytes
• Potassium diffuses through paracellular pathways (passive)
• Sodium actively transported into intercellular spaces by sodium/potassium ATPase
• Cl- and bicarbonate transported into intercellular spaces due to electric potential created by sodium transport
• Water follows as the fluid is hypertonic

Question 7

Describe the absorption of calcium in the large intestine

Answer 7

• Stimulated by vitamin D and parathyroid hormone
• Carried across the apical membrane by intestinal calcium-inding protein (IMcal), and through ion channels
• Bound by calbindin to prevent intracellular signalling
• Pumped out the basolateral membrane by plasma membrane calcium ATPase (or sodium/calcium exchanger in high conc)

Question 8

List the effects of vitamin D on enterocytes

Answer 8

• Enhances the transport of Ca2+ through the cytosol
• Increases the levels of calbindin
• Increases rate of extrusion across basolateral membrane by increasing the level of Ca2+ ATPase in the membrane.

Question 9

Describe absorption of heme into the enterocyte

Answer 9

• Absorbed intact into the enterocyte by heme carrier protein 1
• Fe2+ is liberated by heme oxygenase

Question 10

Describe the process of iron uptake into the enterocytes

Answer 10

• Duodenal cytochrome B catalyses reduction of Fe3+ to Fe2+
• Fe2+ transported by divalent metal transporter 1 (H+ coupled transporter)
• Carried to basolateral membrane, where it moves via ferroportin into the blood

Question 11

Describe the two processes that happen to iron following movement through ferroportin in the intestine wall

Answer 11

• Fe2+ converted to Fe3+ by Hephaestin (copper dependent ferroxidase)
• Fe3+ binds to apotransferrin and travels in the blood as transferrin
  OR
• May also bind to apoferritin in cytosol to form a ferritin micelle in the cytosol
• Fe2+ converted to Fe3+ which crystallises within protein shell
• Single ferritin molecule stores up to 4000 iron ions

Question 12

Describe the action of hepcidin

Answer 12

Regulates iron and suppresses ferroportin

Question 13

What happens following iron/ferritin storage in the enterocyte?

Answer 13

• Irreversibly bound
• No transport to plasma
• Lost in the intestinal lumen and excreted into faeces

Question 14

Define vitamin

Answer 14

Organic compounds that cannot be manufactured by the body but are vital to metabolism

Question 15

How are vitamins transported?

Answer 15

• Passive diffusion

- Fat soluble vitamins (ADEK) transported in micelles. K is taken up by active transport

Question 16

Why is vitamin B12 important?

Answer 16

• Stored in the liver

- Impaired absorption means issues maturing red blood cells (pernicious anaemia)

Question 17

How is vitamin B12 transported through the stomach?

Answer 17

• R protein binds to vitamin B12 to prevent denaturing in the stomach
• Also called haptocorrin (released in saliva and parietal cells)

Question 18

What binds to vitamin B12 after R protein?

Answer 18

• Intrinsic factor
• Binds to B12 following digestion of R protein in the SI
• Cubilin recognises intrinsic factor in the ileum, and the complex is taken up into the cell

Question 19

What happens following entry of the vitamin B12/intrinsic factor complex entering the cell?

Answer 19

• Complex broken down
• B12 binds to transcobalamin II and crosses the basolateral membrane and travels to the liver
• Taken up into the liver by TCII receptors
• Proteolysis breaks down TCII inside the cell