Alimentary system 6 - Ions, minerals and vitamins Flashcards
What is paracellular transport?
Through tight junctions and lateral intercellular spaces
What is transcellular transport?
Through the epithelial cells
List the types of channels
- Voltage gated
- Ligand-gated (intracellular/extracellular)
- Mechanically gated
Describe the process of carbohydrate absorption
- Glucose and galactose by secondary active transport (SGLT-1)
- Fructose by facilitated diffusion (GLUT-5)
- Exit the basolateral membrane by facilitated diffusion (GLUT-2)
Compare daily water absorption in the small intestine and large intestine
- Small intestine 8L
- Large intestine 1.4L
Describe the process of standing gradient osmosis
- 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
Describe the absorption of calcium in the large intestine
- 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)
List the effects of vitamin D on enterocytes
- 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.
Describe absorption of heme into the enterocyte
- Absorbed intact into the enterocyte by heme carrier protein 1
- Fe2+ is liberated by heme oxygenase
Describe the process of iron uptake into the enterocytes
- 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
Describe the two processes that happen to iron following movement through ferroportin in the intestine wall
- 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
Describe the action of hepcidin
Regulates iron and suppresses ferroportin
What happens following iron/ferritin storage in the enterocyte?
- Irreversibly bound
- No transport to plasma
- Lost in the intestinal lumen and excreted into faeces
Define vitamin
Organic compounds that cannot be manufactured by the body but are vital to metabolism
How are vitamins transported?
- Passive diffusion
- Fat soluble vitamins (ADEK) transported in micelles. K is taken up by active transport
Why is vitamin B12 important?
- Stored in the liver
- Impaired absorption means issues maturing red blood cells (pernicious anaemia)
How is vitamin B12 transported through the stomach?
- R protein binds to vitamin B12 to prevent denaturing in the stomach
- Also called haptocorrin (released in saliva and parietal cells)
What binds to vitamin B12 after R protein?
- 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
What happens following entry of the vitamin B12/intrinsic factor complex entering the cell?
- 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
