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Gastroenterology > Ions, Vitamins, Minerals > Flashcards

Ions, Vitamins, Minerals Flashcards

1
Q

What is paracellular transport?

A

through tight junctions and lateral intercellular spaces.

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2
Q

What is Transcellular transport?

A

through the epithelial cells.

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3
Q

What are channel proteins and carrier proteins?

A
  • Channel proteins form aqueous pores allowing specific solutes to pass across the membrane.
    • Carrier proteins bind to the solute and undergo a conformational change to transport it
      across the membrane.
  • Channel proteins allow much faster transport than carrier proteins.
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4
Q

What types of gated channel proteins are there?

A

Voltage gated

Ligand gated - extracellular ligand

Ligand gated - intracellular ligand

Mechanically gated

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5
Q

What is the difference between primary active transport and secondary active transport?

A

Primary active transport is linked directly to cellular metabolism (uses ATP to power the transport).

Secondary active transport derives energy from the concentration gradient of another substance that is actively transported.

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6
Q

Examples of primary active transporters?

A

Na+/K+ ATPase (Pancreatic HCO3- Secretion)

H+/K+ ATPase (Stomach – Parietal Cell)

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7
Q

Examples of secondary active transporters?

A

SGLT-1 co-transport (Small bowel absorption of monosaccharides)

HCO3-/Cl- counter transport (Pancreatic HCO3- Secretion)

Na+/H+ counter transport (Pancreatic HCO3- Secretion

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8
Q

Examples of facilitated diffusion transporters?

A

GLUT-5, GLUT-2 (Small bowel absorption of monosaccharides)

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9
Q

How is glucose and galactose absorbed ( carbohydrates )?

A

Absorption of glucose & galactose is by 2o active transport (carrier protein & electrochemical gradient). Carrier protein = SGLT-1 on apical membrane.

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10
Q

Why is the SGLT1 transporter useful?

A

SGLT1 can transport glucose against its gradient

  • it is effective when glucose at levels in the lumen are below those in the enterocyte.
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11
Q

How is fructose absorbed?

A

Facilitated diffusion : GLUT-5 carrier protein on apical membrane

  • Effective at relatively low concentrations of fructose in the lumen as tissue and plasma levels are low.
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12
Q

How is glucose secreted out?

A

facilitated diffusion.

Carrier protein = GLUT-2, a high-capacity, low-affinity facilitative transporter.

  • Glucose between plasma and tissue/enterocyte generally equilibrated.
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13
Q

Where is water absorbed the most and how?

A

The greatest amount of water is absorbed in the small intestine, esp the jejunum.

The absorption of water is powered by the absorption of ions.

99% of the H2O presented to the GI tract is absorbed.

Many ions slowly absorbed by passive diffusion.

Calcium and iron are incompletely absorbed, and this absorption is regulated.

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14
Q

How much water is absorbed in the small and large bowel?

A

Approximately 8 litres of water a day absorbed in the small bowel.

Approximately 1.4 litres of water a day absorbed in the large bowel.

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15
Q

How much water is absorbed in the small and large bowel?

A

Approximately 8 litres of water a day absorbed in the small bowel.

Approximately 1.4 litres of water a day absorbed in the large bowel.

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16
Q

How is the osmosis gradient created?

A

Driven by Na+

Transport of sodium from lumen into enterocytes becomes more efficient as it travels down the intestine

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17
Q

How is does Na+ absorption change going down the intestine?

A

Becomes more efficient as travel down intestine:

  • Counter-transport in exchange for H+ (proximal bowel)
  • Co-transport with amino acids, monosaccharides (jejunum)
  • Co-transport with Cl- (ileum)
  • Restricted movement through ion channels (colon)
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18
Q

Describe the absorption of Chlorine ions and potassium ions?

A

Cl- co-transported with Na+ (ileum), exchanged with HCO3- (colon) into enterocytes.
Both secondary active transport

K+ diffuses in via paracellular pathways in small intestine, leaks out between cells in colon.
Passive transport.

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19
Q

What happens to intracellular sodium?

A

Active transport of Na+ into the lateral intercellular spaces by Na+K+ATPase transport in the lateral plasma membrane

20
Q

Why is the fluid in between intracellular spaces hypertonic?

A

High concentrations of ions in the fluid

Due to Cl- and HCO30 transported into the spaces by the electrical potential generated by Na+ transport

  • Water distends the intercellular channels and causes increased hydrostatic pressure.

Ions and water move across the basement membrane of the epithelium and are carried away by the capillaries.

21
Q

What absorbs calcium ions?

A

Duodenum and Ileum absorb Ca2+

Ca2+ deficient diet increases gut’s ability to absorb

Vit D and parathyroid hormone stimulate absorption

Diet 1-6g/day, secretions 0.6g. Absorb 0.7g

22
Q

What are the normal calcium concentrations intra and extra cellular?

A

Low intracellular [Ca2+] approx 100 nM (0.1µM)
(but can increase 10 to 100-fold during various cellular functions).

High extracellular fluid [Ca2+] approx 1-3mM.
(Plasma [Ca2+] approx 2.2-2.6mM)
(Luminal [Ca2+] varies inmM range)

23
Q

What is the calcium transporter?

A

Ca2+ carried across apical membrane by 2 methods :

  • Intestinal calcium-binding protein (IMcal)- facilitated diffusion.
  • Ion channel
24
Q

What are the implications for transport of Ca2+ into the cell from the lumen?

A

Ca2+ acts as an intracellular signalling molecule.

25
Q

What are the implications for Ca2+ transport across the cell?

A
  • Need to transport Ca2+ while maintaining low intracellular concentrations.
  • Binds to Calbindin in cytosol, preventing its action as an intracellular signal
26
Q

What transporter maintains the very low concentrations of calcium normally observed within a cell?

A

Ca2+ pumped across basolateral membrane by plasma membrane Ca2+ ATPase (PMCA) against concentration gradient.

PMCA has a high affinity for Ca2+ (but low capacity).

27
Q

What transporter as a low affinity for Ca2+ but a high capacity?

A

The Na+/Ca2+ exchanger has a low affinity for Ca2+ but a high capacity. Requires larger concentrations of Ca2+ to be effective

Ca2+ pumped across basolateral membrane by plasma mem

28
Q

What does Vitamin D do to increase calcium absorption?

A

1, 25-dihydroxy D3 taken up by 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.

29
Q

What can iron act as?

A

Electron donor and electron acceptor

Critical for oxygen transport

Oxidative phosphyrlation

30
Q

How is Iron present in the diet?

A
  • Inorganic iron - ferric, ferrous

- heme group

31
Q

The body cannot absorb Fe3+, what insoluble salts are formed?

A

Hydroxides
Phosphate
HCO3-

  • Vit C reduces Fe3+ to Fe2+
    Heme smaller part of diet, but more readily absorbed (20% of presented, rather than 5%).
32
Q

What transporter absorbs Dietary heme?

A

heme carrier protein 1 (HCP-1), and via receptor-mediated endocystosis.

Heme is absorbed intact into the enterocyte.

Fe2+ liberated by Heme oxygenase.

33
Q

What catalyses the reduction of Fe3+ to Fe2+?

A

Duodenal cytochrome B (Dcytb)

Fe2+ transported via divalent metal transporter 1 (DMT-1), a H+-coupled co-transporter.

Fe2+ binds to unknown factors, carried to basolateral membrane, moves via ferroportin ion channel into blood.

34
Q

What is Hephaestin?

A

Transmembrane copper-dependent ferroxidase that converts Fe2+ into Fe3+

The Fe3+ binds to apotransferrin travelling in the blood as transferrin

35
Q

What does Hepcidin do?

A

the major iron regulating protein, suppresses ferroportin function

to decreases iron absorption

36
Q

How is a ferritin micelle formed?

A

Apoferritin in cytosol binds to Fe2+

Ferritin is a globular protein complex, Fe2+ oxidises to Fe3+ which crystallises within protein shell

37
Q

What happens to this ferritin stored in enterocytes?

A

Prevents absorption of too much iron (toxic)

38
Q

Increase in iron concentration in the cytosol increases x?

A

x = ferritin synthesis

Ferritin is lost in intestinal lumen as it doesn’t go into lumen and is excreted to faeces ( iron removal )

39
Q

What are vitamins?

A

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

40
Q

What are fat soluble vitamins?

A

A, D, E, K

Transported to brush border in micelles * K taken up by active transport

( Passive diffusion predominant mechanism
)

41
Q

What vitamins have specific transport mechanisms?

A

vitamin C (ascorbic acid), folic acid, vitamin B1 (thiamine), vitamin B12

42
Q

What is Pernicious anaemia?

A

Impaired absorption of vit B12 retards the maturation of red blood cells

  • most vitB12 in food is bound to proteins
43
Q

What allows VitB12 to be released from its protein?

A

In the stomach low pH, digestion of the protein by pepsin

44
Q

B12 is easily denatured by HCl, how is this avoided?

A

Binds to R protein (haptocorrin) released in saliva and from parietal cells.

R proteins digested in duodenum.

45
Q

What is needed to allow Vitamin B12 absorption?

A

Intrinsic Factor- IF

Secreted by parietal

Vit B12/IF complex binds to cubilin receptor, taken up in distal ileum.

46
Q

What happens to the VitB12 / IF complex once in cells?

A

It is broken

B12 then binds to a protein - transcobalamin II - TCII and crosses basolateral membrane

47
Q

How does B12 travel to the liver?

A

Bound to TCII

TCII receptors allow the cells to take up the complex

Proteolysis then breaks down TCII inside the cell

Gastroenterology (10 decks)

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