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BRS GASTROENTEROLOGY > Ions, Vitamins and Minerals > Flashcards

Ions, Vitamins and Minerals Flashcards

1
Q

What does molar mean?

A

one mole per litre

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2
Q

Convert these to 10 to the power of something.

A
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3
Q

What is diffusion?

A

The process whereby atoms or molecules intermingle because of their random thermal motion.

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4
Q

What is the speed of diffusion?

A

Diffusion occurs rapidly over microscopic distances, but slowly over macroscopic distances. (diffusion range)

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5
Q

How have multicellular organisms evolved around diffusion?

A

Multicellular organisms evolve circulatory systems to bring individual cells within diffusion range.

The cell membrane acts as a diffusion barrier, enabling cells to maintain cytoplasmic concentrations of substances different from their extracellular concentrations.

Lipid soluble (non-polar) molecules can cross more easily than water soluble (polar) molecules.

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6
Q

What is a hypotonic and hypertonic solution?

A

hypo= lots of water

hyper= less intial water

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7
Q

What type of solutions does osmosis go through?

A

hypotonic-> isotonic-> hypertonic

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8
Q

How can molecules cross the epithelium to enter the bloodstream?

A

Paracellular Transport through tight junctions and lateral intercellular spaces.

Transcellular Transport through the epithelial cells.

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9
Q

How can solutes cross cell membranes?

A
  • simple diffusion
  • facilitated transport
  • active transport
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10
Q

What are the types of transport proteins involved in solutes crossing cell membranes?

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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11
Q

What are the types of carrier-mediated transport for membrane transport?

A
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12
Q

Describe ion channels?

A
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13
Q

How do different membrane transports open?

A
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14
Q

What is primary and secondary active transport?

A

Active transport requires energy:

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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15
Q

What is facilitated transport/ facilitated diffusion?

A

Enhances the rate a substance can flow down its concentration gradient. This tends to equilibrate the substance across the membrane and does not require energy.

  • the passive movement of molecules across the cell membrane via the aid of a membrane protein
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16
Q

Give an overview of the methods of transport. (passive and active)

A
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17
Q

Examples of Primary active transporters already covered in the course?

A

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

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

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18
Q

Examples of Secondary active transporters already covered in the course?

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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19
Q

Examples of Facilitated transport /facilitated diffusion transporters already covered in the course?

A

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

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20
Q

Describe the absorption of glucose and galactose (carbohydrates).

A

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

SGLT1 can transport glucose uphill against its concentration gradient (so effective when glucose at levels in the lumen are below those in the enterocyte).

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21
Q

Describe the absorption of fructose (carbohydrates).

A

Absorption of fructose is by facilitated diffusion.
- Carrier protein = GLUT-5 on apical membrane.

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

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22
Q

Describe the exit of glucose.

A

Exit of glucose at the basolateral membrane is by 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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23
Q

How water in the GI tract is abosrbed?

A

99%

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24
Q

What powers the absorption of water?

A

absorption of ions

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25
Q

Where is the greatest amount of water absorbed?

A

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

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26
Q

How are many ions absorbed?

A

Many ions slowly absorbed by passive diffusion

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27
Q

How are calcium and iron absorbed?

A

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

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28
Q

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

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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29
Q

Where is the water than is being absorbed come from?

A

Ingest 2L
Saliva 1.2L
Gastric secretions 2L
Bile 0.7L
Pancreas 1.2L
Intestinal 2.4L

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30
Q

What drives the standing gradient osmosis?

A

Na+

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31
Q

What is the direction of transport for Na+?

A

Transport of Na+ from lumen into enterocyte- complex and varies between species.

32
Q

How does efficiency of Na+ transport increase and how?

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)

33
Q

What are the secondary active transport mechanisms in the absorption of other ions?

A

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

34
Q

What are the passive transport mechanisms in the absorption of other ions?

A

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

35
Q

What happens to intracellular Na+ in enterocytes?

A

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

36
Q

How are Cl- and HCO3- transported into the intercellular spaces?

A

Cl- and HCO3- transported into the intercellular spaces due to electrical potential created by the Na+ transport.

High conc of ions in the intercellular spaces causes the fluid there to be hypertonic

37
Q

How does the movement of ions cause the movement of water?

A

Osmotic flow of water from the gut lumen via adjacent cells, tight junctions into the intercellular space.

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.

38
Q

Where is Ca2+ absorbed?

A

Duodenum and Ileum absorb Ca2+

39
Q

What does a CA2+ deficient diet lead to?

A

Ca2+ deficient diet increases gut’s ability to absorb.

40
Q

What stimulates absorption of Ca2+?

A

Vit D and parathyroid hormone stimulate absorption.

41
Q

How much of ion is in our diet, secreted and absorbed every day?

A

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

42
Q

Describe Ca2+ absorption.

A
43
Q

What is the concentration of Ca2+ in different areas of the lumen and plasma and cells?

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 2.2-2.6 mM)
- (Luminal [Ca2+] varies in mM range)

44
Q

How is Ca2+ carried across apical membrane?

A

i) Intestinal calcium-binding protein (IMcal)- facilitated diffusion.

ii) Ion channel

45
Q

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

A

Ca2+ acts as an intracellular signalling molecule.

46
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.

47
Q

How is Ca2+ moved across the basolateral membrane?

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).

Maintains the very low concentrations of calcium normally observed within a cell.

and

Ca2+ pumped across basolateral membrane by plasma membrane Na+/Ca2+ exchanger against concentration gradient.

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

48
Q

What does vitamin D deficiency cause and what is it essential for

A

rickets, osteoporosis

It is essential for Ca2+ absorption

49
Q

How is D3 taken up by enterocytes?

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.
50
Q

What can iron acts as?

A

Iron can act as an electron donor and an electron acceptor

51
Q

What processes in the body is iron critical for?

A

Oxygen transport (red blood cells)
Oxidative phosphorylation (mitochondrial transport chain)

52
Q

Describe the transport of iron in enterocytes.

A
53
Q

When is iron toxic?

A

Iron is toxic in excess, but the body has no mechanism for actively excreting iron.

54
Q

How much iron do we ingest/ absorb?

A

Adult ingests approx 15-20 mg/day

But absorbs only 0.5-1.5mg/day

55
Q

How is iron present in the diet?

A

a) inorganic iron (Fe3+ ferric, Fe2+ ferrous)

b) as part of heme (haem) group (haemoglobin, myoglobin and cytochromes).

56
Q

What type of iron can we absorb?

A

Cannot absorb Fe3+, only Fe2+.

Fe3+ insoluble salts with:
- hydroxide,
- phosphate
- HCO3-

57
Q

What reduces F23+ to Fe2+?

A

vitamin C

58
Q

Can hem be absorbed?

A

Heme smaller part of diet, but more readily absorbed (20% of presented, rather than 5%).

59
Q

How is heme absorbed?

A

Dietary heme is highly bioavailable.

Heme is absorbed intact into the enterocyte.

Evidence that this occurs via heme carrier protein 1 (HCP-1), and via receptor-mediated endocytosis.

Fe2+ liberated by Heme oxygenase.

60
Q

Describe iron uptake.

A

Duodenal cytochrome B (Dcytb) catalyzes the reduction of Fe3+ to Fe2+ in the process of iron absorption in the duodenum of mammals.

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

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

Fe2+ moves across basolateral membrane via ferroportin.

Hephaestin is a transmembrane copper-dependent ferroxidase that converts Fe2+ to Fe3+.

Fe3+ binds to apotransferrin, travels in blood as transferrin.

(Hepcidin, the major iron regulating protein, suppresses ferroportin function to decreases iron absorption).

61
Q

What is another way of absorbing iron via ferritin?

A

Binds to apoferritin in cytosol to form ferritin micelle.

Ferritin is globular protein complex. Fe2+ is oxidised to Fe3+ which crystallises within protein shell.

A single ferritin molecule can store up to 4,000 iron ions.

In excess dietary iron absorption, produce more ferritin.

62
Q

What happens to this ferritin stored in enterocytes?

A

Prevents absorption of too much iron (toxic)

63
Q

Describe the role of ferritin.

A

Irreversible binding of iron to ferritin in the epithelial cells.

Iron/Ferritin is not available for transport into plasma.

Iron/Ferritin is lost in the intestinal lumen and excreted in the faeces.

Increase in iron concentration in the cytosol increases ferritin synthesis.

64
Q

What are vitamins?

A

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

Passive diffusion predominant mechanism

Fat soluble vitamins (A,D,E,K) transported to brush border in micelles. K taken up by active transport.

Specific transport mechanisms for vitamin C (ascorbic acid), folic acid, vitamin B1 (thiamine), vitamin B12

65
Q

What contains a large store of vitamin B12?

A

Liver contains a large store (2-5mg).

66
Q

WHat happens with impaired absorption of vit B12?

A

Impaired absorption of vit B12 retards the maturation of red
blood cells - pernicious anaemia. (macrocytic anaemia)

67
Q

What is most vit B12 in food found?

A

bound to proteins

68
Q

Describe transport of vitamin B12.

A

Intrinsic factor (IF) produced in stomach

69
Q

How can B12 be denatured in the stomach?

A

In the stomach, low pH and the digestion of proteins by pepsin releases free vit B12.

But B12 is easily denatured by HCl.

70
Q

How is the denaturation of B12 in the stomach avoided?

A

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

R proteins digested in duodenum.

71
Q

What does IF do for absorption of vitamin B12?

A

Vit B12 binding glycoprotein secreted by parietal cells.

Vit B12/IF is resistant to digestion.

No IF then no absorption of vit B12

Vit B12/IF complex binds to cubilin receptor, taken up in distal ileum (mechanism unknown, but thought to involve receptor-mediated endocytosis).

72
Q

What happens to Vit B12/IF once inside the cell?

A

Once in cell, Vit B12/IF complex broken- possibly in mitochondria

B12 binds to protein transcobalamin II (TCII), crosses basolateral membrane by unknown mechanism

Travels to liver bound to TCII

TCII receptors on cells allow them to uptake complex

Proteolysis then breaks down TCII inside the cell (hepatocytes)

73
Q

Give a summary of water absorption.

A
74
Q

Give a summary of calcium absorption.

A
75
Q

Give a summary of iron absorption.

A
76
Q

Give a summary of B12 absorption.

A

BRS GASTROENTEROLOGY (25 decks)

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