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BRS GASTROENTEROLOGY > Ions, Vitamins and Minerals > Flashcards

Ions, Vitamins and Minerals Flashcards

1
Q

What is diffusion?

A

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

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2
Q

How does diffusion differ across microscopic distances as opposed to macroscopic distances?

A

Diffusion occurs rapidly over microscopic distances, but slowly over macroscopic distances.

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3
Q

How do multicellular organisms bring individual cell within diffusion ranges?

A

By having circulatory systems

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4
Q

What does the cell membrane do between cells?

A

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

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5
Q

Which molecules can easily cross the cell membrane barrier?

A

Lipid soluble (non-polar molecules)

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6
Q

What is meant by a hypotonic solution in terms of water concentration?

A

High water concentration

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7
Q

What is meant by a hypertonic solution in terms of water concentration?

A

Low water concentration

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8
Q

What is meant by paracellular transport?

A

through tight junctions and lateral intercellular spaces.

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9
Q

What is meant by transcellular transport?

A

Transcellular Transport - through the epithelial cells.

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10
Q

What methods can solutes use to cross cell membranes?

A

Simple diffusion

Facilitated diffusion

Active transport

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11
Q

What are the 2 types of transport proteins involved with active transport and describe how each works?

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.

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12
Q

what are four types of channel proteins?

A
  1. voltaged gated
  2. ligand-gated - intracellular and extracellular ligands
  3. mechanically gated
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13
Q

What are uniports?

A

Carrier mediated transport - one molecule moves in one direction

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14
Q

What are examples of uniports?

A

GLUT
VGCC
VGSC

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15
Q

What are symports?

A

Coupled transport mechanisms where the movement of one molecule down its concentration gradient provides energy for the movement of another against its concentration gradient

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16
Q

What are antiports?

A

Coupled transport where the movement of one ion power the movement of another in the opposite direction

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17
Q

What is an example of a symporter?

A

SGLT

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18
Q

What are examples of antiports?

A

Cl-/HCO3-

Na+ / H+

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19
Q

What is the difference between primary and secondary active transport?

A

Primary uses ATP but secondary uses the energy from the movement of another molecule

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20
Q

Give examples of primary active transporters and where they’re found

A

Na+ / K+ ATPase Pump - found in pancreas for pancreatic bicarbonate secretion and stomach for HCl secretion

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21
Q

Give examples of secondary active transporters and where they’re found

A

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

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

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

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22
Q

How are glucose and galactose absorbed?

A

Secondary active transport through SGLT

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23
Q

Where is glucose and galactose absorbed by secondary active transport?

A

enterocyte apical membrane

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24
Q

Why is it essential that glucose is absorbed via secondary active transport into enterocytes?

A

so it is effective when extracellular luminal concentrations are comparatively less than that in enterocytes

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25
Q

How is fructose absorbed?

A

Via facilitated diffusion using the carrier protein GLUT-5 on the apical membrane. Effective at relatively low concentrations of fructose in the lumen as tissue and plasma levels are low.

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26
Q

How does glucose exit through the basolateral membrane and what carrier protein is involved?

A

Facilitated diffusion, carrier protein is GLUT-2 (high capacity, low affinity facilitative transporter).

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27
Q

In what part of the GI system is the greatest amount of water absorbed?

A

Small bowel (especially in the jejunum)

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28
Q

Approximately how many litres of water are absorbed in the small and large bowel daily?

A

Small bowel - 8L

Large bowl - 1.4L

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29
Q

What drives the standing osmosis gradient?

A

Transport of Na+ from lumen into enterocyte.

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30
Q

what happens to the extracellular Na+?

A

Actively transported into the basolateral intercellular spaces by Na+ / K+ ATPase transport

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31
Q

how does standing gradient osmosis change as you travel down the intestine?

A

Becomes more efficient

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32
Q

How is the standing gradient osmosis created in the proximal bowel?

A

Counter transport in exchange for H+

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33
Q

How is the standing gradient osmosis created in the jejunum?

A

Co transport with amino acids and monosaccharides

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34
Q

How is the standing gradient osmosis created in the ileum?

A

Co transport with Cl-

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35
Q

How is the standing gradient osmosis created in the colon?

A

Restricted movement through ion channels

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36
Q

Explain how chloride ions are absorbed.

A

Cl- co-transported with Na+ within ileum, exchanged with HCO3- (Colon) into enterocytes. Co-transporter executed through secondary active transport on apical membrane.

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37
Q

What method do potassium ions use to be reabsorbed?

A

Diffuses via paracellular pathways in the small intestine and leaks out between cells in the colon via passive transport

38
Q

What parts of the GI tract absorb Ca2+?

A

Duodenum and ileum

39
Q

What stimulates absorption of Ca2+?

A

Vitamin D (Calcitriol) and parathyroid hormone

40
Q

What does a Ca2+ deficient diet stimulate the release of?

A

PTH and calcitriol to enhance intestinal ability to absorb Ca2+.

41
Q

Is Ca2+ normally more concentrated extraceullarly or intracellularly?

A

Extracellularly

42
Q

What can Ca2+ be carried by across the apical membrane?

A

Intestinal calcium binding protein

Ion Channel

43
Q

What does calcium bind to inside a cell to prevent its action as an intracellular signal?

A

calbindin

44
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, preventring its action as an intracellular signal

45
Q

What does vitamin D deficiency cause?

A

Rickets - chidren

Osteomalacia - adults

46
Q

How can Ca2+ be transported across the basolateral membrane?

A
  1. PMCA - Plasma Membrane Ca2+ ATPase
  2. Plasma Membrane Na+ / Ca2+ Exchanger
47
Q

What are the difference between the PMCA and Na+/Ca2+ exchanger?

A

PMCA has a higher affinity for Ca2+, but Na+/Ca2+ has a higher capacity than PMCA.

(Na+/Ca2+ requires larger concentrations of Ca2+ to be effective).

48
Q

How does 1,25-dihydroxy D3 taken up by enterocytes effect Ca2+ absorption?

A

Enhances 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.

49
Q

What processes in the body is iron important for?

A

Oxygen transport (RBCs)

Oxidative phosphorylation (mitochondrial transport chain)

(Iron exhibits properties such as an electron donor and acceptor)

50
Q

How much of the 15-20mg of iron which is ingested a day by adults is actually absorbed?

A

0.5-1.5mg/day

51
Q

Does the body have a mechanism for actively excreting iron?

A

No

52
Q

What is iron present in the diet as?

A
  1. Inorganic iron (Fe3+ and Fe2+)
  2. Haem iron (Haemoglobin, myoglobin and cytochromes)
53
Q

Which form of iron is easily absorbed?

A

Ferrous - 2+

54
Q

What reduces Fe3+ to Fe2+?

A

Vitamin C

55
Q

What does Fe3+ form insoluble salts with?

A

hydroxide
phosphate
HCO3-

56
Q

What enzyme catalyses the converts Fe3+ into Fe2+?

A

Duodenal cytochrome B

57
Q

How is heme iron absorbed into enterocytes?

A

Through the apical duodenal membrane into the enterocyte, an action potentiated by activity of haem carrier protein I (HCP-I), and through receptor-mediated endocytosis.

58
Q

What enzyme liberates Fe2+ from erythrocytes?

A

Heme oxygenase

59
Q

What does ferritin bind to in the cytosol to form ferritin micelles?

A

Apoferritin

60
Q

What happens to the ferritin stored in enterocytes?

A

It prevents absorption of to much iron by binding to some of it so it isnt in too high concs in the enterocytes

61
Q

What happens to ferritin levels in excess dietary absorption situations?

A

Body produces more ferritin

62
Q

What happens to iron/ferritin complexes?

A

(Iron/ferritin is lost in the intestinal lumen and excreted in the faeces

63
Q

What are vitamins?

A

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

64
Q

What vitamins are fat soluble

A

ADEK

65
Q

How are the fat-soluble vitamins transported to the brush border?

A

In micelles

66
Q

Which vitamin does the liver contain a large store of?

A

B12

67
Q

What is Cobalamin?

A

Vitamin B12

68
Q

What is pernicious anaemia?

A

Impaired absorption of vitamin B12 which slows the maturation of red blood cells. due to a lack if intrinsic factor

69
Q

Outline vitamin B12 absorption.

A

HCL in stomach releases B12, which combines with haptocorrin. Cleaved by and bound to intrinsic factor (IF) (resistant to digestion) - synthesised in the gastric parietal cells; the B12-IF complex traverses into the SI whereby it binds to receptors in the ileum - complex is taken up by an enterocyte and bound to transcobalamin II - stored in liver or transported to tissues. Crosses the basolateral membrane from MDRI channels into capillaries.

70
Q

What does Haptocorin do?

A

prevents the destruction of vitamin b12 by stomach acid

71
Q

What is intrinsic factor?

A

Vitamin B12 binding glycoprotein secreted by parietal cells. Vitamin B12/IF is resistant to digestion.

72
Q

What does Vitamin B12/IF complex bind to in distal ileum?

A

Cubilin receptor

73
Q

What is R protein?

A

Haptocorrin

74
Q

In the stomach, low pH and the digestions of proteins by pepsin releases free Vitamin B12, which is easily denatured by HCl. How is the denaturation of Vitamin B12 in the stomach avoided?

A

B12 binds to Haptocorrin which is released in saliva from parietal cells

75
Q

What happens to Haptocorrin/B12 complexes in the duodenum?

A

Haptocorin is digested so B12 is released and can bind to intrinsic factor

76
Q

What happens to VB12 after it enters a cell and is therefore no longer bound to intrinsic factor?

A

B12 binds to transcobalamin II (TCII), and they both then cross the basolateral membrane and travel to the liver. TCII receptors on hepatocytes allow them to take up the Vitamin B12/ TCII complex. Proteolysis then breaks down TCII inside the cell.

77
Q

List causes for VB12 deficiency.

A

Inadequate intake of sources containing the compound (Veganism)

Inadequate secretion of IF: pernicious anaemia (an autoimmune disorder) - autoantibodies interfere.

Lack of stomach acid (achlorhydria) - after partial gastrectomy surgery.

Malabsorption - diseases in the ileum reduces B12 absorption.

78
Q

How does Vitamin B12 appear in the plasma?

A

Bound to transcobalamin II

79
Q

In the stomach, what causes release of free vitamin B12?

A

Low pH and digestion of proteins by pepsin.

80
Q

How is inorganic iron transported across the brush border?

A

Via DMT-1

81
Q

How is Fe2+ —> transferrin?

A

Fe2+ is oxidised by transmembrane copper-dependent feroxidase (Hephaestin) → Ferric (Fe3+) → Bound to apotransferrin → transferrin.

82
Q

How is iron transported in the blood?

A

Ferroportin

83
Q

Describe heme iron absorption - ferrous

A

Haem (ferrous) iron absorbed into the apical membrane through haem transporter. Directly stored as mucosal ferritin (protective mechanism for overload), or transported through the basolateral membrane via ferroportin into circulation. Stored ferritin has limited capacity → Shed across epithelial cells.

84
Q

What happens to Fe2+ iron outside the enterocyte?

A

Ferrous iron (Fe2+) converted into ferric (Fe3+) through hephaestin action to be associated with plasma transferrin and circulates towards liver or bone marrow.

85
Q

What effect does ferric iron flow into the liver have?

A

Ferric iron flow into the liver exerts negative feedback to release hepcidin → Reduces ferroportin integration into basolateral membrane in order to reduce systemic overload.

86
Q

Describe ferric sulphate iron absorption

A

Ferric sulphate iron (Fe2+) under action of duodenal cytochrome B is reduced into ferrous iron → Entry through the apical membrane by associated DMT-I.

Then either stored as ferritin or leaves enterocyte as ferroportin

87
Q

What permits the movement of Ca2+ from the apical membrane to the basolateral membrane?

A

Calbindin-D

88
Q

Outline the absorption of Ca2+ in enterocytes.

A

Vitamin-D dependent uptake of Ca2+ across the apical membrane into the enterocyte is mediated by TRPV6 (IMcal). Cytoplasmic Ca2+ binding protein Calbindin-D permits the movement of Ca2+ from the apical membrane to the basolateral membrane. Calbindin proteins transport Ca2+ in cytosol, preventing action as intracellular signal.

Ca2+ extrusion from enterocytes performed by 2 proteins: PMCAI and NCXI (Na+/Ca2+ exchanger). Ca2+ pumped across basolateral membrane by plasma membrane Ca2+ ATPase (PMCA) against concentration gradient.

89
Q

What allows hepatocytes to take up Vitamin B12/TCII?

A

Transcobalamin receptors

90
Q

What process breaks down Transcobalamin inside the cell?

A

Proteolysis

BRS GASTROENTEROLOGY (25 decks)

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