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Y1 Alimentary > Ions, vitamins and minerals > Flashcards

Ions, vitamins and minerals Flashcards

1
Q

How can molecules cross epithelium to enter the blood stream?

A
  • paracellular transport through tight junctions and lateral intercellular spaces
  • transcellular transport through epithelial cells
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2
Q

What are channel proteins?

A

Aqueous pores allowing specific solutes to pass across the membrane

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3
Q

What are carrier proteins?

A

Bind to solute and undergo a conformational change to transport it across the membrane

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4
Q

What transports faster? Channel or carrier proteins?

A

Channel

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5
Q

What is the difference between primary and secondary active transport?

A

Primary - linked directly to cellular metabolism and uses ATP to power transport
Secondary - derives energy from conc gradient of another substance that is actively transported

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6
Q

Example of primary active transporters?

A

Na+/K+ ATPase

H+/K+ ATPase

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7
Q

Example of secondary active transporters?

A

GLUT-1 cotransport
HCO3-/Cl- transport
Na+/H+ counter transport

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8
Q

Example of facillitated transport?

A

GLUT5. GLUT2

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9
Q

How are carbohydrates absorbed? - glucose and galactose

A

Glucose and galactose by secondary active transport, carrier protein SGLT-1 on apical membrane that carries glucose against conc gradient

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10
Q

What is the carrier protein for glucose and galactose absorption

A

SGLT1

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11
Q

What is the carrier protein for fructose? What type of transport does it use?

A

Facillitated diffusion

GLUT5

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12
Q

How does glucose exit the enterocyte at the brush broder?

A

Facillitated diffusion

GLUT-2 high capacity low affinity facilitative transporter

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13
Q

What are specific issues in alimentary absorption?

A

Water and ions
Calcium
Iron
Vitamins B12

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14
Q

Where is most of the water absorbed? How is absorption of water powered?

A

Small intestine - esp jejunum

Powered by absorption of ions that are absorbed slowly by passive diffusion

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15
Q

How much water is absorbed per day in small intestine?

A

8 litres

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16
Q

How much water is absorbed per day in large intestine?

A

1.4 litres

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17
Q

How is Na+ transported into the enterocyte from the lumen?

A

Counter transport in exchange for H+
Co-transport with aa, monosaccharides (jejunum)
Cotransport with Cl- (ileum)
Restricted movement through ion channels (colon)

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18
Q

How is Cl- absorbed into enterocytes?

A

Co-transported with Na+ (ileum) and exchanged with HCO2- (colon) into enterocytes. Both secondary active transport

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19
Q

How is K+ absorbed into enterocytes?

A

Diffuses in via paracellular pathways in small intestine, leaks out between cells and colon. Passive transport

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20
Q

What happens to sodium taken into cells?

A

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

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21
Q

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

A

due to electical potential created by Na+ transport

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22
Q

How does water enter the gut cells?

A

Osmotic flow from 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 epithelium and carried away by capillaries

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23
Q

What parts of the small intestine absorb Ca2+

A

Duodenum and Ileum

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24
Q

What stimulates Ca2+ absorption?

A

Vit D and parathyroid hormone

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25
Q

What does a Ca2+ deficient diet do?

A

Increases gut ability to absorb

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26
Q

What is the intracellular fluid [Ca2+]

A

approx 100nM - low

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27
Q

What is the extracellular fluid [Ca2+]

What conc in plasma or lumen

A

approx 1-3mM
Plasma [Ca2+] 2.2-2.6 mM
Luminal varies

28
Q

How is Ca2+ carried across the apical membrane

A
  1. Intestinal calcium binding protein (IMcal- facillitated diffusion)
  2. Ion channel
29
Q

What is Ca2+ used for in the body?

A

Intracellular signalling molecule

30
Q

What happens to calcium in cytosol and what does that do?

A

Binds to calbindin which prevents its action as an intracellular signal

31
Q

How is Ca2+ pumped across the basolateral membrane?

A

Ca2+ ATPase (PMCA) against conc gradient

Na+/Ca2+ exchanger against conc gradient

32
Q

How does PMCA maintain low conc Ca2+ in cell

A

it has high affinity Ca2+ but low capactiy so maintains the very low conc of calcium normally observed in cell

33
Q

How does the Na+/Ca2+ exchanger work transporting Ca2+?

A

Low affinity for Ca2+ but high capacity so requires larger conc of Ca2+ to be effective

34
Q

What does a deficiency of Vit D do?

A

Cause rickets, osteoporosis

35
Q

What is Vit D essential for?

A

Normal Ca2+ absorption

36
Q

What does 1,25-dihydroxy D3 do when taken up by enterocytes?

A

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

37
Q

What is iron used as in the body?

A

Electron donor and acceptor

38
Q

What processes in the body are iron critical for?

A 
Oxygen transport RBC
Oxidative phosphorylation (mitochondria transport chain)
39
Q

How much iron does an adult ingest and how much do they absorb?

A

Ingest 15-20mg/day

Absorb 0.5-1mg/day

40
Q

What does ion present as in the diet?

A

Inorganic iron eg Fe3+ ferric, Fe2+ ferous

Part of heme/haem group in haemoglobin, myoglobin and cytochromes

41
Q

Which form of inorganic iron can the body absorb?

A

only Fe2+, can’t absorb Fe3+

42
Q

What reduces Fe3+ to Fe2+? What catalyses this reaction?

A

Vit C

Catalysed by duodenal cytochrome B (dcytb)

43
Q

How much of heme/haem is absorbed?

A

20% of presented instead of 5% of rest of iron

44
Q

How is heme/haem absorbed into enterocyte?

A

Heme carrier protein HCP-1 via receptor mediated endocytosis

45
Q

How is Fe2+ liberated?

A

Heme oxygenase

46
Q

How is Fe2+ transported into enterocyte?

A

Divalent metal transporter 1 (DMT-1) which is a H+ coupled co-transporter

47
Q

How does Fe2+ move into blood across basolateral membrane?

A

Via ferroportin ion channel

48
Q

What converts Fe2+ to Fe3+?

A

Transmembrane copper dependent ferroxidase

49
Q

What does Fe3+ bind to? How does it travel in blood

A

Bind to apotransferrin

Travels in blood as transferrin

50
Q

What is a major iron regulating protein? What does it do?

A

Hepcidin

Supresses ferroportin function to decrease iron absorption

51
Q

How else can iron transport in the body? (hint: micelle)

A

Bind to apoferritin in cytosol to form ferritin micelle

52
Q

What is ferritin?

A

Globular protein complex which oxidises Fe2+ to Fe3+ which crystallises within protein shell

53
Q

What happens when too much iron absorbed?

A

Irreversible binding of iron to ferritin in epithelial cells
Iron/Ferritin not available for transport into plasma
Iron/Ferritin lost in intestinal lumen and extreted in faeces
Increase in iron conc in cytosol increases ferritin synthesis

54
Q

What are vitamins?

A

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

55
Q

What is the predominant mechanism to take up vitamins in the body?

A

Passive diffusion

56
Q

What are the fat soluble vitamins

A

A, D, E, K

57
Q

How are the fat soluble vitamins taken up in the body?

A

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

58
Q

What vitamins need specific transport mechanisms?

A

Vit C (ascorbic acid)
Folic acid
Vit B1 (thiamine)
Vit B12

59
Q

Where is a store of Vit B12 found? What happens if there is impaired absorption of Vit B12

A

Liver

Retards maturation of red blood cells - pernicious anaemia

60
Q

How is denaturation of vit B12 avoided in the stomach by HCl?

A

Binds to R protein (haptocorrin) released in saliva and from parietal cells. R proteins are digested in duodenum

61
Q

What is the intrinsic factor IF?

A

Glycoprotein that binds to VitB12, makes it resistant to digestion. No IF then no absorption of vit B12

62
Q

What does the vitB12/IF complex do?

A

Binds to cubilin receptor, taken up in distal ileum - receptor mediated endocytosis

63
Q

What happens to vitB12 once it is in the cel

A

The VitB12/IF complex broken down, possibly in mitochondria. B12 bind to protein transcobalamin II (TCII), crosses basolaterla membrane by unknown mechanism. Travels to liver bound to TCII, TCII receptor on cels allow them to uptake complex and proteolysis breaks down TCII inside cells

64
Q

How much water enters gastrointestinal tract a day?

A

7.5L due to various secretions

65
Q

How much of the H2O in the GI tract is absorbed?

A

99%

Y1 Alimentary (21 decks)

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