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Alimentary system > 6. Ions, vitamins and minerals > Flashcards

6. Ions, vitamins and minerals Flashcards

1
Q

Why do multicellular organisms need to evolve circulatory systems?

A

Diffusion occurs slowly over macroscopic distances

Circulatory system brings individual cells within diffusion range

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2
Q

Describe the movement of water in osmosis

A

Hypotonic to hypertonic

low concentration solution to high concentration solution

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3
Q

How are different proteins localised to different sides of the membrane?

A

By tight junctions

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4
Q

Paracellular transport

A

through tight junctions and lateral intercellular spaces.

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5
Q

Transcellular transport

A

through the epithelial cells

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6
Q

Describe the 2 types of transport protein

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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7
Q

Which transport protein allows faster transport?

A

Channel proteins

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8
Q

How can channel proteins be selective?

A

They may be gated

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9
Q

Describe the 4 types of gated channel proteins

A

Voltage gated: change in membrane potential causes explosive opening of other ion channels that fires the AP
(Extracellular) Ligand gated: e.g. a hormone may bind and cause it to open
(Intracellular) Ligand gated: 2nd messengers e.g. cAMP bind to ion channels causing them to open
Mechanically: Increase in pressure or stretch receptor change ion conductance across cell

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10
Q

Describe the 3 types of carrier proteins

A

Uniport: 1 direction, usually down concentration gradient
Symport: 2 things in same direction
Antiport: 2 things in opposite directions, often to equalise charge

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11
Q

Primary active transport

A

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

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12
Q

Secondary active transport

A

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

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13
Q

Facilitated transport

A

Enhances rate a substance can flow down its concentration gradient.
Tends to equilibrate the substance across the membrane No energy required

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14
Q

List 2 examples of Primary active transporters

A

Na+/K+ ATPase

H+/K+ ATPase

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15
Q

List 3 examples of Secondary active transporters

A

SGLT-1 co-transport
HCO3-/Cl- counter transport
Na+/H+ counter transport

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16
Q

List 2 examples of Facilitated transport transporters transporters

A

GLUT-5

GLUT-2

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17
Q

How much water presented to the GI tract is absorbed and what is this driven by?

A

99%

Driven by absorption Na+

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18
Q

Where is the greatest amount of water absorbed?

A

Small intestine

Especially the jejunum

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19
Q

How are many ions absorbed? What about Ca2+ and Fe2+?

A

Many ions slowly absorbed by passive diffusion.

Ca2+ and Fe2+ are incompletely absorbed, and this absorption is regulated

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20
Q

How many litres of water a day are absorbed in the small intestine?

A

8 L

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21
Q

How many litres of water a day are absorbed in the large intestine?

A

1.4 L

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22
Q

Where does the 8 L absorbed come from?

A

Secretions

Thus we are “reabsorbing”

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23
Q

By what process is water absorbed? What is this driven by?

A

Standing gradient osmosis

Driven by absorption Na+

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24
Q

Describe the lumenal transport of Na+ into enterocytes through the intestine (mention Cl-, K+ and HCO3-)

A

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, exchanged with HCO3- (colon)
Essentially, increasing levels of intracellular Na+
Cl- move in with Na+ (Ileum)
Cl- move in exchange for HCO3- (Colon)
In small intestine K+ diffuses in
In colon K+ diffuses into lumen

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25
Q

Describe the movement of Na+ once inside enterocytes through the intestine (mention Cl-, K+ and HCO3-)

A

Na+ pumped into lateral intracellular spaces in exchange for K+ via ATPase
Increases Na+ concentration in lateral intracellular spaces, changes electrochemical gradient, making fluid around cell more +ve, encourages movement of -ve ions across membrane
So, Cl- and HCO3- ion concentration increase in that fluid
Makes solution hypertonic
Drives absorption of water: water moves via paracellular pathways and transcellular pathways: Increases pressure against basement membrane: Forces water and ions across basement membrane into bloodstream

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26
Q

Where is Ca2+ absorbed?

A

Duodenum and Ileum

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27
Q

What does a Ca2+ deficient diet result in?

A

Increased ability of gut to absorb (adapts)

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28
Q

What stimulates absorption of Ca2+?

A

Vitamin D

Parathyroid hormone

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29
Q

How much Ca2+ is ingested, secreted and absorbed per day?

A

Diet: 1-6g
Secretions: 0.6g
Absorb: 0.7g

30
Q

Intracellular and extracellular concentrations of calcium?

A

Intracellular: Low 100nM
Extracellular: High 1-3 mM

31
Q

Plasma and luminal concentrations of Ca2+

A

Plasma: 2.2-2.6mM
Luminal: varies inmM range

32
Q

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

A

Don’t need to put energy into it because Ca2+ can move down its concentration gradient

33
Q

Via which 2 pathways can Ca2+ be carried across the apical membrane of enterocytes?

A

Intestinal calcium-binding protein (IMcal): allows facilitated diffusion.
Ion channels

34
Q

How does Ca2+ exist in the cytosol without causing an intracellular signal?

A

Binds to calbindin in cytosol, preventing its action as an intracellular signal

35
Q

What happens to Ca2+ at the basolateral membrane?

A

Calbindin dissociates from Ca2+
Ca2+ pumped out via ATPase PMCA (primary active transport)
Ca2+ pumped out via Na+/Ca2+ exchanger against concentration gradient (secondary active transport)

36
Q

PMCA affinity and capacity for Ca2+

A 
High affinity
Low capacity (slow)
37
Q

Na+/Ca2+ exchanged affinity and capacity for Ca2+

A

Low affinity

High capacity

38
Q

What does Na+/Ca2+ exchanger require to be effective?

A

Larger concentrations of Ca2+

39
Q

What does Vitamin D deficiency result in?

A

Rickets (Childhood)

Osteoporosis

40
Q

How does Vitamin D effect the Ca2+ uptake by enterocytes?

A

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.

41
Q

What is special about Iron?

A

Iron can act as an electron donor and an electron acceptor

42
Q

What processes in the body is iron critical for?

A 
Oxygen transport (RBCs)
Oxidative phosphorylation (mitochondrial transport chain)
43
Q

Iron in excess

A

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

44
Q

Iron being toxic in excess has what implications for iron absorption?

A

Need to be able to absorb quickly as required, but also to limit that absorption

45
Q

How much iron is ingested and absorbed per day?

A

Ingest: 15-20mg
Absorb: 0.5-1.5mg

46
Q

How is iron present in the diet?

A 
Inorganic iron  (Fe3+ ferric, Fe2+ ferrous) 
As part of heme (haem) group (haemoglobin, myoglobin and cytochromes).
47
Q

Which is the only form of iron that can be absorbed?

A

Fe2+

48
Q

What does Fe3+ form insoluble salts with?

A

Hydroxide,
Phosphate,
HCO3-

49
Q

What is the effect of Vitamin C on Fe3+?

A

Reduces Fe3+ to Fe2+

thus helps absorption of iron

50
Q

Role of heme in the diet

A

Heme smaller part of diet, but more readily absorbed

20% of presented is absorbed, rather than 5%

51
Q

Describe the absorption of heme into enterocytes

A

Heme absorbed via heme carrier protein 1 (HCP-1), and via receptor-mediated endocystosis
Fe2+ is then liberated by Heme oxygenase

52
Q

Describe the process resulting in absorption of iron from Fe3+

A

Duodenal cytochrome B (Dcytb) catalyzes the reduction of Fe3+ to Fe2+
Fe2+ transported into enterocyte via divalent metal transporter 1 (DMT-1), a H+-coupled co-transporter

53
Q

What happens to Fe2+ in the cytosol of enterocytes?

A

Fe2+ binds to unknown factors
Carried to basolateral membrane
Moves via ferroportin ion channel into blood

54
Q

What happens to Fe2+ after being transported by ferroportin into the blood?

A

Fe2+ is converted to Fe3+ by Hephaestin

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

55
Q

What is the major iron regulating protein and what does it do?

A

Hepcidin

Suppresses ferroportin function to decrease iron absorption

56
Q

What happens to Fe2+ in the cytosol that is not transported into the blood?

A

Fe2+ binds to apoferritin to form ferritin micelle.
Fe2+ is oxidised to Fe3+, which crystallises within protein shell
This makes it biologically inert

57
Q

What happens in excess dietary iron absorption?

A

Produce more ferritin

58
Q

How does irreversible binding of iron to ferritin in the epithelial cells prevent iron reaching toxic levels?

A

Iron/Ferritin is not available for transport into plasma

Iron/Ferritin is lost into the intestinal lumen when enterocytes are sloughed off and excreted in the faeces

59
Q

Vitamins

A

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

60
Q

What is the predominant mechanism of absorption of vitamins?

A

Passive diffusion

61
Q

How are Fat soluble vitamins (A, D, E, K) absorbed?

A

Transported to brush border in micelles

K taken up by active transport.

62
Q

List 4 water soluble vitamins that require specific transport mechanisms

A

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

63
Q

How much Vitamin B12 is stored in the liver?

A

2-5 mg

64
Q

What does impaired absorption of vitamin B12 result in?

A

Retards the maturation of RBCs

Causes pernicious anaemia.

65
Q

How is the majority of Vitamin B12 in food found?

A

Bound to proteins

thus digestion of proteins by pepsin in the stomach releases free B12

66
Q

How is the denaturation of B12 in the stomach avoided?

A

Binds to R protein (haptocorrin) released in saliva and from parietal cells.
Stops B12 being denatured by Hal in the stomach
R proteins digested in duodenum.

67
Q

Describe the passage of Vitamin B12 from the stomach to the distal ileum

A

B12 liberated in stomach by pepsin
B12 binds to R protein released from parietal cells
This complex travels into the duodenum
In duodenum R proteins are broken down, so B12 binds to
Intrinsic factor
This complex moves to the distal ileum, where it binds to cubulin and absorbed

68
Q

What is intrinsic factor secreted by?

A

parietal cells

69
Q

What would lack of IF result in?

A

No absorption of B12

70
Q

What happens to the B12/IF complex when it enters enterocytes?

A

Vit B12/IF complex broken (possibly in mitchondria)

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

71
Q

What happens to the B12/TCII complex when it enters the blood?

A

Travels to liver
TCII receptors on cells allow them to uptake complex.
Proteolysis then breaks down TCII inside the cell.
B12 is stored

Alimentary system (15 decks)

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