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alimentary system > Ions, vitamins and minerals > Flashcards

Ions, vitamins and minerals Flashcards

1
Q

What is molar, mM, µM, nM, pM and fM and thei conversions?

A

Molar = one mole per litre

Millimolar (mM)       10-3
Micromolar (µM)     10-6
Nanomolar (nM)      10-9 
Picomolar  (pM) 	10-12 
Femtomolar  (fM) 	10-15
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2
Q

What is diffusion?

Is it faster over macroscopic or microscopic distances?

A
  • The process whereby atoms or molecules intermingle because of their random thermal motion.
  • Diffusion occurs rapidly over microscopic distances, but slowly over macroscopic distances.
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3
Q

What is a hypotonic solution?

A

Solution that has less solute and more water than another solution

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4
Q

What is the pathway of water in osmosis (tonicity of solutions)?

A

hyptonic ->isotonic->hypertonic

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5
Q

What are the two broad types of transport?

A

Transcellular - through epithelial cells

Paraceullular - through tight junctions and lateral intercellular spaces

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6
Q

Which membrane transporters are faster - channel or carrier protein?

A

channel protein

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7
Q

What are some types of ion channels?

A

Ion channels can be voltage gated, ligand gated, mechanically gated

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8
Q

What are the types of carrier protein mediated transport?

A

Uniport -one
Symport - cotransported
Antiport - counter transport

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9
Q

What is primary active transport and give an example?

A

Linked directly to cellular metabolism (ATP powered)

E.G. Sodium Potassium ATPase

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10
Q

What is secondary active transport and give an example?

A

Derives energy from concentration gradient of another substance actively transported

E.G. Bicarbonate/Chloride counter-transport, SGLT-1

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11
Q

What is the importance of facilitated diffusion?

A

Enhances rate a substance can flow down a concentration gradient
so it can reach equilibrium.

E.G. GLUT-5, GLUT-2

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12
Q

How are glucose and galactose absorbed?

A
  • Absorption of glucose & galactose is by secondary active transport
    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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13
Q

How is fructose absorbed?

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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14
Q

How does glucose exit the enterocyte and enter the blood?

A
  • Exit of glucose at the basolateral membrane is by facilitated diffusion. Carrier protein = GLUT-2, a high-capacity, low-affinity facilitative transporter.
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15
Q

What % and volume of water is absorbed by the GI tract and how?
Which part of the GI tract absorbs the most water?

A
  • 99%
  • It is absorbed by the action of ion absorption
  • Most water is absorbed in the small intestine (especially the jejunum)
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16
Q

How are many ions absorbed and are Ca2+ and iron completely absorbed?

A

Many ions absorbed slowly by passive diffusion and Ca2+ and iron are incompletely absorbed due to regulation.

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17
Q

How much water is absorbed by the small and large intestine?

A

SI - 8L

LI - 1.4 (DAILY)

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18
Q

Where does the water in the GI tract come from?

A
  • Ingestion (2L)
  • Saliva (1.2L)
  • Gastric secretions (2L)
  • Bile (0.7L)
  • Pancreas (1.2L)
  • Intestine (2.4L)
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19
Q

What is the name of the process by which water is reasborbed?

A

Standing gradient osmosis (driven by Na+)

20
Q

How does sodium enter enterocytes?

A

Sodium gets into the cells via different methods depending on location of the enterocyte:

  • Proximal bowel uses counter transport for H+
  • Jejunum uses cotransport with amino acids, monosaccharides
  • Ileum uses cotransport with Cl-
  • Colon uses restricted movement through ion channels
  • Cl- is cotransported with Na+ in the ileum and exchanged with HCO3- in the colon
21
Q

Describe the standing gradient osmosis

A

High intracellular sodium is controlled by:

  • Active transport of Na+ into lateral intercellular spaces by Na+/K+ATPase
  • Cl- and HCO3- transports into the intercellular space by electrochemical gradient of Na+
  • High concentration of ions in intercellular spaces (hypertonic)
  • 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
22
Q

Calcium absorption - where does majority occur, what does a calcium deficiency do to absorption, what stimulates its absorption, how much is absorbed daily, and at rest what are the relative concentration of intra and extracellular calcium?

A
  • Most absorption of calcium occurs in the duodenum and ileum
  • A calcium deficiency increases the gut’s ability to absorb calcium
  • Vitamin D and parathyroid hormone stimulate absorption of calcium
  • Even though we ingest 1-6g of calcium a day, we only absorb about 0.7g of it
  • This is because it is regulated
  • At rest, there is a low (nm) intracellular concentration and a high (micromole) extracellular concentration
23
Q

How is calcium transported across the apical membrane?

A

Absorption via 2 types of transporter:

  1. Facilitated diffusion – IMcal (intestinal calcium binding protein)
  2. Ion channel
24
Q

How is calcium transported whilst maintaining low intracellular concentrations (because it is a signalling molecule)?

A
  • Binds to calbindin in cytosol to make it inactive
  • Calcium pumped out of basolateral surface by:
    Ca2+ATPase (PMCA)
  • PMCA has a high affinity but low capacity
  • Maintains the low intracellular concentration.
  • It is also pumped out by Na+/Ca2+ exchanger against its concentration gradient
  • It has a low affinity but high capacity so needs larger concentrations of calcium to be effective (hence it works before the PMCA to reduced the large calcium amounts)
25
Q

What is vitamin D important for and what does a deficiency cause?

A
  • needed for calcium absorption

- causes rickets or osteoporosis

26
Q

How is vitamin D taken up and what are its functions?

A

Taken up by enterocytes

Functions to:

  • Enhance transport of Ca2+ through cytosol
  • Increase level of calbindin
  • Increase number of Ca2+ATPase in membrane for faster extrusion from cell
27
Q

What is iron needed for?

A

Oxygen transport (in haem) and oxidative phosphorylation (ETC)

28
Q

Why must uptake of iron be regulated?

A

Iron in excess is toxic but there is no mechanism for excreting iron.
Iron is necessary so must be absorbed fast but limited too.

29
Q

How much iron do adults ingest and absorb daily?

A

15-20mg

Absorb 0.5-1.5mg

30
Q

How is iron present in the diet (as what)?

A

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

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

31
Q

Can we absorb Fe3+ or Fe2+?

A

Fe2+

32
Q

How is Fe3+ absorbed then?

A
  • Fe3+ forms insoluble salts with (e.g. hydroxide, phosphate, bicarbonate)
  • Vitamin C reduction from Fe3+ to Fe2+
33
Q

How is iron as part of haem absorbed?

A

We absorb this intact into the enterocyte via haem carrier protein 1 (HCP-1) which endocytoses via receptor mediated endocytosis. Fe2+ is then liberated by haem oxygenases

34
Q

How does iron enter the blood?

A
  • Ferric enters the blood via ferroportin (FP)
  • Hephaestin (HP) is a copper dependant ferroxidase that converts Fe2+ to Fe3+ on the basolateral side
  • Fe3+ binds to apotransferrin and travels in blood as transferrin (TF)
  • Hepcidin supresses FP
35
Q

How does iron get from the duodenum lumen into enterocytes?

A

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

Haem can bind to haem carrier protein 1 (HCP-1) which endocytoses via receptor mediated endocytosis. Fe2+ is then liberated by haem oxygenases

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

36
Q

How are iron ions stored in ferritin?

A
  • Iron can bind to apoferritin in cytosol to form ferritin micelle
  • Ferritin is a blobular protein complex (each can store 400 iron ions)
  • Fe2+ then oxidises to Fe3+ which creates a protein shell.
  • The micelle is lost when the enterocytes are shunted off the tip of the villi and excreted
  • 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
37
Q

What are vitamins?

A

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

38
Q

How are vitamins absorbed mainly?

A

Passive diffusion

39
Q

How are vitamin A, D, E, K uptaken?

A

Fat soluble vitamins (e.g. A, D, E, K) are transported to the brush border by micelles while K is actively taken up

40
Q

How are vitamin C, B12, B1 and folic acid taken up?

A

By specific transport mechanisms

41
Q

Vitamin B12 - where is it stored and what does impaired absorption lead to?

A
  • Liver contains a large store (2-5mg)

- Retards the maturation of RBCs = pernicious anaemia

42
Q

Vitamin B12 release from food and transport to duodenum

A
  • Most Vitamin B12 is bound to proteins in food
  • Stomach low pH and pepsin releases B12 from food but the B12 is easily denatured by HCl
  • B12 therefore binds to R protein (haptocorrin) which is released by parietal cells and in saliva
  • R proteins are then digested in the duodenum
43
Q

How does vit B12 resist digestion in the SI and how does it get into the blood?

A
  • B12 binds to Intrinsic Factor (IF) secreted by parietal cells
  • IF is resistant to digestion.
  • B12/IF complex binds to cubulin (cub) receptor in distal ileum and is endocytosed (possibly by receptor mediated methods)
  • B12/IF complex broken down in mitochondria, B12 binds to transcobalamin II (TCII) and travels to liver in the blood
44
Q

How does vitamin B12 bound to TCll enter the liver?

A
  • TCII receptors on the liver allow uptake of complex.

- Proteolysis then breaks down the TCII inside the cell

45
Q

How does K+ diffuse in the SI and colon?

A

K+ diffuses in paracellularly in the small intestine and out in the colon

46
Q

Is iron in haem or the inorgnaic ion more readily absorbed?

A

Haem - 20% absorbed

Inorganic - 5%

alimentary system (14 decks)

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