Ions Vitamins And Minerals Flashcards
Diffusion
Process of atoms and molecules intermingling because of their random thermal motion
Occurs rapidly over microscopic distances but slowly over macroscopic distances
Define osmosis
Diffusion of water from hypotonic to hypertonic medium
How do molecules cross the epithelium to enter the bloodstream (2 ways)?
- Paracellular transport through tight junctions and lateral intercellular spaces
- Transcellular transport through epithelial cells
How can solutes cross the cell membranes
Simple diffusion
Facillatated diffusion
Active transport
Facilitated transport- describe it
Enhances the rate of a substance down its conc grad
This tends to equilibrate the substance across the membrane and doesn’t require energy
GLUT-5, GLUT-2 (small bowel absorption of monosaccharides)
Active transport
- Primary- linked directly to cellular metabolism (uses ATP to power it)
- Secondary- derives energy from the conc grad of another substance that is actively transported
Primary active transport– Na+/K+ ATPase (pancreatic HCO3- secretion)
- H+/K+ ATPase (stomach parietal cell)
Secondary active transport– SGLT-1 co-transport (small bowel absorption of monosaccharides)
- HCO3-/Cl- counter transport (pancreatic HCO3- secretion)
- Na+/H+ counter transport (pancreatic HCO3- secretion
Carrier proteins
Bind to the solute and undergo a conformational change to transport it across the membrane
Uniport symport and antiport
What do channel proteins do?
Form aqueous pores allowing specific solutes to pass across the membrane
Allow much faster transport then carrier proteins
How are glucose and galactose absorbed at brush border?
- By secondary active transport through SGLT-1 carrier protein on apical membrane
- SLGT-1 can transport glucose uphill against its conc grad (effective when glucose at levels in lumen below that of enterocyte)
How does glucose exit at basolateral membrane?
- Facilitated diffusion through GLUT-2 carrier protein → a high capacity, low-affinity facilitative transporter
- Glucose between plasma and tissue/enterocyte generally equilibrated
How is fructose absorbed at brush border?
- Facilitated diffusion through GLUT-5 carrier protein on apical membrane
- Effective at rel. low concs of fructose in lumen as tissue and plasma levels are low
Where are water and ions absorbed in GI tract?
- 99% of H2O presented to GI tract is absorbed
- Water absorption is powered by ion absorption
- Small intestine absorbs water the most, esp jejunum
- Many ions slowly absorbed by passive diffusion
- Ca2+ and iron are incompletely absorbed and absorption is regulated
How much water do the small and large bowels absorb?
- 8l in small bowel
- 1.4l in large bowel
How is Na+ transported into enterocyte?
- Counter-transport in exchange for H+ in proximal bowel
- Co-transport with amino acids and monosaccharides in jejunum
- Co-transport with Cl- in ileum
- Restricted movement through ion channels in colon
How are other ions absorbed?
- Cl- co-transported with Na+ (ileum), exchanged with HCO3- (colon) → both secondary active transport
- K+ diffuses in via paracellular pathways in small intestine, leaks out between cells in colon → passive transport
What is standing gradient osmosis + how does it lead to water absorption from GI tract?
- How water is absorbed normally in GI tract
- Driven by Na+ → it’s transported from lumen into enterocyte
- This Na+ is actively transported into lateral intercellular spaces by Na+/K+ ATPase transport in lateral plasma membrane
- Cl- and HCO3- transported into intercellular spaces due to electrical potential created by Na+ transport- this high ion conc causes fluid there to be hypertonic
- Osmotic flow of water from gut lumen via adjacent cells, tight junctions into intercellular space
- Water distends intercellular channels- causes it to swell and increases hydrostatic pressure
- Ions and water move across basement membrane of epithelium & are carried away by capillaries
Where is Ca2+ is absorbed?
- Duodenum and ileum absorb it
- Vit D and PTH stimulate absorption
- Ca2+ deficient diet increases gut’s ability to absorb
How much Ca2+ do we have intracellularly vs extracellularly?
- Low intracellular- 100nM (can increase 100x during various cell functions)
- High extracellular- 1-3mM
In plasma we have 2.2-2.6mM
How is Ca2+ moved across apical membrane (to basolateral membrane) of enterocytes? (2 things together)
- Intestinal calcium-binding protein (IMcal) e.g. calbindin - facilitated diffusion
- Ion channel
- What happens to Ca2+ when it’s taken into cell?
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- Ca2+ acts as an intracellular signalling molecule and we need to maintain low intracellular Ca2+ concs
- So Ca2+ binds to calbindin in cytosol, preventing its action as an intracellular signal
How is Ca2+ moved across basolateral membrane? (2 ways)
PMCA - Ca2+ pumped across by plasma membrane Ca2+ ATPase (PMCA) against conc grad
- PMCA has high affinity (attraction) but low capacity for Ca2+
- Maintains v low concs of Ca2+ normally observed in a cell
NA+/CAW+ EXCHANGER - Against conc gradient
- Na+/Ca2+ exchanger has low affinity for Ca2+ but a high capacity
- Requires larger concs of Ca2+ to be effective
What does vit D3 do to enterocytes to help with Ca2+ transport (2 things)?
- Enhances Ca2+ transport through cytosol
- Increases calbindin levels
- Increases rate of extrusion across basolateral membrane by increasing the level of PMCA in membrane
- What processes in the body is iron critical for?
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- Oxygen transport in RBCs
- Oxidative phosphorylation in mitochondria ETC
- It can act as an electron donor and acceptor
How much iron do we absorb and in which form?
- Iron is present in diet as inorganic iron (ferrous Fe2+ and ferric Fe3+ form) and as part of haem group (haemoglobin, myoglobin, cytochromes)
- Can’t absorb Fe3+, only Fe2+
- Fe3+ forms insoluble salts with hydroxide, phosphate, HCO3-
- Vit C reduces Fe3+ to 2+
- Haem (iron) is smaller part of diet but more readily absorbed (20% of presented rather than 5%)
- How is haem absorbed? (give specific details)
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- Absorbed intact into enterocyte via heme carrier protein (HCP-1) and via receptor-mediated endocytosis
- Fe2+ is liberated by heme oxygenase
How are Fe3+ and Fe2+ taken up by enterocytes?
- Duodenal cytochrome B (Dcytb) catalyses reduction of Fe3+ to Fe2+ in process of iron absorption in duodenum
- Fe2+ transported via divalent metal transporter 1 (DMT-1) which is **a H+ coupled co-transporter
What 2 things can then happen to Fe2+ in the cell?
Moves into blood
Gets stored in cell
Fe2+ moves into blood- how does it do this, + describe the action of hepicidin
- Fe2+ binds to unknown factors, carried to basolateral membrane then moves via ferroportin ion channel into blood
- Hephaestin is a transmembrane copper-dependent ferroxidase that converts Fe2+ to Fe3+
- Fe3+ binds to apotransferrin and travels in blood as transferrin
- Hepcidin suppresses ferroportin function to decrease iron absorption
- Fe2+ gets stored in cell- how?
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- 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 4000 iron ions
- In excess dietary iron absorption, more ferritin is made
Stores prevent absorption of too much iron which can be toxic. The iron/fertile is irreversibly bound so not available for transport
- What are vitamins?
Organic compounds that can’t be made by body but vital to metabolism
How are different vitamins taken up by body?
- Fat soluble vitamins (ADEK) transported to brush border in micelles
- K taken up by active transport
- There are specific transport mechanisms for vitamin C (ascorbic acid), folic acid, vitamin B1 (thiamine) and vitamin B12
How is vitamin B12 important and where is it found?
- Liver contains a large store (2-5mg)
- Impaired absorption of vit B12 retards the maturation of RBCs → pernicious anaemia
- Most Vit B12 in food is bound to proteins
What happens to B12 when it reaches stomach?
- Low pH and digestion of proteins it’s attached to by pepsin releases free vit B12
- B12 then binds to R protein (haptocorrin) released in saliva and from parietal cells as B12 is easily denatured by HCl
What happens in duodenum?
- R proteins are digested
- Intrinsic factor which was made by parietal cells in stomach binds to vit B12 and this vit B12/IF complex is resistant to digestion
- The complex binds to cubilin receptor and is taken up in distal ileum
What happens when the complex comes into a cell?
- Vit B12/IF complex broken- possibly in mitochondria
- B12 binds to protein transcobalamin II (TCII), crosses the basolateral membrane and travels to the liver
- TCII receptors on liver cells allow them to uptake the complex
- Proteolysis breaks down TCII inside cell
