Ions Vitamins And Minerals

Question 1

Diffusion

Answer 1

Process of atoms and molecules intermingling because of their random thermal motion

Occurs rapidly over microscopic distances but slowly over macroscopic distances

Question 2

Define osmosis

Answer 2

Diffusion of water from hypotonic to hypertonic medium

Question 3

How do molecules cross the epithelium to enter the bloodstream (2 ways)?

Answer 3

• Paracellular transport through tight junctions and lateral intercellular spaces
• Transcellular transport through epithelial cells

Question 4

How can solutes cross the cell membranes

Answer 4

Simple diffusion
Facillatated diffusion
Active transport

Question 5

Facilitated transport- describe it

Answer 5

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)

Question 6

Active transport

Answer 6

• 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

Question 7

Carrier proteins

Answer 7

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

Question 8

What do channel proteins do?

Answer 8

Form aqueous pores allowing specific solutes to pass across the membrane

Allow much faster transport then carrier proteins

Question 9

How are glucose and galactose absorbed at brush border?

Answer 9

• 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)

Question 10

How does glucose exit at basolateral membrane?

Answer 10

• Facilitated diffusion through GLUT-2 carrier protein → a high capacity, low-affinity facilitative transporter
• Glucose between plasma and tissue/enterocyte generally equilibrated

Question 11

How is fructose absorbed at brush border?

Answer 11

• 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

Question 12

Where are water and ions absorbed in GI tract?

Answer 12

• 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

Question 13

How much water do the small and large bowels absorb?

Answer 13

• 8l in small bowel
• 1.4l in large bowel

Question 14

How is Na+ transported into enterocyte?

Answer 14

• 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

Question 15

How are other ions absorbed?

Answer 15

• 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

Question 16

What is standing gradient osmosis + how does it lead to water absorption from GI tract?

Answer 16

• 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

Question 17

Where is Ca2+ is absorbed?

Answer 17

• Duodenum and ileum absorb it
• Vit D and PTH stimulate absorption
• Ca2+ deficient diet increases gut’s ability to absorb

Question 18

How much Ca2+ do we have intracellularly vs extracellularly?

Answer 18

• Low intracellular- 100nM (can increase 100x during various cell functions)
• High extracellular- 1-3mM
  In plasma we have 2.2-2.6mM

Question 19

How is Ca2+ moved across apical membrane (to basolateral membrane) of enterocytes? (2 things together)

Answer 19

• Intestinal calcium-binding protein (IMcal) e.g. calbindin - facilitated diffusion
• Ion channel

Question 20

• What happens to Ca2+ when it’s taken into cell?
  -

Answer 20

• 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

Question 21

How is Ca2+ moved across basolateral membrane? (2 ways)

Answer 21

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

Question 22

What does vit D3 do to enterocytes to help with Ca2+ transport (2 things)?

Answer 22

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

Question 23

• What processes in the body is iron critical for?
  -

Answer 23

• Oxygen transport in RBCs
• Oxidative phosphorylation in mitochondria ETC
• It can act as an electron donor and acceptor

Question 24

How much iron do we absorb and in which form?

Answer 24

• 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%)

Question 25

• How is haem absorbed? (give specific details)
  -

Answer 25

• Absorbed intact into enterocyte via heme carrier protein (HCP-1) and via receptor-mediated endocytosis
• Fe2+ is liberated by heme oxygenase

Question 26

How are Fe3+ and Fe2+ taken up by enterocytes?

Answer 26

• 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

Question 27

What 2 things can then happen to Fe2+ in the cell?

Answer 27

Moves into blood
Gets stored in cell

Question 28

Fe2+ moves into blood- how does it do this, + describe the action of hepicidin

Answer 28

• 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

Question 29

• Fe2+ gets stored in cell- how?
  -

Answer 29

• 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

Question 30

• What are vitamins?

Answer 30

Organic compounds that can’t be made by body but vital to metabolism

Question 31

How are different vitamins taken up by body?

Answer 31

• 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

Question 32

How is vitamin B12 important and where is it found?

Answer 32

• 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

Question 33

What happens to B12 when it reaches stomach?

Answer 33

• 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

Question 34

What happens in duodenum?

Answer 34

• 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

Question 35

What happens when the complex comes into a cell?

Answer 35

• 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