Ions, minerals and vitamins Flashcards
(34 cards)
What are the 2 overarching ways in which molecules can cross epithelium to enter cells
Paracellular transport- via tight junctions and lateral intercellular spaces.
Transcellular transport - through epithelial cells
Contrast channel and carrier proteins
Channel proteins - form aqueous pores allowing specific solutes to pass across the membrane. They have ion selective filter in aqueous pore
Carrier proteins - bind to solute and undergo a conformational change to transport it across the membrane
Channels proteins allow much faster transport than carrier proteins
What are the different types of channel proteins
- Voltage gated
- ligand gated (extracellular ligand)
- Ligand gated (intracellular ligand)
- Mechanically gated

What are the different types of carrier mediated transport
Uniport, symport and antiport

What are the 2 types of active transport (energy required)
Primary- linked directly to cellular metabolism (ATP)
Secondary - derives energy from the concentration gradient of another substance that is actively transported
Summarise all membrane transport

Give examples of primary active transporters , secondary active transporters and facilitated transporters covered in the GI course
Primary active -
- Na+/K+ ATPase
- H+/K+ ATPase (parietal cell stomach)
SECONDARY active
- SGLT1 cotransport (small bowel absorption of monosaccharides)
- HCO3-/Cl- counter transport (Pancreatic HCO3- secretion)
- Na+/H+ counter transport (Pancreativ HCO3- secretion)
Facilitated Transport
- GLUT5, GLUT 2 (small bowel absorption of monosaccharides)
How is glucose and galactose absorbed by enterocytes
Via secondary active transport
SGLT-1 on apical membrane ; transport glucose against it’s concentration gradient (especially when glucose in lumen is low) by using energy from Na absorption
Exit of glucose at basolateral membrane is by facilitated diffusion via GLUT 2 carrier protein

Contrast the properties of GLUT 2 and GLUT 5
GLUT 2- high capacity, low affinity facilitative expansion transporter. Hence glucose between plasma and tissue is equilibrated
GLUT 5- high affinity, hence effective at relatively low concentrations of fructose in the lumen as the tissue and plasma levels are low
How is fructose absorbed
GLUT 5 carrier protein at apical membrane
Then same as glucose in basolateral membrane
What part of the small bowel absorbs Ca2+
What hormones stimulate absorption
Duodenum and ileum
Vit D and PTH
What increases gut ability to absorb calcium
Ca2+ deficient diet
What’’s the intake of Ca2+ in diet and how much is absorbed and secreted
Diet- 1-6g/day
secretions - 0.6g
Absorbe 0.7g
Summarise calcium absorption. Draw
Ca2+ carried across apical membrane by:
- intestinal calcium-binding protein (iMcal) - facilitated diffusion
- ion channel
Ca2+ pumped across basolateral membrane by:
- PMCA
- Na+/Ca2+ exchanger

Contrast PMCA and Na+/Ca2+ exchanger in terms of properties
PMCA - plasma membrane Ca2+ ATPase
Has high affinity for Ca2+ but low capacity. Hence maintains very low concentration of calcium normally observed within a cell
Na+/Ca2+ exchanger
Low affinity for Ca2+ but high capacity. Hence requires larger concentrations foR Ca2++ to be effective
What is the intracellular [Ca2+] and extracellular. Comment on the diffference between the 2
Intracellular - 100nM; however can increase 10 to 100 fold during various cellular functions
Extracellular
- Plasma - 2,2-2.6mM
- Luminal varies in mM range
Ca2+ entering enterocyte from lumen can act as intracellular signalling molecules; hence it bind to calbindin
What’s the effect of Calcitriol on enterocytes
- Enhances the transport of Ca2+ through cytosol
- increase levels of calbindin
- increase levels of PMCA; hence increasing rate of extrusion of C2+ across basolaterla membrane
What is the property of iron and what processes in the body is iron critical for?
Can act as an electron donor and acceptor
Hence important for:
- Oxygen transport - haemoglobin
- Oxidative phosphorylation
How much iron does an adult in gets each day and in what forms does the iron present
15-20mg/day ingested
But absorbs only 0.5-1.5mg/day
Iron present in diet as:
- inorganic (ferric Fe3+/ ferrous Fe2+)
- part of haem group (haemoglobin, myoglobin and cytochromes)
What form of iron can we not absorb? Why is this the case
Fe3+
as it forms insoluble salts with hydroxide , phosphate or HC03- ions
What type of iron is more readily absorbed than the others
Haem- 20% of presented is absorbed unlike 5% for other forms
Hence highly bioavailability
What reduces Fe3+ to Fe2+ in the blood
Vit C
What transporter transports haem into enterocyte and how is the Fe2+ liberated
Via HCP-1 (heme carrier protein 1) and via receptor mediated endocytosis
the Fe2+ is liberated by Heme oxygenase
Summarise how iron can be absorbed
DMT-1 (divalent metal transporter 1)- this is a H+ coupled co-transporter
Hephaestin- a transmembrane copper dependent ferroixdase
Fe3+ binds to apotransferrin to make transferrin


