Ions, Vitamins and Minerals Flashcards
What are the 2 mechanisms of absorption of nutrients?
- Paracellular —> between cells
- tight junctions
- lateral intercellular spaces
- Transcellular —> through epithelial cells
- channel proteins —> aq pores
- faster
- voltage-gated, ligand-gated,
mechanically gated - carrier —> bind (soluble binding site) and change
- uniport, symport, antiport
- channel proteins —> aq pores
What are the 3 types of transport proteins?
- Primary active transporters
- directly linked to cellular metabolism
- eg. Na+/K+ ATPase (pancreas)
H+/K+ ATPase (stomach parietal)
- Secondary active transporters
- energy from conc grad of another actively
transported substance - eg. SGLT-1 symport (small bowel apical)
HCO3-/Cl- antiport (pancreas)
Na+/H+ antiport (pancreas)
- energy from conc grad of another actively
- Facilitated diffusion
- eg. GLUT2 (small bowel basolateral)
GLUT5 (small bowel basolateral)
- eg. GLUT2 (small bowel basolateral)
How are carbohydrates absorbed?
Glucose and galactose:
- apical —> SGLT-1 - 2° active t
- basolateral —> GLUT2 - fac diff
- high capacity, low affinity —>
prevent blood glucose spikes
Fructose:
- apical —> GLUT5 (fac diff)
How is water absorbed in the GI tract?
Standard gradient osmosis:
- ion absorption —> inc water potential in lumen —>
osmosis —> water ‘follows’ ions
- driven by Na+ absorption:
1. Na+ transport
2. Electrochemical grad —> Cl-, HCO3- transport
3. Water potential grad —> water transport
- 9.4 L/day —> 8L/day in small (mostly jejunum)
(99%) —> 1.4L/day in large
- ingest —> 2L
saliva —> 1.2L
gastric secretions —> 2L
bile —> 0.7 L
pancreas —> 1.2L
intestines —> 2.4 L
How is Na+ absorbed in the GI tract? (3)
Apical - Na+/H+ antiport (proximal)
- Na+/amino acid symport (jejunum)
- SGLT-1 (jejunum)
- Na+/Cl- symport (ileum)
Basolateral - Na+/K+ ATPase
How is Cl- absorbed in the GI tract?
Apical - Na+/Cl- symport (ileum)
- HCO3-/Cl- antiport (large bowel)
∵ Na+ transport —> electrical potential gradient
How is K+ absorbed in the GI tract? (2)
Paracellular —> passive transport
How is calcium absorbed in the GI tract?
Apical —> IMcal fac diff (duodenum and ileum)
—> Ca2+ ion channel
Intracellular - 0.1 µM - needs to stay low (signalling)
—> binds to calbindin
—> pumped out quickly
Basolateral —> PMCA (plasma membrane Ca2+
ATPase)
- high affinity, low capacity
—> NCX antiport (Na+/Ca2+ exchanger)
- low affinity, high capacity
- 1-6g/day
- inc by - vit D —> inc Ca2+ transport through cytosol
—> inc calbindin
—> inc PMCA
- parathyroid hormone
- Ca2+ deficiency
How is vitamin D absorbed in the GI tract?
absorbed as 1,25-dihyrdroxy D3
How is iron absorbed in the GI tract?
Lumen —> Fe3+ to Fe2+ via Dcytb (uses vit C)
Apical - Fe2+ —> DMT-1 (Fe2+/H+ symport)
- heme —> HCP-1 (heme carrier) or endocytosis
—> Fe2+ freed via heme oxygenase
Intracellular —> binds to factors
Basolateral —> ferroportin (ion channel)
—> Fe2+ to Fe3+ via hephaestin (Cu-dep)
Blood —> Fe3+ binds to apotransferrin —> transferrin
- 0.5-1.5 mg/day
- 15-20mg consumed - ionganic - 5% absorbed
- ferric 3+ —> can’t absorb
- ferrous 2+
- heme - 20% absorbed - excess intake: intracellular —> irreversibly binds to
apoferritin —> ferritin micelle —>
Fe2+ to Fe3+ —> Fe3+ crystallise in
protein shell —> excreted - in blood iron conc —> inc ferritin production
How are fat soluble vitamins absorbed?
A, D, E, K —> micelles
K —> active t.
- liver —> stores 2-5mg for 6 months
How is vitamin B12 absorbed in the GI tract?
Mouth - B12 consumed bound to dietary protein
Stomach - pepsin and HCl —> dietry protein broken
down —> B12 bound to R protein.
(haptocorrin) from saliva and parietal —>
protect from denaturation by HCl
Small intestine - R protein digested in duodenum —>
B12 binds to IF
Apical - B12/IF to cubilin receptor —> endocytosis
(distal ileum)
Intracellular - B12/IF breakdown —> B12 binds to TCII
Liver - TCII receptors —> B12-TCII in —> proteolysis of
TCII (transcobalamin II)
