ions vit and minearals Flashcards
what is diffusion
The process whereby atoms or molecules intermingle because of their random thermal motion.
osmosis
diffusion of water from a hypotonic to hypertonic thru a medium
How can molecules cross the epithelium to enter the bloodstream?
Paracellular Transport
through tight junctions and lateral intercellular spaces.
or
Transcellular Transport through the epithelial cells
how can solute cross cell membrane
simple diffusion
- facilitated transport
- active transport
how are they transported
Channel proteins
- Carrier proteins
what is a channel protien
form aqueous pores allowing specific solutes to pass across the membrane.
what are carrier protiens
bind to the solute and undergo a conformational change to transport it
across the membrane.
whats faster
Channel proteins allow much faster transport than carrier proteins.
Types of carrier-mediated transport
uniport
symport (symp and anti are coupled transport)
antiport
what is primary AT
transport is linked directly to cellular metabolism (uses ATP to power the transport).
what is secondary AT
derives energy from the concentration gradient of another substance that is actively transported.
what is Facilitated transport /facilitated diffusion
Enhances the rate a substance can flow down its concentration gradient. This tends to equilibrate the substance across the membrane and does not require energy.
are channel mediated transport active or passive
passive always
and carrier
passive or active
Examples of Primary active transporters already covered in the course?
Na+/K+ ATPase (Pancreatic HCO3- Secretion)
H+/K+ ATPase (Stomach – Parietal Cell)
Examples of Secondary active transporters already covered in the course?
SGLT-1 co-transport (Small bowel absorption of monosaccharides)
HCO3-/Cl- counter transport (Pancreatic HCO3- Secretion)
Na+/H+ counter transport (Pancreatic HCO3- Secretion)
Examples of Facilitated transport /facilitated diffusion transporters already covered in the course?
GLUT-5, GLUT-2 (Small bowel absorption of monosaccharides)
how are glucose and galactose absorbed
is by 2o active transport (carrier protein & electrochemical gradient).
what carrier protien
Carrier protein = SGLT-1 on apical membrane
why sglt 1
SGLT1 can transport glucose uphill against its concentration gradient (so effective when glucose at levels in the lumen are below those in the enterocyte).
fructose absorption
Absorption of fructose is by facilitated diffusion.
what protien
Carrier protein = GLUT-5 on apical membrane.
why is it effective
Effective at relatively low concentrations of fructose in the lumen as tissue and plasma levels are low.
Exit of glucose at the basolateral membrane is by
facilitated diffusion.
what protien
Carrier protein = GLUT-2, a high-capacity, low-affinity facilitative transporter.
where is most water absorbed
The greatest amount of water is absorbed in the small intestine, esp the jejunum.
what 2 ions are not completely absorbed
Calcium and iron are incompletely absorbed, and this absorption is regulated.
how is water absorbed
through standing gradient osmosis
what is it driven by
Na+
how is the transport of na+ from the lumen to the enterocyte in the proximal bowel
Counter-transport in exchange for H+ (proximal bowel)
jejunum
Co-transport with amino acids, monosaccharides (jejunum)
ileum
Co-transport with Cl- (ileum)
colon
Restricted movement through ion channels (colon)
how is cl- co transported with na+ in iluem
Cl- co-transported with Na+ in ileum
colon
exchanged with HCO3- (colon) into enterocytes
what are both above example sof
Both secondary active transport
how does k+ diffuse in colon
in via paracellular pathways in small intestine, leaks out between cells in colon.
what is that an example of
Passive transport.
what happens to intracellular sodium
Active transport of Na+ into the lateral intercellular spaces by Na+K+ATPase transport in the lateral plasma membrane
what strcutures absorb ca2+
Duodenum and Ileum
what affect does calcium have on gut ability to absorb
Ca2+ deficient diet increases gut’s ability to absorb.
what factors can stimulate absoprtion of ca2+
Vit D and parathyroid hormone stimulate absorption.
how can calcium enter our cells from the small bowel
cia paracellular junction
another way
through calcium channels binds to calbindin prevents the prescence of high concentration in our cells so either goes through another channel or swapped for na+
what can vit d do in the cell
enhances absorption and movement of calcium through the cell
what are the 2 ways that ca2+ can be carried across the apical membrane
Intestinal calcium-binding protein (IMcal)- facilitated diffusion.
Ion channel
what are the implications for transport of Ca2+ into the cell from the lumen?
Ca2+ acts as an intracellular signalling molecule.
What are the implications for Ca2+ transport across the cell?
Need to transport Ca2+ while maintaining low intracellular concentrations.
Binds to calbindin in cytosol, preventing its action as an intracellular signal.
how can calcium be trasnported out of the basolateral membrane (and what type of movememnt)
Ca2+ pumped across basolateral membrane by plasma membrane Ca2+ ATPase (PMCA) against concentration gradient.
what is the affinity of PMCA for ca2+ and capacity
PMCA has a high affinity for Ca2+ (but low capacity).
what is an alternate method for ca to be transported through teh basolateral membrane
Ca2+ pumped across basolateral membrane by plasma membrane Na+/Ca2+ exchanger against concentration gradient.
describe the affinity of the na/ca exchanger for ca and capacity
The Na+/Ca2+ exchanger has a low affinity for Ca2+ but a high capacity. Requires larger concentrations of Ca2+ to be effective.
what can 1, 25-dihydroxy D3 when taken up by enterocytes
Enhances the transport of Ca2+ through the cytosol
Increases the levels of calbindin
Increases rate of extrusion across basolateral membrane by increasing the level
of Ca2+ ATPase in the membrane.
is iron an electron donor or acceptor
Iron can act as an electron donor and an electron acceptor
What processes in the body is iron critical for?
Oxygen transport (red blood cells)
Oxidative phosphorylation
mitochondrial transport chain
what are the 2 forms of ions that can be absorbed in oir bodies
heam
non haem
what happens to haem irion
absorbed directly through a transporter and goes to intracellular region
non haem aka
ferrous
problem
ferrous not very absorbable so changed to a ferric
how
by duodenal cytrocrome b agent
then
absorbed via channel to intraceelular area
then
stored as ferritin or absorbed across basolateral membrane through ferroportin channels
how is iron transported
first convert ferric to ferrous by transferin to liver or bone marrow
what happens if there is enough ferritin in the liver
hepcidin is produced
reduces production of ferroportin
how is Iron present in the diet as:
inorganic iron (Fe3+ ferric, Fe2+ ferrous)
as part of heme (haem) group (haemoglobin, myoglobin and cytochromes).
can you absorb fe3+
Cannot absorb Fe3+, only Fe2+.
what reduces fe3+ to fe2+
Vit C reduces Fe3+ to Fe2+
what happens to heam once it is absorbed
Heme is absorbed intact into the enterocyte.
Evidence that this occurs via heme carrier protein 1 (HCP-1), and via receptor-mediated endocystosis.
what liberates the fe2+
Fe2+ liberated by Heme oxygenase.
what enzyme is responsible for
catalyzes the reduction of Fe3+ to Fe2+ in the process of iron absorption in the duodenum of mammals
Duodenal cytochrome B (Dcytb)
Duodenal cytochrome B (Dcytb)
how is fe2+ transported into the cell
Fe2+ transported via divalent metal transporter 1 (DMT-1), a H+-coupled co-transporter.
Fe2+ binds to unknown factors, carried to basolateral membrane, moves via ferroportin ion channel into blood.
how does fe2+ move across basolateeral membrane
ferroportin
what converts fe2+ to fe3+
Hephaestin is a transmembrane copper-dependent ferroxidase that converts Fe2+ to Fe3+.
how does fe3+ travel in blood
what does it bind to
Fe3+ binds to apotransferrin, travels in blood as transferrin.
hepcidin function
(Hepcidin, the major iron regulating protein, suppresses ferroportin function to decreases iron absorption).
what is an alternative method for fe2+ transport
Binds to apoferritin in cytosol to form ferritin micelle.
how is it oxidised
Ferritin is globular protein complex. Fe2+ is oxidised to Fe3+ which crystallises within protein shell.
What happens to this ferritin stored in enterocytes?
Prevents absorption of too much iron (toxic)
what is the binding of iron to ferritin like
Irreversible binding of iron to ferritin in the epithelial cells.
how is ferritn lost
Iron/Ferritin is lost in the intestinal lumen and excreted in the faeces.
what happens when iron sythesis increases
Increase in iron concentration in the cytosol increases ferritin synthesis.
how is vit k taken up
active tr
where is vit b stored
Liver contains a large store (2-5mg).
how is vit b12 freed
In the stomach, low pH and the digestion of proteins by pepsin releases free vit B12.
But B12 is easily denatured by HCl.
How is the denaturation of B12 in the stomach avoided?
Binds to R protein (haptocorrin) released in saliva and from parietal cells.
R proteins digested in duodenum.
what is intrisic factor/Vit B12 binding glycoprotein secreated by
parietal cells.
what is special about Vit B12/IF
it is resistant to digestion.
No IF then
then no absorption of vit B12
where do the Vit B12/IF complex go
binds to cubilin receptor, taken up in distal ileum (mechanism unknown, but thought to involve receptor-mediated endocytosis
what happens once in a cell
Once in cell, Vit B12/IF complex broken- possibly in mitchondria
how does it cross to basolateral membrane
B12 binds to protein transcobalamin II (TCII), crosses basolateral membrane by unknown mechanism
then it travels to
Travels to liver bound to TCII.
function of TCII
TCII receptors on cells allow them to uptake complex.
how is TCII removed
Proteolysis then breaks down TCII inside the cell.
In the epithelial cells, majority of iron is bound to ferritin and is unavailable for absorption.
In the epithelial cells, majority of iron is bound to ferritin and is unavailable for absorption.
