ions vitamins and minerals Flashcards
what are the two main types of transport?
- para cellular
- transcellular
what are the methods via transcellular transport?
what do these also involve?
- facilitated diffusion
- simple diffusion
- active transport
carrier proteins
what is paracellular transport?
when things don’t go through the cells they travel around the cells
what are the two types of channel proteins?
Channel proteins :
channel proteins form aqueous pores allowing specific solutes to pass across the membrane
(much faster)
Carrier proteins:
carrier proteins bind to the solute and undergo an conformational change to transport it across the membrane
what are the two ways of active transport?
- primary
linked directly to cellular metabolism - secondary
derives energy from concentration gradient of another substance actively transported
example of secondary active transport?
- enterocytes absorb GLUCOSE & GALACTOSE via 2nd active transport
- glucose transporter
example of facilitated diffusion?
Exit of glucose then via facilitated diffusion (GLUT-2)
absorption of fructose
example of primary active transport?
sodium potassium ATPase
where does water absorption happen ?
how much water is absorbed?
99% of water is absorbed by GI tract
- Most water is absorbed in the small intestine
mainly jejunem
how are ions absorbed?
- Many ions absorbed slowly by passive diffusion
- Ca2+
and iron are incompletely absorbed
how much water does the small intestine absorb?
8 litres
how much water does the colon absorb?
1.4 litres
where does the water come from?
Ingestion (2L) saliva (1.2L). Gastric secretions (2L) bile (0.7L). The pancreas (1.2L) intestine (2.4L).
what is the name of the process by which the water is absorbed?
- standing gradient osmosis
- is driven by absorption of the sodium ion
how does sodium get into cells in different placed enterocytes?
- proximal bowel - counter transport of Na+ for H+
- jejunum - co-transport with amino acid, monosaccharides
- ileum - co transport with Cl-
- colon - restricted movement through the ion channels
how are chloride ions moved?
Cl- is co-transported with Na+
in the ILEUM and
exchanged with HCO3- in the COLON.
how is potassium chlorides transported?
Cl- is co-transported with Na+
in the ILEUM and
exchanged with HCO3-in the COLON.
so we get potassium in faeces
what happens with high sodium in the cell?
- sodium is pumped into lateral intracellular spaces by Na/K ATPase
- this increases the electrochemical gradient to positive
- so negative ions move Cl- and HCO3
- the solution is hypertonic
- so this drives the absorbtion of water
where is the calcium absorbed?
Most absorption of calcium occurs in the duodenum and ileum.
what stimulates the absorption of calcium?
- calcium deficiency increases the absorbtion
- Vitamin D and parathormone stimulate the absorption of calcium
how much calcium do we ingest
how much calcium do we absorb?
1-6g of calcium a day
we only absorb 0.7g of it
by which transporters is the calcium carried across by?
- intestinal calcium binding protein (IMcal)
- ion channel
what is the issue with absorbing calcium
calcium acts as a secondary transporter
- we need to keep it inactive so we bind it to calbindin to make it inactive
how do we ship the calcium across the cell into the blood stream ?
actively transport it
- using Ca2+ATPase (PMCA).
- HIGH affinity but LOW capacity.
Na+/Ca2+ exchanger.
low affinity but high capacity
so what happens just after the calcium is in the cell?
- calbindin is unbound
- high Ca2+ conc
- The Na+/Ca2+ exchanger will effectively reduce the large calcium concentration
- PMCA will then lower the
calcium concentration even further
why do we need vit D?
Essential for normal Ca2+ absorption - increases level of calbindin - Enhance transport of Ca2+ through cytosol. - Increase number of Ca2+ATPase
why is iron important?
Iron is vital for oxygen transport (in haem) and oxidative phosphorylation (ETC)
what happens to iron in excess
and what do we need to do?
- toxic
- must be regulated
so we absorb 15- 20mg/day - but we only absorb 0.5-1.5mg/day
how do we absorb iron?
method 1 ?
- inorganic iron
– Fe2+ (ferric) or Fe3+ (ferrous) - We CANNOT absorb Fe3+ directly
so we absorb it as Fe3+ + insoluble salt binding - or we reduce it to Fe2+ using vit C
how do we absorb iron?
method 2?
as part of haem
- absorb this intact into the enterocyte via haem carrier protein 1 (HCP-1) which endocytoses via receptor mediated endocytosis.
Fe2+ is then liberated by haem oxygenases.
how is iron uptaken?
method 1
- haem can bind to haem carrier protein 1 and then is absorbed by receptor mediated endocytosis
- Haem Oxygenase
(HO) then liberates Fe2+
how is iron uptaken ?
method 2
Duodenal Cytochrome B (Dcytb – this enzyme can reduce
Fe3+ → Fe2+
which then passes into cytosol via Divalent Metal Transporter 1 (DMT-1) with H+ ions.
how is iron excreted into the blood?
- Ferric enters the blood via ferroportin (FP).
- Hephaestin (HP) is a copper dependant
ferroxidase that converts Fe2+ → Fe3+ - Fe3+ binds to apotransferrin and travels in
blood as transferrin (TF).
-
what suppresses ferroportin?
Hepcidin
how is iron excreted?
- Ferric binds to apoferratin to form ferritin micelle
- Fe2+ then oxidises to Fe3+ which creates a protein shell.
- The micelle is lost when the enterocytes
are shunted off the tip of the villi. - Increase of iron conc. in cytosol increases
ferritin synthesis.
how are most vitamins transported?
- Vitamins are predominantly absorbed by passive diffusion.
how are fat soluble vitamins ADEK transported?
- K is actively taken up.
- ADE transported to the brush border by micelles
where is vit B12 stored?
Liver contains a large store (2-5mg).
what happens when there is impaired absorbtion of B12?
pernicious anaemia.
how is B12 released from food in stomach?
what is the problem and what happens?
most B12 is bound to protein in the food
- the stomach low PH and pepsin releases the B12 from the food
- B12 is easily denatured by HCl
- B12 therefore binds to R protein (haptocorrin) which is released by parietal cells and in saliva.
the R protein is then digested in the duodenum
- then the B12 binds to the intrinsic factor
the intrinsic factor is not digested - B12/IF complex binds to cubulin (cub) receptor in distal ileum and is endocytosed
- then within the mitochondria the complex is broken down
and B12 binds to transcobalamin II (TCII) and travels to liver in the blood. - TCII receptors on the Liver allow uptake of complex.
- Proteolysis then breaks down the TCII inside the cell.
