Ions, vitamins and minerals (41) Flashcards
What is paracellular transport?
diffusion through tight junctions and lateral intercellular spaces
What is transcellular transport?
diffusion through epithelial cells
What are the 2 types of transport proteins?
- channel proteins: form aqueous pores allowing specific solutes to pass across the membrane
- carrier proteins: bind to the solute and undergo a conformational change to transport it across the membrane
- channel proteins allow much faster transport
What types of channel proteins are there?
- voltage gated
- ligand gated (extracellular or intracellular ligand)
- mechanically gated
What are the 3 types of carrier-mediated transport?
- uniport: solute comes in and goes out other side
- symport: transported molecule and co-transported ion go in same direction
- antiport: exchange an ion for another
What are the 2 types of active transport?
- primary AT: linked directly to cellular metabolism (uses ATP to power)
- secondary AT: derives energy from electrochemical gradient of another substance to actively transport something
By what type of membrane transport are glucose and galactose absorbed by enterocytes?
- 2y active transport: uses stored potential energy from Na+ electrochemical gradient
- carrier protein= SLGT-1 on apical membrane –> transports glucose uphill against its conc. gradient (so effective when glucose at low levels in lumen)
By what type of membrane transport is fructose absorbed by enterocytes?
- facilitated diffusion
- carrier protein= GLUT-5 on apical membrane –> effectively low conc. of fructose in lumen as tissue and plasma levels are low, so passive transport can be used
How does glucose exit at the basolateral membrane of enterocytes?
- facilitated diffusion
- carrier protein= GLUT-2
How is water absorbed in the GI tract?
- powered by the absorption of ions
- most water absorbed in small intestine, esp. jejunum
- 99% of water is absorbed
How is sodium transported into enterocytes differently at different stages of the gut?
- counter-transport in exchange for H+ in proximal bowel
- co-transport w/ amino acids and monosaccharides in jejunum
- co-transport w/ Cl- in ileum
- restricted movement through ion channels in colon
How are Cl- and HCO3- ions moved into/out of the gut?
- Cl- co-transported w/ Na+ in ileum, using 2y AT
- Cl- absorbed in large intestine by exchange with bicarbonate–> secretion of bicarbonate ions into lumen helps neutralisation of acids
How is K+ moved into/out of the gut?
- K+ diffuses into small intestine via paracellular pathways
- leaks out between cells in large intestine
- PASSIVE
Where is calcium absorbed in the gut?
duodenum and ileum
What stimulates calcium absorption in the gut?
vitamin D and PTH
How is calcium absorbed into enterocytes?
- directly via intercellular channels OR via facilitated diffusion across IMcals (intestinal calcium-binding protein)
- Ca2+ binds to calbindin, preventing its action as an intracellular signal–> allows it to be transported across cell
What is the role of vitamin D in calcium absorption in the gut?
- inc. paracellular absorption of calcium
- inc. transcription of channels that allow absorption of Ca2+ from lumen
- inc. calbindin levels
- inc. PMCA channels
Why don’t we want a lot of free calcium in enterocytes?
it acts as an intracellular signalling molecule
How is Ca2+ pumped across the basolateral membranes of enterocytes?
- by PMCA (plasma membrane Ca2+ ATPase) against its conc. gradient to maintain a low conc. of Ca2+ in cell
- by Na+/Ca2+ exchanger (requires larger conc. to be effective, so low affinity and high capacity)
How is iron absorbed into enterocytes?
- heme ion directly absorbed by heme transporter–> goes to Fe2+ or stored as ferritin to prevent iron overload
- nonheme iron changes from Fe3+ to Fe2+ via duodenal cytochrome B so we can absorb
- Fe2+ absorbed by DMT1 (a H+ coupled co-transporter) then goes to ferroportin 1 channel at basolateral membrane or becomes stored ferritin
What happens to Fe2+ after it is transported through ferroportin 1 across basolateral membrane of enterocytes?
- converted to Fe3+ by hephaestin
- attached to plasma transferrin in blood–> to be delivered to liver–> activates hepcidin–> inhibits ferroportin 1 function to dec. iron absorption
Which vitamins are transported to brush border in micelles?
fat soluble vitamins A, D, E, K
What is the consequence of impaired absorption of vitamin B12?
retardation of the maturation of red blood cells–> pernicious anaemia
How is vitamin B12 absorbed in the gut?
- dietary B12 is attached to dietary proteins
- in stomach, B12 detaches from protein due to acid and attaches to haptocorrin (to protect from acid)
- after duodenum, detaches and travels through small bowel, then attaches to intrinsic factors
- absorbed at terminal ileum into enterocytes, detached from intrinsic factors
- cross basolateral membrane via MDR1 channel then to capillaries, where it attaches to transcobalamin 2–> travels body
