Ions, vitamins and minerals (41)

Question 1

What is paracellular transport?

Answer 1

diffusion through tight junctions and lateral intercellular spaces

Question 2

What is transcellular transport?

Answer 2

diffusion through epithelial cells

Question 3

What are the 2 types of transport proteins?

Answer 3

• 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

Question 4

What types of channel proteins are there?

Answer 4

• voltage gated
• ligand gated (extracellular or intracellular ligand)
• mechanically gated

Question 5

What are the 3 types of carrier-mediated transport?

Answer 5

• 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

Question 6

What are the 2 types of active transport?

Answer 6

• 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

Question 7

By what type of membrane transport are glucose and galactose absorbed by enterocytes?

Answer 7

• 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)

Question 8

By what type of membrane transport is fructose absorbed by enterocytes?

Answer 8

• 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

Question 9

How does glucose exit at the basolateral membrane of enterocytes?

Answer 9

• facilitated diffusion

- carrier protein= GLUT-2

Question 10

How is water absorbed in the GI tract?

Answer 10

• powered by the absorption of ions
• most water absorbed in small intestine, esp. jejunum
• 99% of water is absorbed

Question 11

How is sodium transported into enterocytes differently at different stages of the gut?

Answer 11

• 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

Question 12

How are Cl- and HCO3- ions moved into/out of the gut?

Answer 12

• 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

Question 13

How is K+ moved into/out of the gut?

Answer 13

• K+ diffuses into small intestine via paracellular pathways
• leaks out between cells in large intestine
• PASSIVE

Question 14

Where is calcium absorbed in the gut?

Answer 14

duodenum and ileum

Question 15

What stimulates calcium absorption in the gut?

Answer 15

vitamin D and PTH

Question 16

How is calcium absorbed into enterocytes?

Answer 16

• 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

Question 17

What is the role of vitamin D in calcium absorption in the gut?

Answer 17

• inc. paracellular absorption of calcium
• inc. transcription of channels that allow absorption of Ca2+ from lumen
• inc. calbindin levels
• inc. PMCA channels

Question 18

Why don’t we want a lot of free calcium in enterocytes?

Answer 18

it acts as an intracellular signalling molecule

Question 19

How is Ca2+ pumped across the basolateral membranes of enterocytes?

Answer 19

• 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)

Question 20

How is iron absorbed into enterocytes?

Answer 20

• 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

Question 21

What happens to Fe2+ after it is transported through ferroportin 1 across basolateral membrane of enterocytes?

Answer 21

• 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

Question 22

Which vitamins are transported to brush border in micelles?

Answer 22

fat soluble vitamins A, D, E, K

Question 23

What is the consequence of impaired absorption of vitamin B12?

Answer 23

retardation of the maturation of red blood cells–> pernicious anaemia

Question 24

How is vitamin B12 absorbed in the gut?

Answer 24

• 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