Ion and Water absorption

Question 1

What is diffusion?

Answer 1

The process of atoms intermingleing due to random thermal motion (mixing)

Question 2

What is osmosis?

Answer 2

movement of H2O across a semipermeable membrane from isotonic to hypertonic solution

Question 3

How can molecules pass epithelium to enter the bloodstream?

Answer 3

Paracelular and transcellular (through cell then into blood stream)
TJ stop paracellular movement (depending on epithelum, like stomach are very tight, while the small intestine are less tight)-. TJ also marks the change from apical and basal membrane of a cell

Question 4

What are two possibilities for membrane transport?

Answer 4

Channel proteins that form aqueous pores-much faster (facilitated or not)-can be gated or shut
Carrier proteins-bring solute in-slower

Question 5

What are different fashions to gate an ion channel?

Answer 5

Voltage gates, extracellular gates (hormones), intracellular ligand, Mechanically gated
What are the different forms of carrier protein channels?
Uniport, symport and antiport

Question 6

What are the main sources of energy for membrane transport?

Answer 6

ATP/primary sources. Secondary transport uses the energy of another transport (like antiporter)-

Question 7

How are carbohydrates like glucose absorebed?

Answer 7

SLGT1 takes in 1 sodium and uses that energy to take glucose as well

Question 8

How is fructose absorbed?

Answer 8

facilitated transport via GLUT5

Question 9

How does glucose exit enterocytes?

Answer 9

Facilitated transport with GLUT2-as the glucose conc gets high in the cell, it will naturally exit through

Question 10

How much water absorbed in the GI tract? Where does it come from?

Answer 10

99% of presented water is absorbed by osmosis-most water absorbed in small intestine (8L), in juejenum. Some in colon (1.4L)
Mostly is made of secretions from the body-saliva, gastric secretion, Bile, pancreas, intestinal secretion

Question 11

What is standing gradient osmosis?

Answer 11

It is how water is absorbed-driven by Na

Question 12

How can Na+ be absorebed at different levels of the GI tract?

Answer 12

Travel of Na to enterocytes is complex and varies, but becomes more efficient as travel down intestine. At first (proximal bowel), antiportport with H+-same charges. then cotransport with glucose and AA (juojenum). In small intestine (illiem)), co transport with Cl- (neutralise charge)
And more distal just Na channels (colon)
Along the GI tract, as you go down, Na conc in the cell increases

Question 13

How are Cl and K+ absorbed?

Answer 13

CL can be transported with Na+ in illieum, or echanged for HCO3-
K+ tends to diffuse paracellularly-diffuse IN in small intestine, but tends to leak out in the colon

Question 14

What happens to the absorbed Na in enterocytes? How does that help Cl- intake?

Answer 14

Exchanged or K+ by Na/K+ ATPase-this means the levels of Na within the interstitial space are high.
So positive charges are high too-causing attraction of the Cl- and HCO3- which pass through

Question 15

How does water move into the blood?

Answer 15

Because of the hypertonic interstitial fluid-water is driven in paracellularly and transcellular
This causes accumulation of water in iterstita-meaning increases pressure-interstitial hydrostatic pressure pushes water into the blood
Where is calcium absorbed?
Small intestine-duodenum and ileum. Low Ca2+ makes body absorb more. VitD and PTH stimulate absorption

Question 16

What are the intracullar and extracellular concentrations of Clacium?

Answer 16

Not high in either, but much more outside the cell than inside (in 0.1uM), outside 1-2 mM

Question 17

How is Calcium carried across the membrane?

Answer 17

IMCaL-intestinal calcium binding protein channels, and ion channels. Both facilitated diffusion
What are implications of Ca transport across the cell?
Normally Ca2+ acts as an intracellular signalling molecule-and want to avoid creating a cascade
Need to maintain low intracellular concentration-binds to calbinin in cytosol, preventing its action

Question 18

What is Calbindin?

Answer 18

A protein in the cytosol that binds Ca2+ to stop it from having signalling effects

Question 19

How is Ca2+ excreted into the blood stream?

Answer 19

Calbindin diffuses across, but there the extracellular gradient isn’t favourable (higher in Intersititial)
Use Plasma membrane Ca2+ ATP ase-PMCA has high affinity (acts at low conc, but not fast)-maintains the low level
Also uses a Na+/Ca2+ antiporter-needs high concentration but can work very fast-great when large Ca2+

Question 20

What is the role of Vitamin D in calcium absorption?

Answer 20

Enchances Transport through cytosol, increase calbindin

Question 21

What are the role of iron in the body? What happens to excess?

Answer 21

Oxygen transport, ETC
Problem is body has no mechanism to secrte iron-and can be toxic in excess
Need to be able to absorb quickly as required but also limit it to not be toxic

Question 22

How is iron present in the diet?

Answer 22

Either Fe2+/Fe3+, or as heme groups. Body can only absorb Fe2+
Fe3 forms insoluble salts
Heme are smaller parts of diet but easier to absorb

Question 23

What is the role of Vit C in iron absorption?

Answer 23

Reduces Fe3+ to Fe2+

Question 24

How is heme taken into enterocytes?

Answer 24

Receptor mediated entocytosis-Heme carrier protein (HCP-1)-which then brings heme intracellularly where its broken down to liberate the iron (heme oxygenase)

Question 25

What is duodenal cytochrome B?

Answer 25

Catalyses the recuction of Fe3+ to Fe2+

Question 26

How is iron absorbed by enterocytes?

Answer 26

Fe3+ is converted to Fe2 by Duodenal cytochrome B
Then DMT1 brings the Fe2+ into the cytosol along with an H+
Fe2+ binds unknown proteins in cytosol, where it goes to basement membrane and Is transported out by ferroportin
But in blood is carried as Fe3+-Hephaestin is a transmembrane copper ferroxidase-then Fe3+ to apotransferin, and travels as transferrin

Question 27

How is iron stored within enterocytes?

Answer 27

Fe2+ can be made to Fe3+ , and with apoferritin, makes Ferritin that traps Fe3+ in the cells and making them innert biochemically-stored

Question 28

What does the iron stored in enterocyte go?

Answer 28

Well as you lose enterocytes, you lose some of those ferritin and as they are in Fe3+ form, the its hard to reabsorbed

Question 29

What is a vitamin?

Answer 29

A biological compound you need to function but that cannot be made in body-fat soluble vitamins (A,D,E,K) are transported in micelles are easily taken in
Water soluble ones need more effort

Question 30

Where is most Vit B12 found ?

Answer 30

Have a large form in body, but if lost-pernicious anaemia. In GI, VitB12 is found binding proteins

Question 31

What is R protein in VitB12 absorption? What happens after in B12 absoprtion

Answer 31

In stomach, Low pH causes dissociated/destruction of proteins-there B12 binds to R proteins (made in liver) that protects it. R protein +B12 go to juojenum, where R protein is digested
That’s where Intrinsic factor binds B12, and travels do distal ileum-where cubulin receptor recognises it and takes it in (maybe receptor mediated endocytosis)

Question 32

What happens to B12 after absorption?

Answer 32

IF+B12(+cubulin?) complex Is broken down, and B12 is released and binds transcobalamin II, which allows to cross membrane, then transport to liver via blood. In liver, transcobalamin II receptor absorbs the complex, then its broken down again, and B12 is stored