Ions, vitamins and minerals

Question 1

How can molecules cross epithelium to enter the blood stream?

Answer 1

• paracellular transport through tight junctions and lateral intercellular spaces
• transcellular transport through epithelial cells

Question 2

What are channel proteins?

Answer 2

Aqueous pores allowing specific solutes to pass across the membrane

Question 3

What are carrier proteins?

Answer 3

Bind to solute and undergo a conformational change to transport it across the membrane

Question 4

What transports faster? Channel or carrier proteins?

Answer 4

Channel

Question 5

What is the difference between primary and secondary active transport?

Answer 5

Primary - linked directly to cellular metabolism and uses ATP to power transport
Secondary - derives energy from conc gradient of another substance that is actively transported

Question 6

Example of primary active transporters?

Answer 6

Na+/K+ ATPase

H+/K+ ATPase

Question 7

Example of secondary active transporters?

Answer 7

GLUT-1 cotransport
HCO3-/Cl- transport
Na+/H+ counter transport

Question 8

Example of facillitated transport?

Answer 8

GLUT5. GLUT2

Question 9

How are carbohydrates absorbed? - glucose and galactose

Answer 9

Glucose and galactose by secondary active transport, carrier protein SGLT-1 on apical membrane that carries glucose against conc gradient

Question 10

What is the carrier protein for glucose and galactose absorption

Answer 10

SGLT1

Question 11

What is the carrier protein for fructose? What type of transport does it use?

Answer 11

Facillitated diffusion

GLUT5

Question 12

How does glucose exit the enterocyte at the brush broder?

Answer 12

Facillitated diffusion

GLUT-2 high capacity low affinity facilitative transporter

Question 13

What are specific issues in alimentary absorption?

Answer 13

Water and ions
Calcium
Iron
Vitamins B12

Question 14

Where is most of the water absorbed? How is absorption of water powered?

Answer 14

Small intestine - esp jejunum

Powered by absorption of ions that are absorbed slowly by passive diffusion

Question 15

How much water is absorbed per day in small intestine?

Answer 15

8 litres

Question 16

How much water is absorbed per day in large intestine?

Answer 16

1.4 litres

Question 17

How is Na+ transported into the enterocyte from the lumen?

Answer 17

Counter transport in exchange for H+
Co-transport with aa, monosaccharides (jejunum)
Cotransport with Cl- (ileum)
Restricted movement through ion channels (colon)

Question 18

How is Cl- absorbed into enterocytes?

Answer 18

Co-transported with Na+ (ileum) and exchanged with HCO2- (colon) into enterocytes. Both secondary active transport

Question 19

How is K+ absorbed into enterocytes?

Answer 19

Diffuses in via paracellular pathways in small intestine, leaks out between cells and colon. Passive transport

Question 20

What happens to sodium taken into cells?

Answer 20

Active transport of Na+ into lateral intracellular spaces by Na+/K+ ATPase transport in the lateral plasma membrane

Question 21

How are Cl- and HCO3- transported into the intercellular spaces

Answer 21

due to electical potential created by Na+ transport

Question 22

How does water enter the gut cells?

Answer 22

Osmotic flow from gut lumen via adjacent cells, tight junctions into the intercellular space. Water distends the intercellular channels and causes increased hydrostatic pressure. Ions and water move across the basement membrane of epithelium and carried away by capillaries

Question 23

What parts of the small intestine absorb Ca2+

Answer 23

Duodenum and Ileum

Question 24

What stimulates Ca2+ absorption?

Answer 24

Vit D and parathyroid hormone

Question 25

What does a Ca2+ deficient diet do?

Answer 25

Increases gut ability to absorb

Question 26

What is the intracellular fluid [Ca2+]

Answer 26

approx 100nM - low

Question 27

What is the extracellular fluid [Ca2+] | What conc in plasma or lumen

Answer 27

approx 1-3mM Plasma [Ca2+] 2.2-2.6 mM Luminal varies

Question 28

How is Ca2+ carried across the apical membrane

Answer 28

1. Intestinal calcium binding protein (IMcal- facillitated diffusion) 2. Ion channel

Question 29

What is Ca2+ used for in the body?

Answer 29

Intracellular signalling molecule

Question 30

What happens to calcium in cytosol and what does that do?

Answer 30

Binds to calbindin which prevents its action as an intracellular signal

Question 31

How is Ca2+ pumped across the basolateral membrane?

Answer 31

Ca2+ ATPase (PMCA) against conc gradient | Na+/Ca2+ exchanger against conc gradient

Question 32

How does PMCA maintain low conc Ca2+ in cell

Answer 32

it has high affinity Ca2+ but low capactiy so maintains the very low conc of calcium normally observed in cell

Question 33

How does the Na+/Ca2+ exchanger work transporting Ca2+?

Answer 33

Low affinity for Ca2+ but high capacity so requires larger conc of Ca2+ to be effective

Question 34

What does a deficiency of Vit D do?

Answer 34

Cause rickets, osteoporosis

Question 35

What is Vit D essential for?

Answer 35

Normal Ca2+ absorption

Question 36

What does 1,25-dihydroxy D3 do when taken up by enterocytes?

Answer 36

Enhances transport of Ca2+ through cytosol Increases levels of calbindin Increases rate of extrusion across basolateral membrane by increasing level of Ca2+ ATPase in the membrane

Question 37

What is iron used as in the body?

Answer 37

Electron donor and acceptor

Question 38

What processes in the body are iron critical for?

Answer 38

``` Oxygen transport RBC Oxidative phosphorylation (mitochondria transport chain) ```

Question 39

How much iron does an adult ingest and how much do they absorb?

Answer 39

Ingest 15-20mg/day | Absorb 0.5-1mg/day

Question 40

What does ion present as in the diet?

Answer 40

Inorganic iron eg Fe3+ ferric, Fe2+ ferous | Part of heme/haem group in haemoglobin, myoglobin and cytochromes

Question 41

Which form of inorganic iron can the body absorb?

Answer 41

only Fe2+, can't absorb Fe3+

Question 42

What reduces Fe3+ to Fe2+? What catalyses this reaction?

Answer 42

Vit C | Catalysed by duodenal cytochrome B (dcytb)

Question 43

How much of heme/haem is absorbed?

Answer 43

20% of presented instead of 5% of rest of iron

Question 44

How is heme/haem absorbed into enterocyte?

Answer 44

Heme carrier protein HCP-1 via receptor mediated endocytosis

Question 45

How is Fe2+ liberated?

Answer 45

Heme oxygenase

Question 46

How is Fe2+ transported into enterocyte?

Answer 46

Divalent metal transporter 1 (DMT-1) which is a H+ coupled co-transporter

Question 47

How does Fe2+ move into blood across basolateral membrane?

Answer 47

Via ferroportin ion channel

Question 48

What converts Fe2+ to Fe3+?

Answer 48

Transmembrane copper dependent ferroxidase

Question 49

What does Fe3+ bind to? How does it travel in blood

Answer 49

Bind to apotransferrin | Travels in blood as transferrin

Question 50

What is a major iron regulating protein? What does it do?

Answer 50

Hepcidin | Supresses ferroportin function to decrease iron absorption

Question 51

How else can iron transport in the body? (hint: micelle)

Answer 51

Bind to apoferritin in cytosol to form ferritin micelle

Question 52

What is ferritin?

Answer 52

Globular protein complex which oxidises Fe2+ to Fe3+ which crystallises within protein shell

Question 53

What happens when too much iron absorbed?

Answer 53

Irreversible binding of iron to ferritin in epithelial cells Iron/Ferritin not available for transport into plasma Iron/Ferritin lost in intestinal lumen and extreted in faeces Increase in iron conc in cytosol increases ferritin synthesis

Question 54

What are vitamins?

Answer 54

Organic compounds that cannot be manufactured by the body but vital to metabolism

Question 55

What is the predominant mechanism to take up vitamins in the body?

Answer 55

Passive diffusion

Question 56

What are the fat soluble vitamins

Answer 56

A, D, E, K

Question 57

How are the fat soluble vitamins taken up in the body?

Answer 57

Transported to brush border in micelles. K taken up by active transport

Question 58

What vitamins need specific transport mechanisms?

Answer 58

Vit C (ascorbic acid) Folic acid Vit B1 (thiamine) Vit B12

Question 59

Where is a store of Vit B12 found? What happens if there is impaired absorption of Vit B12

Answer 59

Liver | Retards maturation of red blood cells - pernicious anaemia

Question 60

How is denaturation of vit B12 avoided in the stomach by HCl?

Answer 60

Binds to R protein (haptocorrin) released in saliva and from parietal cells. R proteins are digested in duodenum

Question 61

What is the intrinsic factor IF?

Answer 61

Glycoprotein that binds to VitB12, makes it resistant to digestion. No IF then no absorption of vit B12

Question 62

What does the vitB12/IF complex do?

Answer 62

Binds to cubilin receptor, taken up in distal ileum - receptor mediated endocytosis

Question 63

What happens to vitB12 once it is in the cel

Answer 63

The VitB12/IF complex broken down, possibly in mitochondria. B12 bind to protein transcobalamin II (TCII), crosses basolaterla membrane by unknown mechanism. Travels to liver bound to TCII, TCII receptor on cels allow them to uptake complex and proteolysis breaks down TCII inside cells

Question 64

How much water enters gastrointestinal tract a day?

Answer 64

7.5L due to various secretions

Question 65

How much of the H2O in the GI tract is absorbed?

Answer 65

99%