Small Intestine Digestion and Absorption of Lipids and other nutrients

Question 1

fat soluble vitamines

Answer 1

A, D, E, K

Question 2

Triglyceride subunits

Answer 2

glycerol and fatty acids…

phospholipids have the same subunits

Question 3

non polar side of bile salts

Answer 3

emulsify the fats

Question 4

polar side of the bile salts

Answer 4

promotes water solubility of the fat droplets

Question 5

bile salts and phospholipids do what

Answer 5

convert large fat globules into smaller pieces with polar surfaces that inhibit reaggregation

increases surface area

now the lipases can gain access to degrade the TGs to monoglycerides and FAs which enter the absorptive cells by simple diffusion or aggregate to form loosely held micelles which readily break down

Question 6

lingual and gastric lipase

Answer 6

resistant to pepsin

inactivated by pancreatic lipases in the SI

15% of fat digestion

Question 7

CCK

Answer 7

secretion is stimulated by the entrance of fat into the duodenum

stimulates the flow of bile into the intestine (sphincter of Oddi relaxes)

also stimulates secretion of pancreatic lipases

Question 8

glycerol ester hydrolase (pancreatic lipase)

Answer 8

operates only at the oil water interface of the TG droplet

surface emulsifiers (phospholipids and proteins) present at the surface inhibit its action

bile salt micelles also inhibit it

TG –> 2-monoglyceride _ fFAs

Question 9

colipase

Answer 9

amphipatic protein secreted by pancreas

reverses the inhibition mechanism for pancreatic lipase by anchoring it to the lipid droplet

Question 10

cholesterol ester hydrolase

Answer 10

active against a wide range of esters (pancreatic)

cholesterol ester –> cholesterol

Question 11

phospholipase A2 (PLA 2)

Answer 11

active against glycerophospholipids (lecithin) to make lysophospholipids (lysolechitin)

Question 12

micelles

Answer 12

3-10nm in diameter

formed by bile acdis with the products of fat digestion
- especially 2MG, FAs, cholesterol, and lysolechitin

surface is mostly covered in bile acids

interior = hydrophobic materials like ADEK, FAs, cholesterol

DO NOT contain TGs

Question 13

critical micelle concentration

Answer 13

minimal concentration of bile acids needed for micelles to form

Question 14

mixed micelles of bile acids and lipid digestion products

absorption

Answer 14

diffuse through the unstirred layer and among the microvilli

Question 15

as digestion products are absorbed from free solution

Answer 15

more digestion products partition out of the micelles while its in the unstirred layer

Question 16

how much of the brush border surface is available for lipid absorption

Answer 16

the whole surface

Question 17

lipid resynthesis after absorption

Answer 17

occurs in the SER of the intestinal epithelial cells

then accumulate into chylomicrons –> exocytosis

Question 18

chylomicrons fate

Answer 18

enter lacteals, leave the intestine in the lymph,

enter the thoracic duct

enter circulation left subclavian vein

Question 19

chylomicron components

Answer 19

2MG, fFAs, TGs, LysoPL, PL, Chol, CholE

fat vitamins

Question 20

rate limiting step for lipid absorption

Answer 20

micelle diffusion through the unstirred layer

Question 21

pH of fluid immediately in contact with the brush border…why important

Answer 21

1 pH unti lower than the luminal fluid

helps protonate the FAs -COO- end

makes them more lipid soluble to cross the membrane

Question 22

FAs, 2MG, chol, and lysolechitin absorption

Answer 22

diffuse across the apical membrane

Question 23

cholesterol esterase

Answer 23

bound to luminal membrane

promotes uptake of cholesterol across the membrane

Question 24

microvillus membrane FA binding protein (MVM-FABP)

Answer 24

specific protein that uses Na+ gradient to transport long chain FAs into the cell

Question 25

how much of cholesterol absorbed is pumped back into the lumen? how?

Answer 25

50% ABC transport protein (ATP binding cassette) present in the intestinal cell

Question 26

I-FABP & L-FABP SCP1 & SCP2

Answer 26

fatty acid and sterol binding proteins within the cytosol important to prevent formation of oil droplets in the cytosol and transport these products to the SER

Question 27

phospholipid resynthesis in cytosol

Answer 27

lysophospholipid + FA in SER

Question 28

lipid droplets in the SER after resynthesis of lipids and some new synthesis of lipids

Answer 28

prechylomicrons surface covered with phospholipids (polar groups out) they are now transported to the Golgi

Question 29

chylomicron processing

Answer 29

in the golgi 60-750 nm

Question 30

where are bile salts reabsorbed

Answer 30

terminal ileum cross via secondary active transport (co transported with Na+) and by simple diffusion

Question 31

water soluble vitamins absorbtion

Answer 31

diffusion or mediated transport exception = B12 (very large charged molecule)

Question 32

vitamin C (ascorbic acid)

Answer 32

Ileum active cotransport with Na+ essential for oxidation reactions

Question 33

folic acid

Answer 33

jejunum facilitated diffusion nucleic acid synthesis, promotes growth and maturation of RBCs

Question 34

vitamin B6 (pyridoxine)

Answer 34

jejunum simple diffusion acts as coenzyme in reactions of amino acid and protein metabolism

Question 35

vitamin B2 (riboflavin)

Answer 35

jejunum facilitated diffusion constituent of flavin nucleotide coenzymes

Question 36

vitamin B1 (thiamine)

Answer 36

jejunum co transport with Na+ metabolism of carbs and some amino acids

Question 37

vitamin B12 (cobalamine)

Answer 37

ileum receptor mediated endocytosis essential for DNA synthesis; promotes growth and maturation of RBCs

Question 38

Niacin (nicotinamide)

Answer 38

jejunum active transport coenzyme in oxidation reactions

Question 39

any vitamin absorption in duodenum?

Answer 39

NO

Question 40

intrinsic factor

Answer 40

glycoprotein secreted by parietal cells in stomach

Question 41

vitamin B12 metabolsim

Answer 41

1. released from ingested proteins by pepsin and acid 2. rapidly bound by a gastric vitamin B12 binding protein (haptocorrin) --> higher affinity for B12 at acidic pH than intrinsic factor 3. in SI, pancreatic proteases digest the binding proteins....then IF binds to it 4. receptors in ileal mucosa for IF --> absorption

Question 42

Ca2+ absorption steps

Answer 42

1. cross apical border via Ca2+ channel 2. in cytosol --> binds to calbindin (CaBP) 3. intracellular vesicles take up Ca2+ and transport it through the cytosol 4. released across basolateral by exocytosis or. ... 5. Ca2+ bound to Calbindin diffusion out via Ca2+ ATPase or Na+/Ca2+ exchanger **this is limited to the duodenum....everywhere else its paracellular diffusion

Question 43

vitamin D and Ca2+ absorption

Answer 43

facilitates active Ca2+ absorption promotes formation of calbindin also enhances all 3 pathways of Ca2+ pathways

Question 44

Ca2+ absorption in stomach

Answer 44

acidic environment solubulizes Ca2+ and its diffused into cells

Question 45

Ca2+ in duodunem (pH)

Answer 45

vitamin C (ascorbic acid) increases acidity of intestine --> enhances absorption

Question 46

calcium citrate (citric acid)

Answer 46

most effective absorption form of Ca2+

Question 47

conditions where iron complexes are most soluble

Answer 47

low pH presence of vitamin C

Question 48

how heme form of iron crosses apical membrane

Answer 48

facilitated diffusion then in cytosol....heme oxidase (heme --> Fe2+)

Question 49

how non heme Fe3+ crosses apical membrane

Answer 49

apical membrane iron reductase Fe3+ --> Fe2+ in lumen then... co transported with H+ by a divalent cation transporter (DCT1) in the brush border membrane

Question 50

fate of Fe2+ in the cytosol

Answer 50

oxidized to Fe3+ and binds to ferritin

Question 51

how Fe3+ (bound to ferritin) crosses the basolateral membrane

Answer 51

IREG1 and hephaestin

Question 52

in the blood....Fe3+ binds to what

Answer 52

transferrin....cells that take up iron have receptors for transferrin

Question 53

iron bound to ferritin in lumen of intestine

Answer 53

cannot be absorbed and lost in feces

Question 54

iron basolateral receptors in crypt cells

Answer 54

receptors for transferrin-iron complex

Question 55

when blood levels are high Fe3+ binds

Answer 55

iron regulatory protein (IRP)

Question 56

when iron is deficient (anemia)

Answer 56

IRP increase the translation of mRNA encoding the iron transport proteins DCT1 and IREG1

Question 57

relative absorption sites for carbs, lipids, and proteins

Answer 57

most --> least duodenum --> jejunum --> ileum

Question 58

relative absorption sites for Ca2+, iron, and folate

Answer 58

all three = duodenum Ca2+ then has some in jejunum and ileum as well

Question 59

relative absorption sites for bile acids

Answer 59

little in duodenum moderate in terminal jejunum high in ileum moderate in ascending colon