Nutrient Absorption

Question 1

salivary enzymes

Answer 1

-amylase and lingual lipase

Question 2

stomach enzymes

Answer 2

• gastric chief cells secrete pepsinogen-pepsin at low pH
• endopeptidase for aromatic L aa
• optimum pH 1-3, inactive at 5
• gastric lipase

Question 3

pancreatic enzymes

Answer 3

• amylase
• endopeptidases-trypsin, chymotrypsin, elastase
• exopeptidases (carboxypeptidase)
• lipase/colipase
• phospholipase A2
• cholesterol esterase

Question 4

intestinal extoenzymes

Answer 4

• membrane bound in brush border, catalytic site faces lumen
• enterokinase-activates trypsin
• disaccharideases
• peptidases

Question 5

activation of pancreatic proteolytic enzymes

Answer 5

• enterokinase activates trypsin from trypsinogen
• trypsin then activates chymotrypsin, eslastase, and carboxypeptidase A and B
• the carboxy are exopeptidases, others are endo

Question 6

protein digestion and absorption of peptides/aa

Answer 6

• proteins yield 40% aa and 60% oligopeptides (2-6)
• single aa carried across via a Na cotransporter
• up to tetra peptides-cleaved by brush border peptidases or pumped across with a H+ cotransporter
• then cleaved by teta/tri/di peptidases inside cell and diffuse out into cap
• newborns can absorb globulins and other whole proteins, allergies to some foods can be whole protein absorption

Question 7

sterospecific absorption

Answer 7

• more L form absorbed that D form
• absorption is sterospecific
• therefore it’s not just diffusion

Question 8

uptake by hamster intestine

Answer 8

• absorption is against a gradient

- everted sacs and aa concentrated against gradient

Question 9

Na coupled aa transport

Answer 9

• aa require inward sodium gradient for (out to in) concentrative uptake
• increasing Na led to increased aa uptake
• pump Na out and K in on baso-lateral
• Na and aa co-transport in on apical membrane

Question 10

transport kinetics

Answer 10

• vmax and km
• the transporters in the membrane become saturated
• MM kinetics
• free diffusion would be linear

Question 11

brush border membrane

Answer 11

• see table- too hard to compact here

- slide 9

Question 12

H/oligo transporter

Answer 12

• PepT1

- di/tri/tetra peptides into enterocyte

Question 13

hartnup disease

Answer 13

• can’t uptake phenylalanine
• can take phenyylalanyl and leucine, and arginine and cysteine
• system B is defective
• high tryptophan in urine because affects kidney too

Question 14

cystinuria

Answer 14

• system B0+ affected
• can’t uptake arginine or cysteine
• basic aa absorption reduced
• forms kidney stones

Question 15

digestion and absorption of sugars

Answer 15

• brush border extoenzymes convert maltose (glu-glu), lactose (glu-gal) and sucrose (glu-fru) to monosaccharides
• carbs absorbed as simple sugars
• Glu and gal compete for same Na coupled carrier
• fructose is independent of Na and cannot be concentrated in cell
• ingested carb is 60% starch, 30% sucrose, 10% lactose
• salivary amylase begins digestion
• ptyalin has pH optimum of 6.7 and is inactivated in stomach
• most starch broken down in intestine by pancreatic amylase
• transport is facilitated diffusion of Na coupled
• SGLT1 is Na coupled transporter for glu or gal
• GLUT 5 mediates diffusion of fructose
• GLUT2 mediates diffusion out

Question 16

undigested fiber

Answer 16

• no cellulase for digesting cellulose of hemi-cellulose

- maintains consistency of stool

Question 17

lactose intolerance

Answer 17

• lactose is osmotically active, osmotic diarrhea
• lactaid is milk containing lactase
• lactose tolerance test- give oral load, measure blood glu
• lactose deficiency will have more H2 breathed out and won’t ingest as much lactose, less lactose to glucose
• bacteria in gut ferment lactose and make lots of gas

Question 18

glucose/galactose malabsorption

Answer 18

• rare genetic disease
• SGLT1 carriers are missing or defective
• accumulation of glu in intestine causes diarrhea, dehydration, and death
• patients die soon after birth if diarrhea isn’t diagnosed
• restricted diet of fructose

Question 19

fats

Answer 19

• neutral fat
• phospholipids
• cholesterol
• fatty acids
• waxes of ingested plant cell walls

Question 20

CCK

Answer 20

• fats cause release
• GIP decreases gastric acid secretion
• CCK:
• slows gastric motility and emptying, stimulates pancreatic enzyme secretion
• stimulates intestinal fluid secretion
• stimulates gallbladder contraction
• relaxes sphincter of Oddi

Question 21

digestion of fats

Answer 21

• starts with lingual lipase
• continues with gastric lipase and food-bearing lipase with acidic pH optima
• digestion occurs mostly in the jejunum and is completed by the mid jejunum via:
• pancreatic phospholipase A2, cholesterol esterase, pancreatic lipase

Question 22

fat globules and micelles

Answer 22

• micelles have long chain FA
• cholesterol
• monoglycerides
• phospholipids
• bile salts
• fat soluble vitamins
• mixed with amphipathic bile salts
• lipases hydrolyze TAGs at surface, core replaces them and they shrink

Question 23

absorption of neutral fat

Answer 23

• mixed micelles carry major part of lipids, once they diffuse, emulsifying more lipids recycles empty bile micelle, bile salt doesn’t diffuse in
• FA with short chain are more hydrophilic, diffuse directly
• for long chain-need FATP takes them to sER to re-esterification
• NPC1L1 is a transporter for cholesterol
• bile salts not absorbed in jejunum, in ileum into portal system for recycling- can be reused even in a single meal

Question 24

formation of chylomicrons

Answer 24

1. LCFA and other products of lipid digestion are converted back to triglycerides, phospholipids, and esters of cholesterol in SER
2. fat droplets form in the cisternae of the SER
3. apoproteins are synthesized in RER and transferred to the SER, where they associated with lipid droplets
4. nascent chylos and VLDLs arrive at the cis face of the Golgi apparatus, apoproteins are glycosylated
5. vesicles carrying chylos or VLDLs bud off from the trans golgi, move to the basolateral membrane in transport vesicles
6. vesicles fuse with the basolateral membrane and release chylos or VLDLs
7. chylos and VLDLs pass through large interendothelial channels of lymphatic capillaries and enter lymph
8. glycerol, SCFA, MCFA pass through enterocyte and enter blood cap