Week 1: Physio -Digestion and Absorption

Question 1

carbohydrate digestion

Answer 1

1. lumenal hydrolysis: a-amylase from pancreas breaks down starch (amylopectin and amylose) to a-limit dextrin, maltose, and maltotetrose
   - cellulose not digested because of beta(1-4) glycosidic bonds
2. surface hydrolysis: maltase hydrolyzes maltotetrose, maltose, and maltotriose to glucose. isomaltase breaks down a-limit dextrins
   - process so that not all carbs hydrolyzed to glucose in lumen-osmotic pressure

Question 2

Osmotic impact of glucose

Answer 2

• Starch has little impact, only draws 3.6mL of H2O/day
• individual glucose molecules can potentially draw 3.6L/day
• osmotic impact of glucose is minimized via: regulating gastric emptying, release of osmotic active subunits near absorptive membrane, transport glucose into cells rapidly, in cells-glucose converted to glycogen and lipids

Question 3

what drive glucose absorption in intestines?

Answer 3

• from electrochemical potential between cytosol and gut lumen of Na+
• Net deltaG(Na)= -12,300J/mole
• Na/K ATPase pump on basolateral side
• Na/glucose co transporter on apical side
• glucose in cell=glucose in interstitial fluid via facilitated diffusion
• intestinal epithelium highly permeable to NaCl, allows NaCl to leak back to lumen paracellularly. Means keeps lumen voltage close to interstitial fluid->largest possible voltage difference across apical membrane
• colon: no paracellular transport of NaCl, means less of a voltage drop between lumen and cell. More glucose stays in colon, which means more water in colon.

Question 4

Protein digestion in intestines

Answer 4

• proteins broken down by pancreatic proteases (trypsin, chymotrypsin, elastase, carboxypeptidase A & B) to oligopeptides and free amino acids
• membrane bound dipeptidases and aminooligopeptidases break them down further
• Na+/amino acid cotransporters bring into cell
• there are different cotransporters for basic, acidic, aromatic/aliphatic, and L,a-amino acids
• dipeptides: can also enter cell through dipeptide/H+ cotransporter which is Na+ dependent. H+ is recycled out via NHE

Question 5

Prolinuria and Na/Iminoacid Cotransporter

Answer 5

• prolinuria is malabsorption of the following amino acids
• transports glycine, proline, hydroxyproline
• coin shaped binding pocket
• alanine wouldn’t fit because of rotation

Question 6

Cystinuria

Answer 6

malabsorption of cystine, lysine, arginine

-transporter is able to transport lysine, cysteine, and cystine (dimer of cysteine)

Question 7

Digestion of lipids

Answer 7

1. gastic mixing emulsifies lipids
   - small intestinal mixing maintains emulsions, but droplets too big to penetrate unstirred layer (mucin)
2. pancreatic and lingual lipase hydrolyze TGs to 2-monoglyceride and 2 FFAs
   - colipase helps anchor lipase to surface of lipid droplet
   - as more FFAs made, droplets get smaller and H2O interface area increases. Larger surface area means more collapse and lipase. More hydrolysis
3. Bile salts form micelles-barrels
   - FFAs and 2-monoglycerides partition between lipid droplets, H2O, and micellar phase
   - micelles can permeate unstirred layer. FFAs and 2-monoglycerides released into diffuse into cell

Question 8

Digestion of lipids- continued

Answer 8

4. Inside cell, FFAs and 2MG re-esterified to TGs so are trapped and won’t diffuse back out enterocytes
   - Apolipoprotein and TGs form chylomicron, which bud from ER and fuse with enterocyte basolateral membrane to exocytose
   - enter lacteals to circulation
5. bile salts move to ileum to be reabsorbed. return to liver via haptic portal vein. Stored in gallbladder or released to duodenum