Digestion and absorption physiology

Question 1

Absorption of carbohydrates 1

Answer 1

• All are absorbed as monomers, which must pass thru the glycocalyx and mucin “unstirred layer” barrier
• Most carbs are in the form of starch which must be broken up into smaller units
• Amylase (part of the juices)breaks starch into many parts: maltotriose, maltose, maltotetrose, and a-limit dextrin

Question 2

Absorption of carbohydrates 2

Answer 2

• a-limit dextrins are broken down by isomaltase nz (on the apical membrane surface) to maltose, maltotriose, maltotetrose
• Maltose, maltotetrose, maltotriose are broken down to glucose by maltase (also on apical membrane surface)
• Thus there are 2 phases for carb absorption: luminal and surface
• Cellulose cannot be broken down by our cells b/c we lack cellulase

Question 3

Osmotic consequences of starch breakdown

Answer 3

• When starch is broken down into many smaller units it has a larger osmotic effect
• There are means of regulating this effect to prevent substantial water loss
• Gastric emptying is regulated
• Release of most osmotically active subunits occurs near the absorptive membrane (for fast absorption into circulation)
• Rapid transport into cells from circulation (insulin)
• Rapid conversion to glycogen or lipids (both have low osmotic impact)

Question 4

Mechanism of glucose absorption 1

Answer 4

• SGLUTs (sodium-dependent GLUTs) on the apical membrane of the SI cells transport glucose into the cell based on the Na electrochemical gradient
• Na/K ATPase on the basal surface maintain the low Na concentration inside the cell and help maintain the membrane potential (-60) of the cell
• Leaky K channels recycle the K pumped into the cell by Na/K ATPase and help maintain the membrane potential

Question 5

Mechanism of glucose absorption 2

Answer 5

• There is paracellular leakage of Cl and Na from ISF to lumen, which keeps the apical membrane potential (-57) close to the basal membrane potential (-60)
• This gives the largest voltage difference across the apical membrane and thus more efficiently moves glucose from the Na electrochemical gradient
• Facilitated diffusion (channels) of glucose across the basal membrane keeps cytosolic [glucose] close to plasma [glucose]
• Fructose absorption: relies entirely on facilitated diffusion transporters both on apical and basal membranes

Question 6

Amino acid absorption

Answer 6

• Pancreatic proteases (trypsin, chymotrypsin, elastase, carboxypeptidase) break proteins into oligopeptides, dipeptides, and free AAs in the lumen
• Oligopeptides are broken down by aminooligopeptidases on the apical membrane
• Dipeptides are broken down by dipeptidases on the apical membrane
• Single amino acids are absorbed in a similar fashion to glucose: utilizing an Na cotransporter
• Once in the cell, the amino acids exit the basal membrane via facilitated diffusion

Question 7

Dipeptide absorption

Answer 7

• Dipeptides can also be absorbed, but they are absorbed based on the H+ electrochemical gradient (H+ cotransporter)
• Dipeptide + H+ enter the cell via cotransporter and the H+ is moved back out into the lumen via NHE (Na-H+ anti porter)
• Thus dipeptide absorption is directly linked to H+ absorption, however it is indirectly controlled by Na gradient (more Na movement by NHE will mean a higher H+ gradient for the dipeptides)
• Once in the cell the dipeptides are broken down to single AAs by cytosolic peptidases and transported via facilitated diffusion out the basal membrane

Question 8

AA malabsorption syndromes

Answer 8

• Prolinuria: inability to absorb glycine, proline, and hydroxyproline
• Problem w/ Na-iminoacid cotransporter
• Cystinuria: inability to absorb cystine, lysine, and arginine
• These are amino acidurias b/c nephrons use the same cotransporters to recover filtered AAs

Question 9

Absorption of fat 1

Answer 9

• Gastric mixing emulsifies lipids but droplets are too large to penetrate unstirred layer
• Lipases break up TAGs into 2 FFA and a monoglyceride (no glycerol released)
• Colipase anchors lipase to droplet surface and allows it to continuously break down TAGs and reduce the size of the lipid droplets (accelerating the action of lipase by increasing surface area of droplets)

Question 10

Absorption of fat 2

Answer 10

• Bile salts (planar molecules w/ hydrophilic and hydrophobic surfaces) combine to form a micelle (tube w/ hydrophobic surface in middle)
• FFAs and 2-monoglycerides will be taken up by the micelle (sit in the center of the tube), and the micelles will diffuse thru the unstirred layer
• At the apical membrane the FFAs and 2-monoglycerides leave the micelle center and diffuse across the membrane into the cell

Question 11

Formation of chylomicron

Answer 11

• The FFAs and 2MGs in the enterocyte migrate to the ER where they are re-esterified to TAGs
• Apolipoprotein is synthesized (to stabilize the lipid droplets) and the TAGs/apolipoprotein are packaged into chylomicrons
• Transport vesicles containing chylomicrons bud from the ER and travel to the basal membrane and dump the chylomicrons into lymph via exocytosis
• Bile salts are not reabsorbed until terminal ileum; where they return to the liver via hepatic portal vein for recycling