Digestion & Absorption

Question 1

General characteristics of digestion and absorption

Answer 1

• blood ==> villi via arteriole ==> capillaries beneath epithelial cells
  
  • villus also has venule and lacteal (lymphatic vessel)
• nutrients ==> venule
• fat products ==> lacteal
• huge SA of intenstinal epithelium aids in absorptions
  
  • plicae circulares
  • villi
  • microvilli

Question 2

Enzymatic role in digestion (general)

Answer 2

• proteolytic enzymes/other pancreatic enzymes are secreted as zymogens or pro-enzymes
  
  • @ stomach: pepsinogen ==> pepsin via stomach acid
  • @ duodenum:
    
    • trypsinogen ==> trypsin via brush border enteropeptidase
    • other pro-enzymes ==> active form via trypsin
• enzymes @ saliva, stomach, upper small intestine ==> convert nutrients to small polymers
• enzymes @ brush border ==> convert polymers to monomers

Question 3

Mechanism of carbohydrate digestion

Answer 3

• plant starch = glucose polymer w/a-1,4 and a-1,6 linkages
  
  • amylase hydrolyzes a-1.4 links==> maltose and a-Dextrins
  • free glucose is never product of amylase digestion
  • brush border enzymes convert polysaccharides to monomers
    
    • SI (sucrase-isomaltase)
    • MGA (maltase-glucoamylase)
• cellulose = b-1,4 linked polymer
  
  • cannot be digested
    *

Question 4

Carbohydrate substrate/Brush-border enzyme/Product

Answer 4

1. a-Limit dextrin ==Sucrase-isomaltase (SI) breaks 1,6 linkages==> Glucose
2. Maltotriose ==Maltase-Glucoamylase (MGA)==> Glucose
3. Lactose==Lactase==>Glucose
4. Sucrose==Sucrase==> Glucose

Question 5

Absorption of carbohydrates

Answer 5

• Na⁺-dependent glucose transporter (SGLT1)
  
  • located @ brush border
  • transports glucose and galactose (w/Na⁺) from lumen ==> cytosol [apical]
• Na⁺-independent fructose transporter (GLUT5)
  
  • facilitative glucose transporter
  • fructose from lumen ==> cytosol [apical]
• Na⁺-independent fructose transporter (GLUT2)
  
  • transports all three monosaccharides from cytosol ==> blood [basolateral}

Question 6

Characteristics of lactose intolerance

Answer 6

• caused by absence of brush border enzyme lactase
• unabsorbes lactose ==> water into intestinal lumen ==> osmotic diarrhea

Question 7

Mechanism of protein digestion (general)

Answer 7

• protein breakdown occurs between stomach and small intestine via:
  
  • gastric, pancreatic, enterocyte brush-border and cytoplasmic peptidases
• categories of enzymes: endopeptidases, exopeptidases, brush-border proteases
  
  • also: gastric vs. pancreatic proteases

Question 8

Mechanism of protein digestion @ stomach

Answer 8

1. chief cells secrete pepsinogen
2. @ pH 1 - 3: pepsinogen ==> pepsin

Question 9

Mechanism of protein digestion @ small intestine

Answer 9

1. trypsinogen ==> trypsin via brush border protease enterokinase
2. Activation of all other precursors by trypsin
3. Trypsin, chymotrypsin, elastase, carboxypeptidase A & B all hydrolyze protein to amino acids and small polypeptides
4. The brush border proteases hydrolyze oligopeptides to amino acids
5. Pancreatic proteases digest themselves and each other

Question 10

Endopeptidases vs. Exopeptidases

Answer 10

• endopeptidase = hydrolyze inner peptide bonds
  
  • Pepsin - aromatic amino acids
  • Trypsin - arginine (R) and lysine (K)
  • Chymotrypsin - aromatic amino acids
  • Elastase - neutral aliphatic amino acids
• exopeptidase = hydrolyze single amino acids from C-terminus
  
  • Carboxypeptidases A
  • Carboxypeptidases B

Question 11

Characteristics of brush border proteases

Answer 11

• Aminopeptidase is an exoprotease that removes one amino acid at a time from the N(amino)-terminus
• Dipeptidyl aminopeptidase removes dipeptides from the N-terminus
• Dipeptidase converts dipeptides to amino acids.

Question 12

Major pancreatic proteases

Answer 12

• **secreted as precursors
• Trypsinogen
• Chymotrypsinogen
• Pro-elastase
• Pro-carboxypeptldase A
• Pro-carboxypeptidase B

Question 13

Mechanism of protein absorption

Answer 13

• apical transport via Na⁺-dependent co-transporter
• di- and tri-peptides absorbed intact
• amino acid carriers =
  
  • neutral
  • basic
  • acidic
  • proline/glycine transporter
• w/in enterocytes, peptides hydrolyzed to amino acid by cytoplasmic peptidases
• amino acids ==> blood via facilitated diffusion @ basolateral membrane

Question 14

Mechanism of fat digestion

Answer 14

• major dietary lipid = triglyceride
  
  • insoluble in water; cannot be efficiently absorbed

1. fat droplets emulsified by bile salts and lecithin to form smaller particles ==>
2. increased SA for digestion by lipase and colipase (colipase helps ancor lipase to drops)
   
   1. triglycerides ==> monoglycerides + fatty acids
3. after lipase digestion; products are solubilized in bile-salt micelles
   
   1. w/hydrophillic groups contacting outside and hydrophobic groups contacting inside

Question 15

Mechanism of fat absorption

Answer 15

1. Micelles move towards enterocytes/brush border
2. lipids eventually contact cell membrane ==> diffuse passively into cell
3. @ cytoplasm: triglycerides resynthesized
4. triglycerides repackaged into lipoprotein particles = “chylomicrons”
   
   1. also contain phospholipids, cholesterol, and apolipoproteins
5. chylomicrons packaged into secretory vesicles @ golgi ==> exit via exocytosis ==> lacteals

Question 16

Characteristics of steatorrhea

Answer 16

• steatorrhea = excess fat loss in stool
  
  • due to inadequate triglyceride digestion
• w/fat malaborption comes fat-soluble vitamin malabsorption
  
  • ==> deficiency @ vit A, D, E, K

Question 17

Mechanism of absorption of water/electrolytes @ upper small intestine

Answer 17

• H₂O flows readily across intestinal epithelium and is absorbed isotonically (follows solute absorption)
• Na⁺ absorbed via co-transport w/nutrients
  
  • Cl^- follows [electrical drive] via paracellular pathway
  • H₂O follows [osmotic drive] via paracellular pathway

Question 18

Mechanism of absorption of water/electrolytes @ ileum

Answer 18

• ileum performs special absorption tasks (bile salts and vitamin B12) and continues to absorb H₂O
• Cl^- absorbed via transcellular pathway
  
  • via Cl^-/HCO₃^- exchange @ apical membrane

Question 19

Mechanism of absorption of water/electrolytes @ colon

Answer 19

• no sugars/amino acids absorbed
• Na absorption via apical Na⁺ channels (epithelial sodium channel, ENaC)
  
  • Aldosterone ==> increased Na⁺ absorption (+ increased K⁺ secretion) ==> increased H₂Oabsorption

Question 20

Mechanism of absorption of water-soluble vitamins

Answer 20

• absorbed either by co-transport with Na+, or by passive diffusion
• absorption is virtually complete in the upper small intestine
• exception = vitamin B12
  
  • absorbed @ distal ileum in a complex with intrinsic factor
  • brush border membrane of ileal enterocytes contains a specific receptor for the B12-IF complex.
  • Impaired absorption of B12 ==> pernicious anemia

Question 21

Water soluble vitamins vs. fat soluble vitamins

Answer 21

• Water soluble vitamins
  
  • B vitamins (B12 + IF)
  • vitamin C
  • niacin
  • folic acid
  • pantothenic acid
  • biotin
• Fat soluble vitamins
  
  • A
  • D
  • E
  • K