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DEMS: Unit 1 > Digestion & Absorption > Flashcards

Flashcards in Digestion & Absorption Deck (21):
1

General characteristics of digestion and absorption

  • 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

2

Enzymatic role in digestion (general)

  • 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 

3

Mechanism of carbohydrate digestion

  • 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
  •  

4

Carbohydrate substrate/Brush-border enzyme/Product

  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

5

Absorption of carbohydrates

  • 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}

6

Characteristics of lactose intolerance

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

7

Mechanism of protein digestion (general)

  • 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

8

Mechanism of protein digestion @ stomach

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

9

Mechanism of protein digestion @ small intestine

  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

10

Endopeptidases vs. Exopeptidases

  • 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

11

Characteristics of brush border proteases

  • 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.

12

Major pancreatic proteases

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

13

Mechanism of protein absorption

  • 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

14

Mechanism of fat digestion

  • 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

15

Mechanism of fat absorption

  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

16

Characteristics of steatorrhea

  • 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

17

Mechanism of absorption of water/electrolytes @ upper small intestine

  • 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

18

Mechanism of absorption of water/electrolytes @ ileum

  • 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

19

Mechanism of absorption of water/electrolytes @ colon

  • no sugars/amino acids absorbed
  • Na absorption via apical Na+ channels (epithelial sodium channel, ENaC)
    • Aldosterone ==> increased Na+ absorption (+ increased K+ secretion) ==> increased H2O absorption

20

Mechanism of absorption of water-soluble vitamins

  • 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

21

Water soluble vitamins vs. fat soluble vitamins

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