Digestion & Absorption Flashcards
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
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
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
*
- cannot be digested
Carbohydrate substrate/Brush-border enzyme/Product
- a-Limit dextrin ==Sucrase-isomaltase (SI) breaks 1,6 linkages==> Glucose
- Maltotriose ==Maltase-Glucoamylase (MGA)==> Glucose
- Lactose==Lactase==>Glucose
- Sucrose==Sucrase==> Glucose
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}
Characteristics of lactose intolerance
- caused by absence of brush border enzyme lactase
- unabsorbes lactose ==> water into intestinal lumen ==> osmotic diarrhea
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
Mechanism of protein digestion @ stomach
- chief cells secrete pepsinogen
- @ pH 1 - 3: pepsinogen ==> pepsin
Mechanism of protein digestion @ small intestine
- trypsinogen ==> trypsin via brush border protease enterokinase
- Activation of all other precursors by trypsin
- Trypsin, chymotrypsin, elastase, carboxypeptidase A & B all hydrolyze protein to amino acids and small polypeptides
- The brush border proteases hydrolyze oligopeptides to amino acids
- Pancreatic proteases digest themselves and each other
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
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.
Major pancreatic proteases
- **secreted as precursors
- Trypsinogen
- Chymotrypsinogen
- Pro-elastase
- Pro-carboxypeptldase A
- Pro-carboxypeptidase B
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
Mechanism of fat digestion
- major dietary lipid = triglyceride
- insoluble in water; cannot be efficiently absorbed
- fat droplets emulsified by bile salts and lecithin to form smaller particles ==>
- increased SA for digestion by lipase and colipase (colipase helps ancor lipase to drops)
- triglycerides ==> monoglycerides + fatty acids
- after lipase digestion; products are solubilized in bile-salt micelles
- w/hydrophillic groups contacting outside and hydrophobic groups contacting inside
Mechanism of fat absorption
- Micelles move towards enterocytes/brush border
- lipids eventually contact cell membrane ==> diffuse passively into cell
- @ cytoplasm: triglycerides resynthesized
- triglycerides repackaged into lipoprotein particles = “chylomicrons”
- also contain phospholipids, cholesterol, and apolipoproteins
- chylomicrons packaged into secretory vesicles @ golgi ==> exit via exocytosis ==> lacteals
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
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
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
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 H2Oabsorption
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
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