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As a ruleof thumb, ∼25% to 30% of digestion occurs preduodenally,and ∼70% to 75% occurs in the duodenum and jejunum.The upper half of the villi is the site of membrane-boundenzymes (brush border enzymes) for final digestion of carbohydrates and proteins. This is also the site of absorption. Thebottom part of the villi form the crypts of Lieberkühn, fromwhich buffers and mucus are secreted, as previously discussedin Chapter 23. Finally, because there is laminar flow throughthe lumen, the slowest movement is near the enterocytes, andthis is slowed even further because of the mucus secreted toprotect the cells. This creates an unstirred water layer thatmolecules must pass through to access the enterocytes.
Thisis no problem for movement of most nutrients but creates aproblem for hydrophobic, lipid-based molecules.CARBOHYDRATE DIGESTION AND ABSORPTIONMuch of the carbohydrate in our diet is in the form of starch,sucrose (table sugar), lactose (milk sugar), and fructose.Starch is a large, branched, long-chain polysaccharide synthesized by plants; it represents a large portion of the carbohydrates consumed in most diets. The glucose moieties withinthe starch molecule are bound by α-1,4 glycosidic linkages,with α-1,6 linkages at branch points. Sucrose and lactose aredisaccharides.
We also ingest large quantities of the monosaccharide fructose, which is present in fruits and many foodsusing “high-fructose corn syrup” as a sweetener.Although we ingest long-chain sugars and oligosaccharides,we can only absorb monosaccharides. Carbohydrate digestionbegins in the mouth and continues through to the small intestine (Fig.
25.2):Starch, glycogen, and cellulose are all polymers ofglucose. Starch is a storage form of glucose in plants andis a major source of energy in our diet. Although animals storeglucose in their muscles in the form of glycogen, the meat weconsume contains little or no glycogen, because it is brokendown after animals are slaughtered.
Cellulose is the structuralcomponent of the cell walls of green plants and has no caloricvalue to humans. Cellulose is plant fiber and is composed ofglucose molecules bound by β-1,4 linkages in large, straightchains. Humans do not have an enzyme that can digest βlinkages, so this “fiber” remains in the gastrointestinal (GI)lumen.
Fibers attract water, loosening stools and adding bulk.High-fiber diets have other health-related advantages, includinga blood cholesterol–lowering effect and enhanced glucose tolerance (antidiabetogenic effect). In contrast to other mammals,ruminants such as cows are able to digest cellulose due to thepresence of symbiotic prokaryotic organisms in their GItracts.■■■Mouth: Salivary α-amylase begins starch digestion,breaking α-1,4 linkages and creating smaller oligosaccharides (maltose and isomaltose, both disaccharidesconsisting of two linked glucose molecules, as well aslarger oligosaccharides and polysaccharides). It is inactivated by the acid pH in the stomach.Small intestine: Pancreatic α-amylase continues thedigestion, forming more maltose and isomaltose.Small intestine brush border: As the chyme contacts thevillous brush border, specific brush border saccharidases digest the maltose, isomaltose, sucrose, and lactoseto their constitutive monosaccharides (Table 25.1).In the small intestine, glucose and galactose are transportedinto the enterocytes with sodium via SGLT-1 transporters(secondary active transport); fructose has its own transportprotein, GLUT-5 (through facilitated transport).
The monosaccharides leave the basolateral membranes by facilitatedtransport through GLUT-2, diffuse through the interstitialspace into the capillary, and are transported through theportal vein to the liver for processing and release into thesystemic circulation. Carbohydrates are efficiently digestedand quickly absorbed.Gastrointestinal PhysiologyValve ofKerckring292EpitheliumVillusLamina propriaLymph noduleCrypt of LieberkühnMuscularis mucosaeSubmucosaCircular muscleLongitudinal muscleSerosaJejunum (low power)EpitheliumVillusLamina propriaCrypt of LieberkühnMuscularis mucosaeAggregated lymph noduleSubmucosaCircular muscleLongitudinal muscleIleum (low power)SerosaFigure 25.1 Small Intestine Surface Area The small intestine has circular folds, villi, and microvilli(hairlike projections on villi), which increase the surface area for absorption to about 600 times the area ofa straight tube. This allows efficient absorption of nutrients.
The upper half of the villi are the site of thebrush border saccharidases and proteases (for final digestion of carbohydrates and proteins), and the upperhalf is also where absorption occurs.Table 25.1Brush Border Saccharidasesand ProductsBrush Border EnzymeSubstrateProduct(s)MaltaseMaltoseGlucoseIsomaltaseIsomaltoseGlucoseSucraseSucroseGlucose + FructoseLactaseLactoseGlucose + GalactoseBecause glucose is absorbed into the enterocytes bySGLT-1 transporters, the rapid absorption of glucosefacilitates sodium absorption, and hence chloride and waterabsorption. This is the principle behind oral rehydrationtherapy (ORT) for dehydration or enteric diseases such ascholera.Digestion and Absorption293Salivaryamylase(ptyalin)StarchSucrcLaosesetoelosD-xyMaltoseveresnguVaSecretin andcholecystokininPancreaticamylasePancreastinaIntesalllwMaltoseMaltasechSucroseLactaseLactoseGlycocalyxSucraseStarGlucoseFructosePortalveinGalactoseEpithelial cellsFigure 25.2 Carbohydrate Digestion and Absorption Starch digestion begins in the mouth withsalivary α-amylase, which produces smaller malto-oligosaccharides.
In the small intestine lumen, pancreaticα-amylase continues to digest the starches. At the same time, brush border saccharidases perform the finaldigestion of maltose and isomaltose (by maltase and isomaltase, producing glucose molecules), and thedisaccharides sucrose (by sucrase, producing glucose and fructose) and lactose (by lactase, producingglucose and galactose). The monosaccharides then are transported into the enterocytes by secondary activetransport with sodium (glucose and galactose) and facilitated transport (fructose; not shown on diagram).PROTEIN DIGESTION AND ABSORPTIONDietary protein digestion begins in the stomach and continuesin the small intestine:■Stomach: Pepsinogens are secreted from the gastricchief cells and are activated to pepsins by stomach acid.Pepsins are endopeptidases (such as trypsin and chymotrypsin) and hydrolyze inner peptide bonds, creatingsmaller oligopeptides.
Pepsins are inactivated in thehigher pH of the duodenum.■Small intestine: Pancreatic proteases are responsible fordigesting the oligopeptides down to smaller peptides. Asstated in Chapter 23, the pancreatic proteases are releasedas zymogens into the duodenum primarily in responseto cholecystokinin (CCK). The brush border enzymeenterokinase activates trypsinogen to trypsin, and thetrypsin activates the other endopeptidases (chymotrypsin, elastase) and the exopeptidases (carboxypeptidase Aand carboxypeptidase B). Once activated, trypsin canalso activate trypsinogen.
The pancreatic proteases294Gastrointestinal PhysiologyGaPepsinogenstrinH⫹Cl⫺Intfac rinsictorBicIntrinsB 12 factor12PepsinProteinPeptidesProcarboxypeptidaseChymotrypsinogenTrypsinogenrves neugaVSecretin andcholecystokininPancreasEnterokxPeptidesypeptidasepsinChymotrypsinTryGlycocalyxoCarbinaseallal wAminopolypeptidaseIntestin ptideseDipeptidasepyPeptidaseslPoCarboxypeptidaseEndopeptidaseLymphatics(to thoracicduct andthence tovenous system)Dipeptides ⫹ tripeptidesAminoacidsPortal vein (to liver)Epithelial cellsFigure 25.3 Protein Digestion and Absorption Protein digestion begins in the stomach with HCland pepsins, which break protein down to smaller polypeptides.
In the lumen of the small intestine, pancreatic proteases (trypsin, chymotrypsin, carboxypeptidase) are activated and continue the digestion tosmaller peptide chains. Final digestion occurs by the brush border proteases, which produce dipeptides,tripeptides, and single amino acids. These molecules are transported into the cells via secondary activetransport with sodium (amino acids), or coupled with H+ (dipeptides and tripeptides). In the cells, the dipeptides and tripeptides are digested to amino acids by cytoplasmic peptidases.■■continue hydrolysis of peptide bonds, making smalleroligopeptides (Fig.
25.3).Small intestine brush border: A variety of brushborder peptidases hydrolyze the peptides to amino acidsand dipeptides and tripeptides, which can then beabsorbed.Enterocytes: Cytoplasmic peptidases digest the dipeptides and tripeptides to amino acids, which can exit thecells.In the small intestine, most of the proteins are absorbedinto the enterocytes in dipeptide and tripeptide form, via H+As previously stated, storing and secreting the pancreaticproteolytic enzymes as zymogens helps prevent digestion of the pancreatic tissue. However, because trypsin canautoactivate itself, the pancreas also produces trypsin inhibitor,which deactivates trypsin in the pancreas and ducts.
In the smallintestine, the large amount of active trypsin is also preventedfrom damaging the intestinal mucosa by the presence of othertrypsin inhibitors (PSTI, pancreatic secretory trypsin inhibitors), which can compete with proteins for trypsin. The PSTIhave also been shown to stimulate growth of intestinal epithelialcells.Digestion and Absorptionsymporters that are specific for the peptides. Amino acidshave different Na+-dependent transporters for basic, acidic,neutral, and imino acids. Once inside the intestinal cells, thecytoplasmic peptidases hydrolyze the dipeptides and tripeptides to amino acids, which leave the cells by facilitated transport into the capillaries. A small amount of the dipeptides andtripeptides can be transported through the cell and into theblood, but the mechanism is unclear.LIPID DIGESTION AND ABSORPTIONAlmost all (98%) of dietary lipids are triglycerides (TG),with the remainder cholesterol esters and phospholipids(PL).
Lipids are easily hydrolyzed to molecules that can beabsorbed; however, their hydrophobicity does not allow easyaccess to the absorptive cells of the small intestine brushborder. As a result, there is a complex mechanism for efficiently moving lipids through the unstirred water layer to theenterocytes. It should be noted that although there is not a lotof lipid digestion in the upper GI tract in adults, lingual andgastric lipases do have a role in hydrolysis of lipids in theneonate:■■■Mouth: Lingual lipase is secreted from VonEbner’s glands of the tongue into the saliva andbegins hydrolysis of TGs to diglycerides and free fattyacids (FFA). The enzyme remains active in thestomach.Stomach: Gastric lipase is secreted from the chief cellsof the gastric pits and also hydrolyzes TGs to diglyceridesand FFA. Again, in the adult this appears to be a minorfactor in lipid digestion.Small intestine: Various lipases are secreted in activeform from the pancreas in response to the action of CCKon the pancreatic acinar cells.
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