Digestion 2

• Professor Shivananda Nayak, The University of the West Indies, Faculty of Medical Sciences, Department of Preclinical Sciences, EWMSC, Mount Hope, Trinidad
• Diet contains carbohydrates, fat and proteins
• They are high molecular weight complex compounds and must be digested for absorption
• Digestion of carbohydrates


  1. Polysaccharides (starch and glycogen) hydrolyzed to simple sugars through enzymes

  2. Salivary amylase hydrolyses α-1,4-glycosidic linkages of polysaccharide chain to produce mono and disaccharides

  3. Further digestion in small intestine by intestinal enzymes which hydrolyze terminal α-1,4-glycosidic linkages

  
• Entry of acidic contents of stomach into duodenum

  1. Stimulates mucosal cells to release secretin and cholecystokinin

  2. Secretin stimulates pancreas to release bicarbonate and water to neutralize acidic chyme

  3. Cholecystokinin stimulates production of digestive enzymes including pancreatic amylase

  4. Pancreatic amylase digests polysaccharides to maltose, isomaltose and limit dextrin

  5. Disaccharidases digest disaccharides into monosaccharides

  
• Humans do not produce β 1,4-endoglucosidase to digest cellulose

  
• Cellulose helps in easy peristalsis and provides bulk to feces
• Lactase deficiency leads to lactose intolerance
• Absorption of monosaccharides


  • Galactose and glucose absorbed rapidly by active process
  • Fructose and mannose absorbed by Na+ independent facilitative transport mechanism

  
• Digestion of proteins in stomach

  1. Protein does not undergo digestion in mouth

  2. Enters stomach and stimulates secretion of hormone gastrin

  3. Gastrin stimulates release of gastric juice containing HCl and pepsinogen

  4. HCl unfolds proteins and activates proteolytic enzyme pepsin

  5. Pepsin digests protein polypeptides into tripeptides, dipeptides and amino acids

  
• Digestion of proteins in intestine

  1. Entry of acidic contents from stomach into intestine triggers secretion of hormones cholecystokinin and secretin

  2. Secretin stimulates release of bicarbonate and pancreatic juice from pancreas

  3. Cholecystokinin stimulates secretion of pancreatic endopeptidases and exopeptidases

  4. Endopeptidases (trypsin, chymotrypsin, elastase) cleave internal peptide bonds

  5. Carboxypeptidase and aminopeptidase hydrolyze peptide bonds from carboxyl and amino terminal ends respectively

  
• Trypsin


  Hydrolyzes peptide bonds whose carboxyl groups are contributed by lysine and arginine
• Chymotrypsin


  Specifically hydrolyzes peptide bonds involving carboxyl group of aromatic amino acids (phenylalanine, tyrosine, tryptophan) and also splits bonds of leucine, methionine, asparagine, histidine
• Elastase


  Hydrolyzes peptide bonds formed by non-polar amino acids like alanine, serine, glycine
• Absorption of amino acids

  Includes Na+ dependent active transport mechanism requiring ATP as energy source
• Digestion of lipids


  1. Depends on bile salts for emulsification

  2. Lipids inhibit gastric motility and retard evacuation of stomach

  3. Gastric and lingual lipase active only at neutral pH, no digestion in adult stomach due to acidic pH

  4. Hydrophilic short and medium chain fatty acids absorbed via mucosal cells and enter portal vein

  5. Longer chain fatty acids dissolve in diet and pass into duodenum

  
• Entry of acidic chyme from stomach into duodenum

  1. Stimulates secretion of hormones secretin and cholecystokinin

  2. Cholecystokinin helps contract gallbladder to release bile salts, acts on pancreas to release digestive enzymes including lipase, decreases gastric motility

  3. Secretin causes pancreas to release bicarbonate-rich solution to neutralize acidic chyme and change pH to alkaline

  
• Digestion of lipids in intestine

  1. Bile provides emulsifying action, pancreatic lipase, phospholipase A2 and cholesterol esterase hydrolyze lipids

  2. Dietary glycerophospholipids digested by pancreatic phospholipase-A2

  3. Fats resynthesized into chylomicrons in mucosal cells and transported to blood via lymphatic vessels

  4. Fatty acids less than 10 carbon atoms carried by portal blood to liver

  
• Mixed micelles


  Formed by long chain free fatty acids, free cholesterol, 2-monoglyceride, 1-monoglyceride, lysophospholipid together with bile salts
• Absorption and transport of lipids


  1. 1-monoacylglycerol further hydrolyzed by intestinal lipase, 2-monoacylglycerol reconverted to triglyceride

  2. Absorbed lysophospholipids and cholesterol recycled with acyl-CoA to regenerate phospholipids and cholesterol esters

  3. Triglycerides, phospholipids, cholesterol esters and fat soluble vitamins transported from mucosal cells to lymph as chylomicrons

  4. Triglycerides hydrolyzed by lipase to release glycerol and fatty acids, glycerol converted to glucose, fatty acids oxidized for energy

  
• Absorption sites of vitamins and minerals

  • Duodenum: Fe, Ca, Mg, Zn
  • Jejunum: Vit C, B1, B2, B6, Folic Acid
  • Ileum: Fat soluble vitamins via micelles, B12
  • Colon: Na, K, Vit K from bacterial action
• Absorption of minerals


  1. Phase 1 - Intraluminal: Chemical reactions and interactions in stomach and intestines, cations influenced by pH and composition of chyme, kept available for absorption by ligands

  2. Phase 2 - Translocation: Passage across membrane into mucosal cell, small anions by simple diffusion, cations by facilitated diffusion or active transport

  3. Phase 3 - Mobilization: Transport across serosal surface into blood or sequestered within cell, Fe and Zn bound to proteins or added to intracellular pool