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Chapter 1 Digestion, Absorption, Transport, and Excretion of Nutrients Digestive System Reduces particle size Helps to absorb micronutrients and trace elements Sets a physical and immunologic barrier © 2004, 2002 Elsevier Inc. All rights reserved. The Digestive System Sites of Secretion, Digestion, and Absorption Digestion in the Stomach Different nutrients leave the stomach at different rates: —Carbohydrates first, followed by protein, fat, and fibrous foods © 2004, 2002 Elsevier Inc. All rights reserved. Summary of Enzymatic Digestion and Absorption Secretion Enzyme Substrate Action Saliva Ptyalin Starch Gastric juice Pepsin Protein Gastric lipase Fats Hydrolysis to form dextrins Hydrolysis of peptide bonds Hydrolysis into free fatty acids © 2004, 2002 Elsevier Inc. All rights reserved. Final Product Summary of Enzymatic Digestion and Absorption —cont’d Secretion Enzyme Substrate Action Final Product Pancreatic exocrine secretion Lipase Fat Fatty acids Cholesterol esterase Cholesterol alphaAmylase Starch, dextrins Hydrolysis to monoglycerides Hydrolysis to esters of cholesterol and fatty acids Hydrolysis © 2004, 2002 Elsevier Inc. All rights reserved. Cholesterol Dextrin, maltose Summary of Enzymatic Digestion and Absorption —cont’d Secretion Enzyme Substrate Action Final Product Pancreatic exocrine secretion Trypsin Protein Hydrolysis Polypeptides Chymotrypsin Protein Hydrolysis Polypeptides Carboxypeptidase Ribonuclease Polypeptides Ribonucleic acids Fibrous protein Hydrolysis Amino acids Hydrolysis Mononucleotides Amino acids Elastase © 2004, 2002 Elsevier Inc. All rights reserved. Hydrolysis Summary of Enzymatic Digestion and Absorption —cont’d Secretion Enzyme Substrate Action Final Product Brush border enzymes Carboxypeptidase; aminopeptidase; dipeptidase Enterokinase Polypeptides Hydrolysis Amino acids Trypsinogen Activates to trypsin Sucrase Sucrose Hydrolysis Polypeptidases and peptides Glucose, fructose © 2004, 2002 Elsevier Inc. All rights reserved. Summary of Enzymatic Digestion and Absorption —cont’d Secretion Enzyme Substrate Brush border enzymes Isomaltase Dextrin Maltase Lactase Maltose Lactose Nucleotidases Nucleic acid Nucleosidases and phosphorylase Nucleosidases Hydrolysis Purine and pyrimidine bases © 2004, 2002 Elsevier Inc. All rights reserved. Action Final Product Hydrolysis Glucose Hydrolysis Glucose Hydrolysis Glucose, galactose Hydrolysis Nucleotides Features of Digestion and Absorption 92% to 97% of the diet is absorbed Small intestine—key role in digestion and absorption Intestines: 7 meters long —Villi and microvilli/brush border © 2004, 2002 Elsevier Inc. All rights reserved. Diagram of Villi of the Human Intestine Refeeding after Starvation GI tract compromised after starvation Feeding results in cellular proliferation, which restores normal gastrointestinal architecture and function after a few days © 2004, 2002 Elsevier Inc. All rights reserved. Lipids The major portion of fat digestion takes place in the small intestine as a result of pancreatic lipase © 2004, 2002 Elsevier Inc. All rights reserved. Summary of Fat Absorption Transport Pathways through the Cell Membrane Large Intestine 5 feet long Bacterial action to produce gases and organic acids Nutrients formed here—vitamin K, thiamin, vitamin B12, riboflavin © 2004, 2002 Elsevier Inc. All rights reserved. Colonic Fermentation of Malabsorbed Carbohydrate and Fiber The Gradual Breakdown of Large Starch Molecules Process of Digestion of Carbohydrate Sites of Secretion and Absorption into Gastrointestinal Tract Neuropeptide Hormones There are more than 100 peptide hormones —GABA —Norepinephrine —Acetylcholine —Neurotensin —Neuropeptide Y —Serotonin (5-HT) —Nitric oxide —Substance P © 2004, 2002 Elsevier Inc. All rights reserved. Hormones and Gastrointestinal Function Gastrin Secretin Cholecystokinin (CCK) Gastric inhibitory polypeptide (GIP) Motilin Somatostatin © 2004, 2002 Elsevier Inc. All rights reserved.