HUN3224 Intermediate Metabolism of Nutrients I Exam I Study Guide I Digestion and Absorption of Carbohydrates Lipids and Proteins A Physiology of the gastrointestinal GI tract 1 List the parts of the GI tract in the proper sequence and note the purpose function of each Also list the accessory organs and their purpose function in relation to digestion and absorption o main structurs of GI tract include the oral cavity esophagus and stomach collectively referred to as upper and the small and large intestines lower GI o Accessory organs pancreas liver and gallbladder o Accessory organs provide or store secretions that ultimately are delivered to the lumen of the digestive tract and aid in the digestive and absorptive processes o Interior passageway of GI tract lumen has four main tunics o Mucosa submucosa muscularis externa serosa or adventitia From ppt o Saliva o 1 Oral Cavity digestive processes begin in the oral cavity Enzymes salivary amylase carbohydrates Lingual lipase milk fat o Lingual lipase is produced by lingual serous glands on tongue and back of mouth and hydrolyzes dietary tiracyglycerols in the stomach Lysozymes Kill bacteria o Ex immunoglobulin A IgA o Mastication chewing breaks down food Increases surface area o principal enzyme of saliva is alpha amylase also called ptyalin it hydrolyzes internal alpha 1 4 bonds within starch becomes inactivated in stomach by low pH of gastric juices o Digestion of Starch Amylopectin dextrin maltose and maltotriose Amylose maltose and maltotriose Amylase hydrolyzes alpha 1 4 links Amylopectin 1 6 link Starch most commonly digested polysaccharide in plants Exists in two forms amylose and amylopectin o 2 Esophagus o Muscular tube Connects the back of the mouth and stomach Movement of food via peristalsis o Lower esophageal sphincter circular muscular valve Relaxes to let food into the stomach Constricts to keep food from moving back up into esophagus o GERD Gastro Esophageal Reflux Disease o Due to reflux of digestive juices from stomach into esophagus o Gastroesophageal sphincter LES is weak or inappropriately relaxes o Relieved by antacids alka selzter o Prevented by Histamine 2 H2 blockers ex peptid ac tagamet HB Zantac 75 Proton pump inhibitors ex Prilosec Nexium o Substances which aggravate heartburn o Decrease LES pressure cause relaxation of the gastroesophageal sphincter and increase liklihood of acid reflux into the esophagus Chocolate high fat foods peppermint alcohol o Increase Acid o Irritate Coffee caffeine alcohol calcium 3 Stomach Holds food Spicy foods and citrus o Mechanical mixing and breaking down of food o Gastric secretion o Absorption of alcohol and aspirin o Digestion in the stomach o Lipids limited hydrolysis via lingual gastric lipase Gastric lipase hydrolyzes short and medium chain triacylglycerols and is responsible for 20 lipid digestion in humans o Proteins some hydrolysis to smaller polypeptides proteases peptones via pepsin an example of an endopeptidase o PTN digestion in stomach o Secretion of pepsinogen By chief peptic cells mainly in response to acetylcholine acetylcholine is released from vagus nerve for action on parietal cells Pepsinogen is a zymogen an inactive protein digesting enzyme Pepsin is made by the chief cells and functions as the principal proteolytic enzyme in the stomach Pepsin is derived from either of two pepsinogens I or II Pepsinogen I has been associated with an increase in peptic ulcers o Activation of pepsinogen By HCl and by pepsin autocatalytic Pepsinogens are secreted in granules into the gastric lumen from chief cells where they are stimulated by acetylcholine acid or both o Pepsin functions as protease an ezyme that hydrolyzes proteins It is an endopeptidase it hydrolyzes interior peptide bonds within proteins o Secretion of HCL Via parietal oxyntic cells contains both potassium chloride transport system and a hydrogen potassium ATPase exchange system Some HCl secreted even when stomach empty Regulated by Gastrin a peptide hormone Release stimulated by presence of protein or by gastrin releasing peptide GRP gastrin releasing peptide or bombesin in response to acetylcholine o GRP released from enteric nerves and stimulates gastrin and HCl release Acetylcholine o A neurotransmitter relased by vagus nerve or in response to local reflex of stomach distention o Stimulates release of gastrin via GRP release or directly by acting on gastrin cells o Also acts directly on parietal cells Histamine o Vasoactive amine produced by mast cells o Release stimulated by vagus nerve and by o Like gastrin and Ach has receptor on parietal gastrin cell Stimulate acid secretion Somatostatin o A hormone inhibits HCl release o Ach inhibits the secretion of somatostatin whereas H promotes secretion Pyloric Sphincter o Circular muscular valve separating the stomach from duodenum o Regulates movement of food and acid from stomach to small intestine o Chyme acid and food o 4 Small intestine o Small intestine o Three sections duodenum jejunum Ileum o Digestion in Small intestine o Digestive events occur In the lumen CH2O hydrolysis of starch or dextrin maltose and maltotriose via pancreatic amylase PTN hydrolysis of proteins and smaller peptides to di and tripeptides and amino acids via pancreatic proteases Lipid Digestion in Lumen o Pancreatic proteases are released as zymogens proenzymes or inactive enzymes which must be chemically altered to function as an enzyme and activated in lumen of duodenum o Trypsinogen converted to trypsin via enteropeptidase duodenal enzyme o Trypsin activates other zymogens autocatalytic o Lipids hydrolysis of triacylglycerols TAG to monacylglycerols MAGs free fatty acids FFAs and glycerol via pancreatic lipase o Emulsification of fats via bile acids bile salts increase surface area of fats o Co lipase provides lipase access to TAG o Formation of micelles Bile acids and phospholipids in bile engulf FFAs MAGs and free cholesterol End products of fat digestion thus transported across unstirred water layer to brush border Most of the digestive enzymes produced by intestinal mucosal cells are found embedded on the brush border and they hydrolyze already partially digested nutrients mainly carbs and proteins o Bile synthesized in liver concentrated and stored in gall bladder o Bile composed mainly of bile acids and salts but also has cholesterol phospholipids and bile pigments bilirubin and biliverdin dissolved in alkaline solution o Bile acids and bile salts act as
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