Biology 9: Digestive System Overview Feeding Behavior Hormones Intracellular Digestion: The oxidation of glucose and fatty acids to make energy. Extracellular Digestion: Process by which nutrients are obtained from food. Occurs in alimentary canal. Mechanical Digestion: Physical breakdown of large food molecules into smaller particles. Chemical Digestion: The enzymatic cleavage of chemical bonds such as the peptide bonds of proteins or the glycosidic bonds of starches. Peristalsis: Rhythmic contractions of the gut tube. !parasympathetic nervous system ¯sympathetic nervous system Small Intestine Glucagon & Ghrelin: !hunger Leptin & Cholecystokinin: !satiety Duodenum First part of small intestine. A basic (pH = 8.5) environment. Site of the majority of chemical digestion. Enzymes in Duodenum Disaccharidases: Brush-border enzymes that break down maltose, isomaltose, lactose, and sucrose into monosaccharides. Aminopeptidase & Brush-border peptidases. Dipeptidase: Digestive Pathway Oral Cavity ADH & Aldosterone: !thirst Pharynx Esophagus Large Intestine Stomach Rectum Oral Cavity Mastication starts the mechanical digestion. Salivary amylase and lipase start the chemical digestion of food. Food is formed into a bolus and swallowed. Pharynx Connects the mouth to the esophagus. The epiglottis prevents food from entering the larynx. Esophagus Propels food to the stomach using peristalsis. Top third has skeletal muscle and is under somatic control. Bottom third has smooth muscle, middle third has combo of both. The middle & bottom are under autonomic control. Stomach An acidic (pH = 2) environment. Four parts: fundus, body, antrum and pylorus. The enzyme pepsin chemically breaks down proteins. Enteropeptidase: Activates trypsinogen and procarboxypeptidases. Hormones in Duodenum Secretin: Peptide hormone. Stimulates release of pancreatic juices and slows motility. Cholecystokinin: Stimulates bile release from gallbladder, release of pancreatic juices, and satiety. Absorption and Defecation The jejunum and ileum of the small intestine are primarily involved in absorption. The small intestine is lined with villi, which are covered with microvilli. Villi: Capillary Bed: Absorbs water-soluble nutrients. Lacteal: Absorbs fat, sends to lymphatic system. Vitamin Absorption: Fat-Soluble: Only A,D,E,K; enter lacteal. Water-Soluble: All others; enter plasma directly. Large Intestine – absorbs H2O and salts, forms feces Cecum: Outpocketing that accepts fluid from small intestine through ileocecal valve. Site of attachment of the appendix. Structure of Colon: Ascending / transverse / descending / sigmoid Gut Bacteria: Produce vitamin K and biotin (vitamin B7 ). Secretory cells that line the stomach Mucous Cells: Produce bicarbonate-rich mucus to protect stomach wall from acid. Chief Cells: Secrete pepsinogen, a protease activated by the acidic environment. Parietal Cells: Secrete HCl and intrinsic factor, which is needed for vitamin B12 absorption. G-Cells: Secrete gastrin, a peptide hormone that !HCl secretion & gastric motility. After processing in the stomach, food particles are now called chyme. Chyme exits through pyloric sphincter ® duodenum. Accessory Organs - Originate from endoderm Pancreas: Acinar Cells produce pancreatic juices that contain bicarbonate, pancreatic amylase, pancreatic peptidases, and pancreatic lipase. Liver: Synthesizes bile, albumin and clotting factors. Process nutrients. Detox: NH3 Urea, as well as alcohol & drugs. Liver receives blood from the abdominal portion of digestive tract via Hepatic Portal Vein. Gallbladder: Stores & concentrates bile. CCK stimulates bile release into biliary tree, which merges with pancreatic duct.