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GI Tract: The Basics • • • • Stages of digestion Mechanical versus chemical digestion Composition of GI tract Exocrine glands Step by step through the digestive tract • explore the digestive tract • UPenn animation • animation of organs of digestion Stages of digestion Mechanical digestion: Peristalsis Figure 24.4 Chemical digestion: Acids Bile Enzymes Why do we need enzymes for digestion? The GI tract is composed of four layers The GI tract has many exocrine gland cells Exocrine glands (salivary, stomach, pancreas): acinus, duct, secretory epithelium: secrete products into GI tract Pancreatic cells (adapted for secretion) Cells are sandwiched together by tight junctions Digestion/Absorption: The stomach Structure of the stomach Rugae: rough ridges which grind food (mechanical digestion) What does your stomach do? Main function is storage and digestion (minimal absorption) Hydrochloric acid lowers pH to 2, killing bacteria and denaturing proteins 1. Mechanical breakdown of food through muscular action and rugae 2. Chemical breakdown of food with acids and enzymes (pepsin begins protein digestion) Gastrin (hormone) increases acid synthesis and release Also produces other hormones (Ghrelin, GIP…) • Produces chyme Control of exocrine (gastric juice) secretion in the stomach Gastric juice secretion (enzyme -pepsin, acid HCl and hormone gastrin is controlled by: 1. Cephalic factors 2. Gastric factors 3. Intestinal factors Control of gastric function The Stomach Lining Figure 24.13c, d Serendipity and the stomach… Saliva/ Gastric juice/ pancreatic juice Site of action pH Enzymes Function Saliva Mouth/ throat 7-8 amylase Starch - maltose Gastric juice stomach 1-2 pepsinogen Proteins – polypeptides/ peptides Gastric juice Pancreatic juice Duodenu m/ small intestine 8 Trypsin Amylase lipase Proteins – peptides Starch – maltose Lipids – fatty acids/ glycerol GI problems 1: Stomach ulcers…. In 1990’s it was discovered that most GI ulcers are associated with Helicobacter infection Helicobacter survives by producing urease and protease enzymes It secretes urease which neutralises the gastric acid to lower the acidity of the stomach for further colonisation It also secretes proteases (e.g. mucinase) to degrade the mucosal lining of the stomach wall, allowing it to burrow into this lining The degradation of this protective lining by H. pylori allows for damage to the stomach wall by gastric acids (causing ulcers) The prolonged presence of stomach ulcers may lead to the formation of stomach cancers There is a correlation between chronic H. pylori infection over a number of years (20 - 30 years) and the development of stomach cancer Barry Marshall was awarded Nobel prize "for discovery of the bacterium Helicobacter pylori and its role in gastritis and peptic ulcer disease" Stomach problems 2 – ‘heartburn’… ‘GERD’ is characterised by acid reflux into the oesophagus Contributing factors causing weakening of the cardiac sphincter include: • Obesity • Diet – fatty foods, chocolate (!), alcohol (!) • Smoking • pregnancy ONE treatment for GERD blocks acid production by the parietal cells Proton pumps in the stomach • Proton pumps are integral proteins in the membrane of gastric epithelial (and many other) cells • Using ATP, they pump H+ into the lumen of the stomach in exchange for K+ Proton pump inhibitors are controversial drugs for treatment of heartburn