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Digestion 3 types of digestion: 1. Mechanical – breaking food into smaller pieces; increases surface area of food for… 2. Chemical – breaking bonds in food into smaller molecules for… 3. Absorption – where nutrients enters blood stream • villi: projections increasing surface area of organ many times • microvilli: microscopic projections of individual cells • capillaries in villi absorb amino acids and sugars • lacteal: extension of lymph system into villi = absorption of fats Overview of digestive tract organs Oral Cavity (mouth): mastication of food = increases surface area - mechanical digestion saliva = lubrication of food, protection from bacteria, prevent tooth decay with buffers ball of saliva and partially digested food = bolus …swallow using skeletal muscle Epiglottis = flap-like structure made of cartilage (connective tissue) Covers wind pipe when swallowing directing food to the correct tube – esophagus Before entering the esophagus proper, the bolus hits the upper esophageal sphincter triggering it to open 1 Upper esophageal sphincter = stops contents of esophagus from backing up into the pharynx Esophagus Lined with smooth muscle Peristalsis = smooth muscle contractions moves bolus to – esophageal sphincter before entering stomach Lower esophageal sphincter (cardiac sphincter) = blocks stomach contents from entering esophagus Stomach Lined with smooth muscle Contraction = churning of bolus = mechanical digestion HCl and enzymes released – enzymes = chemical digestion Partially digested food is liquid = chyme Acidic chyme triggers – pyloric sphincter to open as chyme enters small intestine can take 2 to 4 hours for a meal to enter the small intestine – rugae = folds in stomach to increase surface area; stomach can hold up to 2L of food and fluid! Wow! Pyloric sphincter Stops intestinal contents from backing up into the stomach regulates acid chyme entering into small intestine (one squirt at a time) Small intestine 1st part of the small intestine = duodenum Main site of digestion and absorption (when food comes in contact with blood for the first time) 2 – microvilli = brush border (looks like a shag carpet) most digestion occurs here mixes enzymes with acid chyme from stomach 3 pancreas release pancreatic juice – to duodenum via pancreatic duct liver makes bile – stored in gall bladder – to duodenum via bile duct. – liver: first organ for digestive blood to enter (mostly) – stores glucose as glycogen – filters and detoxifies poisons and drugs – other storage and conversion functions Cells of the duodenum itself = brush boarder cells Enzymes of brush boarder are bound to the surface of the epithelial cells (chem. digest.) 2nd part = jejunum - more absorption 3rd part = ileum – even more absorption Ileocecal valve Separates small and large intestine; stops back flow between these two organs. Large intestine (colon) Re-absorb water (about 90%) and maintains the fluid balance of the body. vitamins and minerals are also absorbed Rectum stores waste before removal As the rectal walls expand due to the materials filling it from within, stretch receptors from the nervous system located in the rectal walls stimulate the desire to defecate. If the urge is not acted upon, the material in the rectum is often returned to the colon where more water is 4 absorbed. If defecation is delayed for a prolonged period, constipation and hardened feces results. Anal sphincter Defecation = removal of solid waste from body Specific Macromolecule digestion Carbohydrate Digestion: Oral cavity Mastication = mech. digest. salivary amylase: hydrolysis of starch and glycogen from other animals (chemical digest.) products = smaller polysaccharides Pancreas Pancreatic amylase secreted into duodenum – Smaller disaccharides from starch = maltose (c.d.) Duodenum maltase: finishes hydrolysis of starch--maltose (glucose + glucose) sucrase = glucose + fructose lactase = glucose + galactose – all chemical digest. After sugars from carbs are absorbed into blood stream from the small intestine, blood moves to liver first and then to the rest of the body via the circulatory system. 5 Lipid Digestion: Oral Cavity - mastication Small intestine Only site of lipid digestion 1. Liver – makes bile Bile salts aid in the digestion and absorption of lipids Emulsifies fat = mechanical digest. – Separates globs into globlets – Increases surface area How? – Bile has polar and non-polar ends – non-polar ends clings to lipid - - polar ends cling to water = pulls globs apart. 6 2. Pancreas – makes lipase Enters duodenum via pancreatic duct Hydrolyzes lipids into fatty acids and glycerol (chem. digest.) After monomers are absorbed by small intestine, they move directly to the lymphatic system and then to the circulatory system. (Not to the liver first) – lacteal: extension of lymph system into villi = absorption of fats Note: Lymph is the fluid that is formed when interstitial fluid (between cells) enters the initial lymphatic vessels of the lymphatic system. The lymph is then moved along the lymphatic vessel network. Eventually, the lymph vessels empty into the lymphatic ducts, which drain into specific veins. Protein Digestion: Oral cavity = mastication Stomach Churning of food HCl – secreted by parietal cells – dissolves connective tissue of protein (mechanical) – pepsinogen (a zymogen) is secreted by chief cell of gastric pits in stomach Zymogen – an inactive enzyme precursor – acid environment activates pepsinogen into pepsin – pepsin: hydrolyzes proteins into smaller polypeptides (chem. digest.) 7 Example of a positive feedback mechanism – when pepsin is activated, it can also activate additional molecules of pepsinogen—self accelerating process Mucus = secreted by goblet cells; protects stomach cells from digestion (acidic environment and chem. digest. of self-cellular protein) Into Duodenum (from pancreas) **zymogen = inactive enzyme form **proteases (from pancreas) = active form Zymogen a. Trypsinogen b. Chymotrypsinogen c. Procarboxypeptidase Activated by Enterokinase (from small intestine Trypsin Trypsin all chemical digest. Protease from Trypsin pancreas Chymotrypsin Carboxypeptidase pancreas pancreas a, b, and c all make peptide chains smaller and smaller Aminopeptidase cleave off individual a.a. Small intestine After a.a. from protein are absorbed into blood stream from the small intestine, blood moves to liver first and then to the rest of the body via the circulatory system. 8