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The Digestive System 1 6.1.1 Why digest food? Food consists of: – Carbohydrates – Lipids – Proteins – Nucleic acids – Minerals – Vitamins – Water These are large, insoluble polymer molecules. They must be digested into monomers small enough to pass through the cell membrane of the cells lining the digestive tract so they can be absorbed in to the blood stream Can be absorbed directly (because of small size), so do not need to be “digested” 6.1.2 Enzymes and digestion • Digestion involves hydrolysis of food molecules breaking apart molecules by adding water Hydrolysis of lactose (a disaccharide) into glucose and galactose (both monosaccharides) with the addition of water Increase the rate • Digestive enzymes catalyze the hydrolysis of insoluble food molecules to soluble end products The enzyme sucrase catalyzes the hydrolysis of sucrose (a disaccharide) into glucose and fructose (both monosaccharides) with the addition of water 6.1.2 Enzymes and digestion Digestive Enzymes Digestive Enzymes Break down complex substances into simpler substances that can be absorbed by the body Complex proteins Complex sugars Fat molecules Amino acids Glucose Fatty acids 7 Macromolecules need to be digested – Carbohydrates: main source of energy (short term) Complex sugars → Simple sugars – Proteins: growth, antibodies, hormones, pumps, enzymes, carriers Polypeptides → Amino acids – Fats: source of energy (long term)/ storage, absorption, help produce hormones, steroids, part of cell membrane Triglyceride → Fatty acids + Glycerol Enzymes: protease amylase lactase nuclease lipase Speed up reactions by lowering activation energy (chemical reaction requires less energy to occur) Fig. 5.6 How enzymes work The enzyme remains unchanged! Important Digestive Enzymes • • • • • • Salivary Amylase Pepsin Trypsin Dipeptidase Pancreatic Amylase Lipase 6.1.4 Digestive System Parts of the Alimentary Canal The Digestive System Consists of the alimentary canal (the long tube) and several accessory organs. anal canal mouth pharynx Parts of the Alimentary Canal esophagus stomach large intestine small intestine 12 Organs of the Digestive System Parotid salivary gland Pharynx Organs of the Digestive System Tongue Mouth Tooth Sublingual salivary gland Submandibular salivary gland Esophagus Liver Stomach Gall bladder Duodenum Pancreas Large intestine Small intestine Anal canal Rectum 14 toast mouth Tracing Toast pharynx epiglottis esophagus stomach anus feces rectum duodenum Tracing a piece of toast through the alimentary canal jejunum sigmoid colon ileum descending colon transverse colon ascending colon cecum 15 Animation of digestive system Types of Digestion: • Mechanical: chewing, mashing, cutting, pounding (mouth / stomach) • Chemical: enzymes (speed up chemical reactions) The Digestive Tract • Parts of the Digestive Tract – Mouth – Esophagus – Stomach – Small Intestine – Large Intestine – Rectum & Anus Accessory Parts • Organs that are not in the digestive tract but help in digestion – Teeth – Tongue – Salivary glands – Liver – Gall bladder – Pancreas The Digestive System Mouth • Functions: – Food enters the mouth or oral cavity – Tasting – Mechanical breakdown of food – Secretion of saliva (by the salivary glands) Anatomy of the Mouth Mouth • Tongue – Mixes and rolls food into tiny mashed up bits (Bolus) – Pushes the bolus toward the esophagus when swallowing. Salivary Glands • Salivary glands – produce and secrete saliva into the oral cavity. – Parotid (beneath the cheeks) – Submaxillary (below the jaw bone) – Sublingual (below the tongue) Saliva Saliva moistens the food and contains enzymes that begin digestion of starch into smaller polysaccharides. • From wikipedia: – Salivareferred to in various contexts as spit, spittle, drivel, drool, or slobber, is the watery substance produced in the mouths of humans and most animals. Important Digestive Enzymes • Salivary Amylase is the 1st enzyme and begins working on starches to break them down into simpler sugars. • They are secreted by the salivary glands and active in the mouth. • It’s optimal pH range is about 7.0. Anatomy of the Mouth and Throat Esophagus • A straight muscular tube that is about 10 inches (25 cm) long which connects the mouth to the stomach • Food takes about 4 to 8 seconds as it passes through to the stomach. • Its walls contain smooth muscles that contracts in wavy motion (Peristalsis). • Peristalsis propels food and liquid slowly down the esophagus into the stomach. • Cardiac Sphincter (ring-like valve) relaxes to allow food into the stomach. Peristalsis in the Esophagus Peristalsis Video Stomach • J-shaped muscular sac, where most digestion occurs. • Has inner folds (rugae) that increase the surface area of the stomach. Stomach Gastric Juices •Food is mixed with gastric juices secreted by the stomach wall •Hydrochloric acid (HCl) activates enzymes, breaks down food molecules and kills bacteria •Enzymes (proteases) break down proteins •Mucus lubricates food and protects the stomach wall from HCl. Stomach • Churns and grinds together the bolus into smaller pieces. • Converts the bolus into a liquid (chyme) after 4 hrs of mechanical and chemical digestion • Chyme passes through the pyloric sphincter into the small intestine. AKA Cardiac Sphincter Movements in Stomach Important Digestive Enzymes • Proteases are a group of enzymes that hydrolyze proteins into amino acids (or smaller pieces of proteins). • Pepsin • Trypsin • Dipeptidase Important Digestive Enzymes • Pepsin is the major stomach enzyme and begins working on proteins to break them down into smaller polypeptides. • It is secreted by the stomach cells and is active in the stomach. • It’s optimal pH range is about 2.0. The Stomach Label the Stomach (a) Lumen of the stomach which stores the food from a meal (b) Gastric pits from which mucus, enzymes and acid are secreted (c) Mucus secreting cells. Mucus protects the surface of the stomach from auto-digestion (d) Parietal cells that produce HCL which kills microorganisms that enter the digestive system (food & tracheal mucus). This also converts inactive pepsinogen to active pepsin (e) Chief cells: produces pepsinogen, a protease enzyme Small Intestine • Long (20 ft), coiled tube beneath the stomach. • This is where digestion is completed and products are absorbed into the bloodstream. • Has three parts: • Duodenum – upper part; about 10 in; connected to the stomach. Where the digestive juices from the pancreas and the liver combine with chyme. Sodium bicarbonate (pH 9) is secreted to neutralize the acid. • Jejunum – about 8 ft; completion of digestion • Ileum – about 12 ft; absorption of nutrients Small Intestine • Site of greatest amount of digestion and absorption Important Digestive Enzymes • Trypsin breaks down polypeptides into smaller peptides. • They are secreted by the pancreas and active in the duodenum of the small intestine. • It’s optimal pH range is about 8.0. Important Digestive Enzymes • Dipeptidase breaks down peptides into amino acids. • They are secreted by the small intestinal cells and active in the small intestine. • It’s optimal pH range is about 8.0. Important Digestive Enzymes • Pancreatic Amylase finishes carbohydrate digestion by breaking down starch into the disaccharide maltose. • They are secreted by the pancreas and active in the small intestine. • It’s optimal pH range is about 7.0. Important Digestive Enzymes • Lipase enzymes work on lipids to break them down into glycerol and fatty acid chains. • Pancreatic lipase is secreted by the pancreas. • It’s optimal pH is 7.2. Small Intestine • Takes about 4 – 8 hrs to complete its journey. • Mucosa (inner wall) – secretes several enzymes that acts on the food. • Where the pancreatic enzymes are emptied into. • Digested nutrients are absorbed through intestinal walls into the blood supply. • Peristalsis moves the undigested food along. Small Intestine • Has folded inner walls covered with fingerlike projections (villi; sing. – villus) • Each villus has tinier projections called microvilli that absorb digested food. • Villi and microvilli increase the surface area of the small intestine for greater absorption. The Small Intestine Label the Small Intestine (a) Villus which increases the surface area for absorption of the products of digestion (b) Microvilli border of the epithelial cell increases the surface area for absorption. (c) Lacteals are vessels that transport lipids out of the small intestine into the lymphatic system. (d) In the wall of the small intestine are the blood vessels to transport absorbed products to the general circulation. There are also muscles to maintain peristalsis Absorption vs. Assimilation • Soluble products are taken up into the epithelial cells of the small intestine. • The products then diffuse into the bloodstream. • Soluble products are transported through the blood to body tissues. • The cells of these tissues absorb them for their own use or storage. Absorption vs. Assimilation How absorption happens • The structure of the villi increases the surface area for the absorption of digested food molecules. (a) folds increase SA:VOL ration by X 3 (b) Villi project into the lumen of the gut increasing the surface area by X 10 (c) Microvilli are outward folds of the plasma membrane increasing the surface area another X10 Diagram a villus • Using the red textbook (page 860), draw and label a villus from the human small intestine. • Include the following labels: villi, microvilli, capillaries, lacteals, epithelial cells, lymph vessel Large Intestine • a.k.a. Colon • larger diameter, but shorter (5 ft) • Water is absorbed from the undigested food making the waste harder until it becomes solid. • Waste stays for 10 – 12 hours. Large Intestine Large Intestine • Waste is pushed into the expanded portion (rectum) of the large intestine. • Solid waste stays in the rectum until it is excreted through the anus as feces. • Appendix hangs on the right side of the large intestine. The Large Intestine The “Colon” • (a) The lumen of the colon • (b) The mucus producing goblet cells • (c) Muscular walls to maintain peristalsis Accessory Organs Produce or store enzymes that helps in digestion. Accessory Organs • Liver – Largest solid organ of the body – It weighs 1.6 kg. and is 20 cm. across. – Produces bile (watery, greenish substance), which aids in the absorption of fats. – Secretes bile to the gall bladder The Liver – Stores vitamins A,D,E,K – Stores sugar and glycogen – Synthesizes proteins – Eliminates biochemical products (RBCs) – Detoxifies drugs, alcohol, and environmental toxins. Accessory Organs • Gall bladder – Stores bile in between meals – Secretes bile to the duodenum through the bile duct during mealtime. • Bile contains bile salts, pigments, cholesterol and phospholipids. • Bile is an emulsifier NOT an enzyme. • Emulsifier – dissolves fat into the watery contents of the intestine. Gall Bladder Accessory Organs • Pancreas – Produces a juice that contains enzymes to break down carbohydrates, fats and protein. – Secretes the juice into the duodenum through the pancreatic duct. –Also secretes hormones, like insulin. The Pancreas Path of Digestion • • • • • • Mouth Esophagus Stomach Small Intestine Large Intestine Anus Let’s see what happens!