BIOL 100 Chapter 15: Digestive System & Nutrition

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University of Nevada - Las Vegas
Biological Sciences
BIOL 100
Michael Webber

Chapter 15: Digestive System & Nutrition 15.1 The Gastrointestinal Tract • the digestive system takes the food we eat and breaks it down into chemical subunits • the subunits are molecules small enough to be absorbed into the bloodstream and delivered to body cells, • where they provide either materials for growth and repair of the body or energy for daily activities Table 15.1 Review of Structures of the Digestive System Structure Description/ Mechanical Chemical Digestion Functions Digestion Mouth receives food; teeth tear and crush digestion of contains teeth and food into smaller carbohydrates tongue; tongue pieces begins manipulates food and monitors quality Pharynx passageway for none none both food and air Esophagus tube that transports none none food from mouth to stomach Stomach J-shaped muscular churning of stomach protein digestion sac for food storage mixes food with begins gastric juice, creating liquid chyme Small Intestine long tube where segmental carbohydrate, digestion is contractions mix protein, and fat completed and food with intestinal digestion is nutrients are enzymes, completed absorbed pancreatic enzymes, and bile Large Intestine final tubular region none some digestion is of GI tract; absorbs carried out by water and ions; bacteria houses bacteria; forms and expels feces 1 o 0 Structure Description/ Mechanical Chemical Digestion Functions Digestion Anus terminal outlet of none none digestive tract • digestive system consists of a long, hollow tube, called the gastrointestinal (GI) tract, into which various accessory glands release their secretions • lumen- the hollow area of the tube through which food and fluids travel • 4 layers of the walls of the GI tract: • (1) mucosa • the innermost layer of the GI tract • moist, mucus-secreting layer • lubricates and protects the GI tract • (2) submucosa • consists of connective tissue containing blood vessels, lymph vessels, and nerves • (3) muscularis • responsible for movement of materials through the GI tract and for mixing ingested materials with digestive secretions • 2 layers of smooth muscle (one circular and one longitudinal) • (4) serosa • a thin layer of epithelial tissue supported by connective tissue that wraps around the GI tract • lubricates the outside of the GI tract • Organs: • mouth • entrance to digestive system • teeth chew food • tongue positions and tastes food • pharynx • passageway for food (and air) • plays a role in swallowing • esophagus • muscular tube • moves food fro pharynx to stomach • stomach • J-shaped muscular sac • stores food • secretes gastric juice (pepsin and HCI) 2 of 0 • mixes food with gastric juice • protein digestion begins • small intestine • long, muscular tube • mixes food with bile and with intestinal and pancreatic enzymes • digests most nutrients • absorbs most nutrients and water • large intestine • colon • muscular tube • absorbs water and some nutrients • stores waste materials (feces) • cecum • blind pouch at junction of small and large intestines • rectum • region of large intestine • passageway for feces • stretching of wall stimulates the defecation reflex • anal canal • regulates defecation • anus • opening at end of system • expels feces • accessory structures • salivary glands • 3 pairs of glands that secrete saliva • saliva moistens food • enzyme (amylase) in saliva begins starch digestion • liver • large organ in abdominal cavity • secretes bile, which emulsifies fats • plays role in processing and storing certain nutrients • gallbladder • small sac • stores bile • releases bile into small intestine • pancreas • gland located behind stomach 3 of 0 • secretes enzymes that digest all major nutrients • secretes buffers that neutralize HCI from stomach • releases secretions into small intestine 15.2 Specialized Compartments for Food Processing • Mouth: • mouth- the entryway to the digestive system • a.k.a. oral cavity • several functions of the mouth: • (1) it begins mechanical and chemical digestion • (2) it monitors food quality • (3) it moistens and manipulates food so that it can be swallowed • Teeth and Mechanical Digestion: • Types of teeth: • Incisors • in front of mouth • slice the food • Canines • tear the food • Premolars & Molars • ground, crushed, and pulverized food • teeth are alive • pulp is located in the middle and contains blood vessels and nerves • dentin-bonelike structure that surrounds the pulp • crown- the part of the tooth visible above the gum line • is covered with enamel • enamel-a nonliving material hardened with calcium salts • root- the part of the tooth below the gum line • is covered with cementum • cementum- a calcified, yet living and sensitive connective tissue • tooth decay results from acid produced by bacteria living in the mouth • plaque- an invisible film of bacteria, mucus, and food particles • promotes tooth decay because it holds the acid against the enamel 4 of 0 • gingivitis- an early stage of gum disease; occurs when plaque that has formed along the gum line causes the gum to become inflamed and swollen • gingiv= the gums • itis= inflammation of • periodontitis- the bacteria in the plaque can then attack the bone and soft tissues around the tooth • peri= around • dont= teeth • itis= inflammation of • Salivary glands and chemical digestion: • 3 pairs of salivary glands: • (1) sublingual-below the tongue • (2) submandibular-below the jaw • (3) parotid-in front of the ears • salivary amylase- an enzyme in saliva that begins to chemically digest starches into shorter chains of sugar • Tongue:Taste and food manipulation • the tongue is a large skeletal muscle studded with taste buds • bolus- a small soft mass • the tongue initiates swallowing by pushing the bolus to the back of the mouth • Pharynx • pharynx-shared by the respiratory and digestive systems • aka throat • the movement of the larynx causes a cartilaginous flap known as the epiglottis to move • swallowing involves a voluntary and involuntary phase: • voluntary phase: • the tongue pushes food • involuntary phase: • reflex movement of the soft palate prevents food from entering the nasal cavity • muscles contract, forcing the bolus toward the esophagus • the epiglottis closes the opening to the respiratory system, preventing food from entering • a muscle ring at the top of the esophagus relaxes, opening the esophagus 5 of 0 • Esophagus • esophagus- the tube that connects the pharynx to the stomach • food is moved along the esophagus and all the rest of the GI tract by rhythmic waves of muscle contraction called peristalsis • Stomach • stomach-a muscular sac that is well designed to carry out it’s functions • 3 functions of the stomach: • (1) storing food and regulating the release of food to the small intestine • (2) liquefying food • (3) carrying out the initial chemical digestion of proteins • storage of food and regulation of the release of food to the small intestine: • bands of circular muscle called sphincters guard the openings of each end of the stomach and regulate the release of food to the small intestine • contraction of a sphincter closes the opening • relaxation of a sphincter allows material to pass through • heartburn- occurs when the pressure of the stomach contents overwhelms the sphincter at the lower end of the esophagus, causing a burning sensation behind the breastbone • chronic heartburn- heartburn that occurs at least once per week • people with this are at risk of developing cancer of the esophagus • liquefaction of food: • food is generally stored and processed within the stomach for 2 to 6 hours • the additional mechanical digestion occurs as the food is churned and mixed with secretions produced by the glands of the stomach until it is a soupy mixture called chyme • initial chemical digestion of proteins: • the lining of the stomach has millions of gastric pits, within which are gastric glands containing several types of secretory cells • pepsin-a protein digesting enzyme • once activated by HCI, pepsinogen becomes pepsin • gastric juice-the mixture of pepsin and HCI • is released into the stomach, the pepsin begins the chemical digestion of the protein in food 6 of 0 • intrinsic factor- a protein necessary for the absorption of vitamin B12 rom the small intestine • secreted by the gastric glands • Small Intestine • has 2 major functions: • (1) chemical digestion • (2) absorption • duodenum- the first region of the small intestine • most chemical digestion and absorption occur in the jejunum and the ileum • chemical digestion within the small intestine: • most digestion occurs in the small intestine is actually performed by pancreatic enzymes • bile-a mixture of water, ions, cholesterol, bile pigments, and bile salts; plays an important role in the mechanical digestion of fats, which assists lipase in chemically digesting fats • is rich in cholesterol Table 15.2 Major Digestive Enzymes Enzzymee Site of Production Site of Action Substrate and Products Carbohydrate digestion salivary amylase salivary glands mouth polysaccharides into shorter molecules amylase pancreas small intestine polysaccharides into disaccharides maltase small intestine small intestine maltase into glucose units sucrase small intestine small intestine sucrose into glucose and fructose lactase small intestine small intestine lactose into glucose and galactose Protein digestion pepsin stomach stomach proteins into protein fragments (poly 7 of 0 Enzyme Site of Production Site of Action Substrate and Products trypsin pancreas small intestine proteins and polypeptides into smaller fragments chymotrypsin pancreas small intestine proteins and polypeptides into smaller fragments carboxypeptidase pancreas small intestine polypeptides into amino acids Lipid digestion lipase pancreas small intestine triglycerides (fats) into fatty acids and glycerol • structure of the small intestine: • villi- are tiny 1 mm projections that cover the entire lining surface of the small intestine • microvilli- microscopic projections; the absorptive epithelial cells covering the surface of each villus • increase the surface area of the small intestine • brush border- a fuzzy surface formed by the microvilli • its wall contains accordion-like pleats called circular folds • the core of each villus is penetrated by a network of capillaries and a lacteal • lacteal- a lymphatic vessel • monosaccharides, amino acids, water, ions, vitamins, and minerals diffuse across the capillary wall into the bloodstream and are delivered to body cells • the products of fat digestion, glycerol and fatty acids, combine with bile salts in the small intestine, creating particles called micelles • the glycerol and fatty acids are reassembled into triglyceride, are mixed with cholesterol and phospholipids, and are coated with special proteins, thus becoming part of a complex known as a chylomicron • Accessory Organs: Pancreas, Liver, and Gallbladder 8 o 0 Table 15.3 Review of Accessory Structures of the Digestive System Structure Secretions/Functions Site of Action of Chemical Secretion Salivary glands (sublingual, Secrete saliva, a liquid that Mouth submandibular, parotid) moistens food and contains an enzyme (amylase) for digesting carbohydrates Pancreas digestive secretions include small intestine bicarbonate ions that neutralize acidic chyme and enzymes that digest carbohydrates, proteins, fats, and nucleic acids Liver digestive function is to small intestine produce bile, a liquid that emulsifies fats, making chemical digestion easier and facilitating absorption Gallbladder stores bile and releases it small intestine into small intestine • Pancreas • pancreas-an accessory organ that lies behind the stomach, extending from the small intestine toward the left side of the body • Liver • liver- the largest internal organ in the body, which has a variety of metabolic and regulatory roles • converts the breakdown products of amino acids into urea, which can then be excreted by the kidney • diseases of the liver: • cirrhosis- a condition in which the liver becomes fatty and gradually deteriorates, its cells eventually being replaced by scar tissue • is sometimes caused by prolonged, excessive alcohol use • hepatitis- is inflammation of the liver • most commonly caused by one of six viruses (A,B,C,D,E, and G) • all forms, liver cells injured by hepatitis viruses stop filtering bilirubin from the blood 9 of 0 • jaundice- the condition in which the accumulating bilirubin is deposited in the skin and the whites of the eyes, causing a yellow tint • found in any disease that damages the liver • a portal system transports blood from one capillary bed to another: • (1) products of digestion are absorbed into the capillaries within the villi of the small intestine • (2) digested food molecules then travel through hepatic portal veins to the liver • (3) the liver monitors blood contents • (4) Hepatic veins deliver blood to the circulatory system • Gallbladder • gallbladder- a pear-shaped sac where bile is stored, modified, and concentrated • gallstones- consist primarily of cholesterol that has precipitated from bile during storage in the gallbladder • Large Intestine • the material that wasn’t absorbed in the small intestine moves to the large intestine • the primary functions of the large intestine are: • (1) to absorb most of the water remaining in the indigestible food residue • (2) to store the feces • (3) to eliminate them from the body • feces- the material left in the large intestine after passing through the colon • consists primarily of undigested food, sloughed-off epithelial cells, water, and millions of bacteria • the brown color comes from bile pigments • people who are lactose intolerant lack the enzyme lactase, which is normally produced by and acts in the small intestine • regions of the large intestine: • large intestine has 4 regions: • (1) cecum
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