ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Forty-Five Outline INTRODUCTION Vertebrates Contain Many Cells with Specialized Functions No Vertebrate Cells Are Specialized for Photosynthesis Plants are self-sustaining autotrophs Animals are heterotrophs Cells must be nourished by food obtained from outside the body Many major organ systems are associated with acquisition of food energy THE NATURE OF DIGESTION Animals Obtain Energy by Degrading Chemical Bonds of Organic Molecules Process acts on simple molecules: amino acids, lipids and sugars Organisms rarely contain large amounts of simple molecules Eating an organism does not provide immediate energy Simple molecules incorporated into long macromolecular chains Macromolecules include proteins, fats and starches First must degrade macromolecules into simple constituent parts fig 45.1 Process Called Digestion ORGANIZATION OF VERTEBRATE DIGESTIVE SYSTEMS General Organization of the Vertebrate Digestive System Consists of a tubular gastrointestinal tract and accessory digestive organs fig 45.2 Initial components are mouth and pharynx Common passage of oral and nasal cavities Pharynx leads to esophagus Esophagus is a muscular tube leading to stomach Preliminary digestion occurs in stomach Food passes into duodenum, upper part of small intestine Battery of digestive enzymes continue digestion Products pass across small intestine wall into bloodstream Tubular gastrointestinal tract has a layered structure fig 45.3 Mucosa is innermost layer Epithelial layer Separates interior (lumen) from blood vessels in next layer Submucosa is next layer of connective tissue Next outermost layer is the muscularis Double layer of smooth muscle Inner muscles have circular orientation Outer layer are arranged longitudinally Serosa connective tissue layer covers external surface Nerve plexuses regulate activities of gastrointestinal tract Specializations of Digestive Systems Indicate Different Ways of Living Fish have large pharynx with gill slits Air-breathing vertebrates have reduced pharynx Many vertebrates have teeth and chew food particles Birds lack teeth, break up food in two-chambered stomach Gizzard grinds material with small pebbles Seeds and hard materials ground up for digestion in second chamber Carnivores have shorter intestines than herbivores Most animal macromolecules are readily digested Herbivores eat cellulose, have convoluted intestines to prolong digestion Herbivorous mammals have multiple-chambered stomachs with cellulose-degrading bacteria FOOD ENTERS THE DIGESTIVE TRACT THROUGH THE MOUTH Teeth Are Important to Animal Digestion Capture food in different ways, teeth specialized for such capture Carnivores possess pointed teeth for capture, cutting and shearing Herbivores have large, flat teeth suited for grinding plant materials Omnivores have both types, front like carnivores, back like herbivores fig 45.5 Incisors: four front teeth, used for biting Canines: one on each side of incisors, used for tearing food Premolars: two on either side behind canines, chewing teeth Molars: three on either side behind canines, chewing teeth Food Is Moistened and Lubricated in the Mouth Tongue mixes food with saliva Saliva secreted by three pairs of salivary glands Empty through mucosal lining of mouth Contains salivary amylase to initiate breakdown of starch Secretion of saliva controlled by the nervous system Continuous secretion to keep the mouth moist Secretion stimulated by presence of food FOOD PASSES TO THE STOMACH THROUGH THE ESOPHAGUS Food Passes Beyond the Teeth to the Back of the Mouth Palate elevates, pushes against back wall of pharynx fig 45.6 Seals off nasal cavity Prevents entry of food into nasal cavity Pressure on pharynx stimulates receptors to signal swallowing center Swallowing center signals respiratory tract Inhibits respiration Seals trachea by raising larynx and closing glottis with epiglottis Food Enters Esophagus Connecting Pharynx and Stomach Upper portion of esophagus enveloped in skeletal muscle Lower two-thirds enveloped in smooth muscle Food propelled to stomach by peristaltic waves Exit of food from esophagus to stomach controlled by a sphincter Muscular constriction at junction of two organs Prevents food in stomach from re-entering esophagus Rodents and horses have true sphincter cannot regurgitate, humans can PRELIMINARY DIGESTION OCCURS IN THE STOMACH Stomach Is a Saclike Portion of the Digestive Tract fig 45.7 Interior of stomach is highly convoluted Folds up when empty, expands when full of food Carnivores that gorge sporadically can distend stomachs greatly Stomach has extra layer of smooth muscle to churn food Gastric Glands of Mucosa Are Exocrine Glands that Produce Secretions fig 45.8 Parietal cells secrete hydrochloric acid (HCl) Chief cells secrete pepsinogen, acid-loving ,weak protein-digesting enzyme Activated pepsinogen molecules cleave fragment from each other, make pepsin Pepsin is more active molecule Production of inactive molecule, converted to active enzyme outside Chemical generically called a zymogen Prevents chief cells from self-digestion Stomach produces 2 liters of acid and gastric secretions per day Produces pH of 2, compared to blood pH of 7.4 Low pH helps denature proteins, keeps pepsin active Proteins denatured into polypeptides Digestion to amino acids occurs in small intestine No digestion of carbohydrates or fats in stomach Chyme: mix of partly digested food and gastric juice Acid solution also kills bacteria ingested with food Overproduction of acids may occur In stomach, cause gastric ulcers Are rare due to protective alkaline mucus produced by mucosa Mucosal cells readily replaced when damaged Duodenal ulcers are more common Produced when excessive amount of acidic chyme delivered into duodenum Alkaline secretions of pancreas cannot neutralize chyme Parietal cells also produce intrinsic factor Polypeptide needed for intestinal absorption of vitamin B12 Required for formation of red blood cells Deficiency causes pernicious anemia Little absorption occurs in stomach, all other absorption in intestine A little water Substances like aspirin and alcohol TERMINAL DIGESTION AND ABSORPTION TAKE PLACE IN THE SMALL INTESTINE Food Passes From Stomach to Small Intestine Controlled by muscular pyloric sphincter fig 45.7 Capacity of small intestine limited, digestion takes time Relatively small amounts of chyme can enter at a time Coordination regulated by neural and hormonal signals Small Intestine Is the Primary Location of Digestion Length is approximately six meters Duodenum comprises first 25 centimeters, or 4% Jejunum and ileum comprise rest of small intestine Duodenum receives chyme, pancreatic enzymes, bile from liver and gallbladder Absorption occurs in all three regions of small intestine Digestion of Food in the Intestine Epithelial wall covered with small projections called villi fig 45.9 Epithelium of villi covered with microvilli, cytoplasmic projections fig 45.10 Seen clearly with electron microscope Epithelial wall also called brush border Both increase the absorptive surface of the small intestine Microvilli also participate in digestion Digestive enzymes embedded in epithelial cell plasma membranes fig 45.11 Brush border enzymes hydrolyze lactose, sucrose and others tbl 45.1 Adult humans lose ability to produce lactase Condition called lactose intolerance Absorption of Food in the Intestine Components of protein and carbohydrate digestion transported across brush border Amino acids and monosaccharides cross to intestinal epithelial cells fig 45.12 Transported across intestinal epithelium to capillaries in villi Blood carries digestion products to liver Travel via hepatic portal vein fig 45.13 Products of fat digestion absorbed by different mechanism fig 45.12 Fats hydrolyzed into fatty acids and monoglycerides Absorbed by intestinal epithelium Reassembled into triglycerides Combine with proteins to form water-soluble chylomicrons Absorbed into lymphatic capillaries, not hepatic portal system Contents of lymphatic system enter blood stream in veins near neck Total volume of food and water equals 2 liters (800 grams of solids) Body adds 7.0 liters of its own fluids making a total of 9.0 1.5 liters salivary enzymes 2.0 liters of gastric secretions 1.5 liters of pancreatic secretions 0.5 liters of bile from the liver 1.5 liters of intestinal secretions Nearly all fluids and solids are absorbed 8.5 liters reabsorbed in the small intestine 350 milliliters reabsorbed in the large intestine Only 50 grams of solids and 100 milliliters of liquid leave as feces Fluid absorption efficiency = 99% THE PANCREAS SECRETES ENZYMES, BICARBONATE AND HORMONES The Pancreas Makes Digestive Enzymes Pancreas located at junction of stomach and small intestine fig 45.2 Fluid secreted into duodenum via pancreatic duct Pancreas is thus an exocrine organ Fluid contains Protein digesting trypsin and chymotrypsin Starch digesting pancreatic amylase Fat digesting lipase Enzymes released primarily as zymogens, activated by brush border Also contains bicarbonate to neutralize HCl from stomach Chyme in intestine is slightly alkaline Bicarbonate produced by acini, clusters of secretory cells Pancreas Also Serves as an Endocrine Gland Produces hormones that regulate levels of blood sugar and other nutrients Produced in islets of Langerhans clustered throughout pancreas Most important hormones are insulin and glucagon THE LIVER PRODUCES BILE AND REGULATES BLOOD COMPOSITION Bile Production Liver is largest internal organ of body fig 45.2 Main secretion of liver is bile Mixture of bile pigments and bile salts delivered into duodenum Bile pigments do not participate in digestion Are waste products from liver's destruction of old red blood cells Eliminated with feces Accumulation of pigments result in jaundice Bile salts are lipid and water soluble Disperse fat droplets in chyme into emulsion of smaller droplets Emulsification increases surface area for lipase to work on Bile is stored and concentrated in gall bladder Fatty food in duodenum triggers contraction of gallbladder to release bile Regulation of Blood Composition Hepatic portal vein carries blood from stomach and intestine to liver fig 45.13 Liver absorbs or chemically modifies substances before they reach rest of body Ingested alcohol and drugs metabolized by liver cells Toxins, pesticides, carcinogens, poisons detoxified Ammonia from intestinal bacteria converted into urea Controls level of substances produced in body Steroid hormones converted into less active water-soluble forms Molecules included in bile, eliminated in feces or through kidneys Produces proteins found in blood plasma Includes most blood clotting factors Maintains blood protein concentration within narrow limits Imbalance can cause edema Regulation of Blood Glucose Levels Constant concentration of blood glucose must be maintained Brain cells totally dependent on blood for supply of glucose Brain cells store little glucose, cannot convert fat or amino acids into glucose Maintaining level requires active control by various body organs Vertebrates eat sporadically, ingestion followed by fasting Most food digested rapidly, metabolites enter blood stream Without control, level of metabolites would change drastically Liver removes glucose from blood, converts it into glycogen Glycogen stored in liver tissue and Also stored in skeletal muscle fibers, but can only be used in muscles Process stimulated by pancreatic hormone, insulin If blood glucose level is low, liver secretes glucose into blood Occurs between meals, during fasting Glucose partly obtained from breakdown of glycogen Conversion stimulated by glucagon, other pancreatic hormone Only liver can secrete glucose into blood Liver stores enough glycogen for 10 hours of fasting For greater fasting liver converts amino acids, lactic acid into glucose Process called gluconeogenesis Amino acids come from muscle protein THE LARGE INTESTINE CONCENTRATES SOLIDS Large Intestine or Colon Comprises Last Meter of Digestive Tract Has no digestive function, absorbs 4% of fluids Shorter in length than the small intestine Lies in three relatively straight segments Surface is not convoluted Inner surface lacks villi Significantly less surface area over which to absorb Absorb sodium, vitamin K, other products of bacterial metabolism Primary function is a refuse dump Undigested material compacted and stored Bacteria live and reproduce and are incorporated into feces Bacterial fermentation produces gas within the colon Human colon evolved to process food with high fiber content Low fiber diets result in slower passage of food through colon May be associated with high level of colon cancer in U. S. The rectum is the terminal portion of the large intestine Feces pass into rectum by peristaltic contractions Material exits anus through two sphincters First sphincter is smooth muscle, opens involuntarily Second sphincter is striated muscle, under voluntary control All Vertebrates Except Placental Mammals Possess a Cloaca A collective cavity for digestive, reproductive and urinary functions Placental mammals have separate exits for each tract NEURAL AND HORMONAL REGULATION OF DIGESTION Coordination by Nervous System Stimulates salivary and gastric secretions in response to sight and smell of food Food in stomach stimulates secretion of gastrin by stomach tbl 45.2 In turn stimulates secretion of pepsinogen and HCl in stomach fig 45.14 Negative feedback loop controls HCl secretions Decreased stomach pH reduces secretion of gastrin Decrease in gastrin decreases HCl production Passage of chyme from stomach inhibits stomach contractions No more chyme enters intestine until previous amount processed Process mediated by neural impulses and gastric inhibitory peptide (GIP) GIP released by duodenum Production stimulated most strongly by fat in chyme Fatty meals take longer to process Coordination by Hormones Cholecystokinin (CCK) is secreted in response to fat in chyme Stimulates contraction of gallbladder to release bile Bile emulsifies fats, increases efficiency of digestion Stimulates secretion of pancreatic digestive enzymes Secretin released in response to acidity of chyme Stimulates pancreas to release bicarbonate Neutralizes acidity of chyme First hormone ever discovered SYMBIOSIS WITHIN THE VERTEBRATE DIGESTIVE SYSTEM Bacterial Digestion of Cellulose Within Animals Vertebrates lack enzymes to digest plant material Some bacteria can do so and are harbored by animals fig 45.15 Plays relatively small role in human nutrition Essential nutrition for termites, cockroaches and some herbivores Cows and related ruminants possess two stomachs fig 45.16 First stomach has two chambers: rumen and reticulum Second stomach has two chambers, omasum and abomasum Capacity of rumen is 50 gallons Provides a fermentation vat for bacteria and protozoa to process cellulose Allows cows to regurgitate and rechew their food (cud) Re-chewed food swallowed, goes into reticulum, omasum and abomasum Abomasum released gastric juices Is equivalent to human stomach Leads to a very efficient digestion of cellulose Horses, rodents and lagomorphs retain bacteria in the caecum Cannot regurgitate material from caecum Rats and rabbits redigest cellulose another way Eat feces and literally redigest them a second time Efficiency approaches that of ruminants Additional Digestive Activity of Intestinal Bacteria Wax digested by bacteria in gut of honey guide birds Intestinal bacteria provide mammals with vitamin K Birds lack bacteria and must consume vitamin K in food Prolonged antibiotic treatment depletes bacteria Must supplement vitamin K until bacteria are re-established NUTRITION Ingestion of Food Has Dual Purpose Provides source of energy Provides raw materials the animal cannot manufacture for itself fig 45.17 Intake of food required to maintain glycogen stores in the liver Excess glucose metabolized by muscles or converted to fat Basal metabolic rate (BMR): rate energy is consumed at complete rest Food energy -(energy used at rest+exercise energy) = energy in glycogen and fat BMR is relatively constant within an individual Balance between food energy and exercise energy determines energy storage in fat Wealthy countries exhibit obesity from overeating and imbalanced diet Obese = 20% more than normal weight for a certain height fig 45.18 Obesity correlated with coronary heart disease, other disorders Obesity in children related to increase in number of fat cells Adult obesity related to change in size of fat cells, number does not decrease Essential Nutrients Over time many vertebrates have lost ability to synthesize substances Substances that cannot be manufactured must be obtained from diet Vitamins: essential organic substances required in trace amounts tbl 45.3 Humans, apes, monkeys, guinea pigs cannot make vitamin C Humans require at least thirteen vitamins Essential amino acids: eight of the total twenty Lysine, tryptophan, threonine, methionine, phenylalanine, leucine, isoleucine valine Must be obtained from proteins in food fig 45.19 Vertebrates synthesize cholesterol, insects cannot Essential minerals: calcium, phosphorus and other trace elements tbl 2.1