Bài giảng Biology - Chapter 41: Animal Nutrition

Chapter 41Animal NutritionOverview: The Need to FeedEvery mealtime is a reminder that we are heterotrophs Dependent on a regular supply of foodFigure 41.1In general, animals fall into one of three dietary categoriesHerbivores eat mainly autotrophs (plants and algae)Carnivores eat other animalsOmnivores regularly consume animals as well as plants or algal matterRegardless of what an animal eats, an adequate diet must satisfy three nutritional needsFuel for all cellular workThe organic raw materials for biosynthesisEssential nutrients, substances such as vitamins that the animal cannot make for itselfAnimals feed by four main mechanismsFigure 41.2BaleenSUSPENSION FEEDERSFecesSUBSTRATE FEEDERSBULK FEEDERSFLUID FEEDERSCaterpillarConcept 41.1: Homeostatic mechanisms manage an animal’s energy budgetNearly all of an animal’s ATP generationIs based on the oxidation of energy-rich molecules: carbohydrates, proteins, and fatsGlucose Regulation as an Example of HomeostasisAnimals store excess caloriesAs glycogen in the liver and muscles and as fatGlucose is a major fuel for cellsIts metabolism, regulated by hormone action, is an important example of homeostasisFigure 41.31 When blood glucose level rises, a gland called the pancreas secretes insulin,a hormone, into the blood. Insulin enhances the transport of glucose into body cells and stimulates the liver and muscle cells to store glucose as glycogen. As a result, blood glucose level drops.2STIMULUS:Blood glucoselevel risesafter eating.Homeostasis:90 mg glucose/100 mL bloodSTIMULUS:Blood glucoselevel dropsbelow set point. Glucagon promotesthe breakdown ofglycogen in theliver and therelease of glucoseinto the blood,increasing bloodglucose level.4 When blood glucose level drops, the pancreas secretes the hormone glucagon, which opposes the effect of insulin.3When fewer calories are taken in than are expendedFuel is taken out of storage and oxidizedCaloric ImbalanceUndernourishmentOccurs in animals when their diets are chronically deficient in caloriesCan have detrimental effects on an animalOvernourishmentResults from excessive food intakeLeads to the storage of excess calories as fatFigure 41.4100 µmObesity as a Human Health ProblemThe World Health OrganizationNow recognizes obesity as a major global health problemObesity contributes to a number of health problems, includingDiabetes, cardiovascular disease, and colon and breast cancerResearchers have discoveredSeveral of the mechanisms that help regulate body weightOver the long term, homeostatic mechanismsAre feedback circuits that control the body’s storage and metabolism of fatSeveral chemical signals called hormonesRegulate both long-term and short-term appetite by affecting a “satiety center” in the brainFigure 41.5Produced by adipose (fat) tissue, leptin suppresses appetite as its level increases. When body fat decreases, leptin levels fall, and appetite increases.LeptinPYYInsulinGhrelinSecreted by the stomach wall, ghrelin is one of the signals that triggers feelings of hunger as mealtimes approach. In dieters who lose weight, ghrelin levels increase, which may be one reason it’s so hard to stay on a diet.The hormone PYY, secreted by the small intestine after meals, acts as an appetite suppressant that counters the appetite stimulant ghrelin.A rise in blood sugar level after a meal stimulates the pancreas to secrete insulin (see Figure 41.3). In addition to its other functions, insulin suppresses appetite by acting on the brain.The complexity of weight control in humansIs evident from studies of the hormone leptinMice that inherit a defect in the gene for leptinBecome very obeseFigure 41.6Obesity and EvolutionThe problem of maintaining weight partly stems from our evolutionary pastWhen fat hoarding was a means of survivalA species of birds called petrelsBecome obese as chicks due to the need to consume more calories than they burnFigure 41.7Concept 41.2: An animal’s diet must supply carbon skeletons and essential nutrientsTo build the complex molecules it needs to grow, maintain itself, and reproduceAn animal must obtain organic precursors (carbon skeletons) from its foodBesides fuel and carbon skeletonsAn animal’s diet must also supply essential nutrients in preassembled formAn animal that is malnourishedIs missing one or more essential nutrients in its dietHerbivorous animalsMay suffer mineral deficiencies if they graze on plants in soil lacking key mineralsFigure 41.8MalnutritionIs much more common than undernutrition in human populationsEssential Amino AcidsAnimals require 20 amino acidsAnd can synthesize about half of them from the other molecules they obtain from their dietThe remaining amino acids, the essential amino acidsMust be obtained from food in preassembled formA diet that provides insufficient amounts of one or more essential amino acidsCauses a form of malnutrition called protein deficiencyFigure 41.9Most plant proteins are incomplete in amino acid makeupSo individuals who must eat only plant proteins need to eat a variety to ensure that they get all the essential amino acidsCorn (maize)and other grainsBeansand other legumesEssential amino acids for adults MethionineValineThreoninePhenylalanineLeucineIsoleucineLysineTryptophanFigure 41.10Some animals have adaptationsThat help them through periods when their bodies demand extraordinary amounts of proteinFigure 41.11Essential Fatty AcidsAnimals can synthesize most of the fatty acids they needThe essential fatty acidsAre certain unsaturated fatty acidsDeficiencies in fatty acids are rareVitaminsVitamins are organic moleculesRequired in the diet in small amountsTo date, 13 vitamins essential to humansHave been identifiedVitamins are grouped into two categoriesFat-soluble and water-solubleTable 41.1MineralsMinerals are simple inorganic nutrientsUsually required in small amountsMineral requirements of humansTable 41.2Concept 41.3: The main stages of food processing are ingestion, digestion, absorption, and eliminationIngestion, the act of eatingIs the first stage of food processingDigestion, the second stage of food processingIs the process of breaking food down into molecules small enough to absorbInvolves enzymatic hydrolysis of polymers into their monomersAbsorption, the third stage of food processingIs the uptake of nutrients by body cellsElimination, the fourth stage of food processingOccurs as undigested material passes out of the digestive compartmentThe four stages of food processingFigure 41.12Piecesof foodSmallmoleculesMechanicaldigestionFood Chemical digestion(enzymatic hydrolysis)Nutrient moleculesenter body cellsUndigested materialINGESTION1DIGESTION2ELIMINATION4ABSORPTION3Digestive CompartmentsMost animals process food In specialized compartmentsIntracellular DigestionIn intracellular digestionFood particles are engulfed by endocytosis and digested within food vacuolesExtracellular DigestionExtracellular digestionIs the breakdown of food particles outside cellsAnimals with simple body plansHave a gastrovascular cavity that functions in both digestion and distribution of nutrientsFigure 41.13 GastrovascularcavityFoodEpidermisMesenchymeGastrodermisMouthTentaclesMesenchymeFood vacuolesGland cellsFlagellaNutritivemuscularcellsAnimals with a more complex body planHave a digestive tube with two openings, a mouth and an anusThis digestive tubeIs called a complete digestive tract or an alimentary canalThe digestive tube can be organized into specialized regionsThat carry out digestion and nutrient absorption in a stepwise fashionEsophagusMouthPharynxCropGizzardIntestineAnusTyphlosoleLumen of intestineEsophagusAnusRectumMouthCropGastric cecaAnusIntestineGizzardCropStomachMouthEsophagusForegutMidgutHindgutEarthworm. The digestive tract ofan earthworm includes a muscular pharynx that sucks food in through themouth. Food passes through the esophagus and is stored and moistened in the crop. The muscular gizzard, whichcontains small bits of sand and gravel, pulverizes the food. Digestion and absorption occur in the intestine, which has a dorsal fold, the typhlosole, that increases the surface area for nutrient absorption.(b) Grasshopper. A grasshopper has several digestive chambers grouped into three main regions: a foregut, with an esophagus and crop; a midgut; and a hindgut. Food is moistened and stored in the crop, but most digestion occurs in the midgut. Gastric ceca, pouches extending from the midgut, absorb nutrients.(c) Bird. Many birds have three separate chambers—the crop, stomach, and gizzard—where food is pulverized and churned before passing into the intestine. A bird’s crop and gizzard function very much like those of an earthworm. In most birds, chemical digestion and absorption of nutrients occur in the intestine.Figure 41.14a–cConcept 41.4: Each organ of the mammalian digestive system has specialized food-processing functionsThe mammalian digestive system consists of the alimentary canalAnd various accessory glands that secrete digestive juices through ductsIIeumof small intestineDuodenum of small intestineAppendixCecumAscendingportion of large intestineAnusSmall intestineLarge intestineRectumLiverGall-bladderTongueOral cavityPharynxEsophagusStomachPyloricsphincterCardiacorificeMouthEsophagusSalivaryglandsStomachLiverPancreasGall-bladderLarge intestinesSmall intestinesRectumAnusParotid glandSublingual glandSubmandibular glandSalivaryglandsA schematic diagram of the human digestive systemPancreasFigure 41.15Food is pushed along the digestive tract by peristalsisRhythmic waves of contraction of smooth muscles in the wall of the canalThe Oral Cavity, Pharynx, and EsophagusIn the oral cavity, food is lubricated and digestion beginsAnd teeth chew food into smaller particles that are exposed to salivary amylase, initiating the breakdown of glucose polymersThe region we call our throat is the pharynxA junction that opens to both the esophagus and the windpipe (trachea)The esophagusConducts food from the pharynx down to the stomach by peristalsisFrom mouth to stomachEsophagusEpiglottis downTonguePharynxGlottisLarynxTracheaBolus of foodEpiglottisupTo lungsTo stomachEsophageal sphinctercontractedGlottis upand closedEsophageal sphincterrelaxedGlottisdown and openEsophageal sphinctercontractedEpiglottisupRelaxedmusclesContractedmusclesRelaxedmusclesStomachFigure 41.161 When a person is not swallowing, the esophageal sphincter muscle is contracted, the epiglottis is up, and the glottis is open, allowing air to flow through the trachea to the lungs. The swallowingreflex is triggeredwhen a bolus offood reaches thepharynx.2 The larynx, theupper part of therespiratory tract,moves upward andtips the epiglottisover the glottis,preventing foodfrom entering thetrachea.3 The esophagealsphincter relaxes,allowing thebolus to enter theesophagus.4After the foodhas entered theesophagus, thelarynx movesdownward andopens thebreathingpassage.5 Waves of muscularcontraction (peristalsis)move the bolus down the esophagus to the stomach.6The StomachThe stomach stores foodAnd secretes gastric juice, which converts a meal to acid chymeGastric juiceIs made up of hydrochloric acid and the enzyme pepsinThe lining of the stomachIs coated with mucus, which prevents the gastric juice from destroying the cellsFigure 41.17Pepsin (active enzyme)HClParietal cellChief cellStomachFolds of epithelial tissueEsophagusPyloric sphincterEpitheliumPepsinogen321 Interior surface of stomach.The interior surface of the stomach wall is highly folded and dotted with pits leading into tubular gastric glands.Gastric gland. The gastric glands have three types of cells that secrete different components of the gastric juice: mucus cells, chief cells, and parietal cells.Mucus cells secrete mucus,which lubricates and protectsthe cells lining the stomach.Chief cells secrete pepsino-gen, an inactive form of thedigestive enzyme pepsin.Parietal cells secretehydrochloric acid (HCl).1Pepsinogen and HCIare secreted into thelumen of the stomach.2HCl convertspepsinogen to pepsin.3Pepsin then activatesmore pepsinogen,starting a chainreaction. Pepsinbegins the chemicaldigestion of proteins.5 µmSmall intestineCardiac orificeGastric ulcers, lesions in the liningAre caused mainly by the bacterium Helicobacter pyloriFigure 41.181 µmBacteriaMucuslayer of stomachThe Small Intestine The small intestineIs the longest section of the alimentary canalIs the major organ of digestion and absorptionEnzymatic Action in the Small IntestineThe first portion of the small intestine is the duodenumWhere acid chyme from the stomach mixes with digestive juices from the pancreas, liver, gallbladder, and intestine itselfFigure 41.19LiverBileAcid chymeStomachPancreatic juicePancreasIntestinaljuiceDuodenum of small intestineGall-bladderThe pancreas produces proteases, protein-digesting enzymesThat are activated once they enter the duodenumPancreasMembrane-boundenteropeptidaseTrypsinActive proteasesLumen of duodenumInactivetrypsinogenOther inactiveproteasesFigure 41.20Enzymatic digestion is completedAs peristalsis moves the mixture of chyme and digestive juices along the small intestineFigure 41.21Oral cavity,pharynx,esophagusCarbohydrate digestionPolysaccharides(starch, glycogen)Disaccharides(sucrose, lactose)Salivary amylaseSmaller polysaccharides,maltoseStomachProtein digestionNucleic acid digestionFat digestionProteinsPepsinSmall polypeptidesLumen of small intes-tine PolysaccharidesPancreatic amylasesMaltose and otherdisaccharidesEpitheliumof smallintestine(brushborder)DisaccharidasesMonosaccharidesPolypeptidesPancreatic trypsin andchymotrypsin (These proteasescleave bonds adjacent to certainamino acids.)SmallerpolypeptidesPancreatic carboxypeptidaseAmino acidsSmall peptidesDipeptidases, carboxypeptidase, and aminopeptidase (These proteases split off one amino acid at a time, working from opposite ends of a polypeptide.)Amino acidsDNA, RNAPancreaticnucleasesNucleotidesNucleotidasesNucleosidesNucleosidasesandphosphatasesNitrogenous bases,sugars, phosphatesFat globules (Insoluble inwater, fats aggregate asglobules.)Bile saltsFat droplets (A coating ofbile salts prevents small drop-lets from coalescing intolarger globules, increasingexposure to lipase.)Pancreatic lipaseGlycerol, fattyacids, glyceridesHormones help coordinate the secretion of digestive juices into the alimentary canalFigure 41.22Amino acids or fatty acids in the duodenum trigger the release of cholecystokinin (CCK), which stimulates the release of digestive enzymes from the pancreas and bile from the gallbladder.LiverGall-bladderCCKEntero-gastroneGastrinStomachPancreasSecretinCCKDuodenumKeyStimulationInhibitionEnterogastrone secreted by the duodenum inhibits peristalsis and acid secretion by the stomach, thereby slowing digestion when acid chyme rich in fats enters the duodenum.Secreted by the duodenum, secretin stimulates the pancreas to release sodium bicarbonate, which neutralizes acid chyme from the stomach.Gastrin from the stomach recirculates via the bloodstream back to the stomach, where it stimulates the production of gastric juices.Absorption of NutrientsThe small intestine has a huge surface areaDue to the presence of villi and microvilli that are exposed to the intestinal lumenThe enormous microvillar surfaceIs an adaptation that greatly increases the rate of nutrient absorptionEpithelialcellsKeyNutrientabsorptionVein carrying blood to hepatic portal vesselVilliLargecircularfoldsIntestinal wallVilliEpithelial cellsLymph vesselBloodcapillariesLactealMicrovilli(brush border)Muscle layersFigure 41.23The core of each villusContains a network of blood vessels and a small vessel of the lymphatic system called a lactealAmino acids and sugarsPass through the epithelium of the small intestine and enter the bloodstreamAfter glycerol and fatty acids are absorbed by epithelial cellsThey are recombined into fats within these cellsThese fats are then mixed with cholesterol and coated with proteinsForming small molecules called chylomicrons, which are transported into lactealsFigure 41.24 Large fat globules are emulsified by bile salts in the duodenum.1 Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides, which then form micelles.2 Fatty acids and mono-glycerides leave micelles and enter epithelial cells by diffusion.3Fat globuleLactealEpithelialcells ofsmallintestineMicelles madeup of fatty acids,monoglycerides,and bile saltsFat dropletscoated withbile saltsBile salts Chylomicrons containing fattysubstances are transported out of the epithelial cells and into lacteals, where they are carried away from the intestine by lymph.4The Large IntestineThe large intestine, or colonIs connected to the small intestineFigure 41.25A major function of the colonIs to recover water that has entered the alimentary canalThe wastes of the digestive tract, the fecesBecome more solid as they move through the colonPass through the rectum and exit via the anusThe colon houses various strains of the bacterium Escherichia coliSome of which produce various vitaminsConcept 41.5: Evolutionary adaptations of vertebrate digestive systems are often associated with dietSome Dental AdaptationsDentition, an animal’s assortment of teethIs one example of structural variation reflecting dietMammals have specialized dentitionThat best enables them to ingest their usual dietFigure 41.26a–c(a) Carnivore(b) Herbivore(c) OmnivoreIncisorsCaninesPremolarsMolarsStomach and Intestinal AdaptationsHerbivores generally have longer alimentary canals than carnivoresReflecting the longer time needed to digest vegetationFigure 41.27CarnivoreHerbivoreColon(largeintestine)CecumStomachSmall intestineSmall intestineSymbiotic AdaptationsMany herbivorous animals have fermentation chambersWhere symbiotic microorganisms digest celluloseThe most elaborate adaptations for an herbivorous dietHave evolved in the animals called ruminantsFigure 41.28 Reticulum. Some boluses also enter the reticulum. In both the rumen and the reticulum, symbiotic prokaryotes and protists (mainly ciliates) go to work on the cellulose-rich meal. As by-products of theirmetabolism, the microorganisms secrete fatty acids. The cow periodically regurgitates and rechews the cud (red arrows), which further breaks down thefibers, making them more accessible to further microbial action. Rumen. When the cow first chews andswallows a mouthful of grass, boluses(green arrows) enter the rumen.1Intestine2 Omasum. The cow then reswallowsthe cud (blue arrows), which moves tothe omasum, where water is removed.3 Abomasum. The cud, containing great numbers of microorganisms, finally passes to the abomasum for digestion by the cow‘s own enzymes (black arrows).4Esophagus