LE 41-2aBaleenLE 41-2bCaterpillarFecesHomeostatic mechanismsmanage an animal’s energybudget• Nearly all of an animal’s ATP generation isbased on oxidation of energy-rich molecules:carbohydrates, proteins, and fatsGlucose Regulation as anExample of Homeostasis• Animals store excess calories as glycogenin the liver and muscles and as fat• Glucose is a major fuel for cells• Hormones regulate glucose metabolism• When fewer calories are taken in than areexpended, fuel is taken from storage andoxidizedLE 41-3STIMULUS:Blood glucoselevel risesafter eating.STIMULUS:Blood glucoselevel dropsbelow set point.Homeostasis:90 mg glucose/100 mL bloodAn animal’s diet must supplycarbon skeletons and essentialnutrients• An animal must obtain carbon skeletonsfrom its food to build complex molecules• Besides fuel and carbon skeletons, adiet must supply essential nutrients inpreassembled form• A malnourished animal is missing oneor more essential nutrients in its dietHow do we know whichnutrients are essential?Beriberi• Japanese sailors around the turn of thecentury fell victim, in large numbers, tothis disease• Characterized by emaciation,weakness, emotional disturbance, andeventually heart failureBeriberi• Sailors adhering to the British naval dietdidn’t develop the disease• In a Manila prison, prisoners that hadbeen switched from the crude brownrice diet to a more pleasing polishedwhite rice diet developed beriberi• Beriberi is caused by a lack of thevitamin Thiamine• Scurvy is caused by a lack of Vitamin CLE 41-10MethionineValineThreoninePhenylalanineLeucineIsoleucineTryptophanLysineEssential amino acids for adultsBeansand otherlegumesCorn (maize)and other grainsLE 41-12Pieces of foodChemical digestion(enzymatic hydrolysis)FoodNutrientmoleculesenter bodycellsSmallmoleculesUndigestedmaterialELIMINATIONABSORPTIONDIGESTIONINGESTIONMechanicaldigestionDigestive Compartments• Most animals process food inspecialized compartments• These compartments reduce risk ofself-digestionIntracellular Digestion• In intracellular digestion, food particlesare engulfed by endocytosis anddigested within food vacuolesExtracellular Digestion• Extracellular digestion is the breakdownof food particles outside of cells• It occurs in compartments that arecontinuous with the outside of theanimal’s body• Animals with simple body plans have agastrovascular cavity that functions in bothdigestion and distribution of nutrientsLE 41-13GastrovascularcavityMouthFoodTentaclesEpidermisGastrodermisMesogleaGland cellsFlagellaNutritivemuscularcellsFood vacuolesMesoglea• More complex animals have a digestive tube withtwo openings, a mouth and an anus• This digestive tube is called a complete digestivetract or an alimentary canal• It can have specialized regions that carry outdigestion and absorption in a stepwise fashionLE 41-14aGizzardIntestineAnusCropEsophagusPharynxMouthEarthwormTyphlosoleLumen of intestineLE 41-14bForegutMidgut HindgutRectumAnusEsophagusMouthCropGastric cecaGrasshopperLE 41-14cAnusEsophagusMouthCropBirdStomachGizzardIntestineEach organ of the mammaliandigestive system has specializedfood-processing functions• The mammalian digestive system consists of analimentary canal and accessory glands thatsecrete digestive juices through ducts• Mammalian accessory glands are the salivaryglands, the pancreas, the liver, and the gallbladder• Food is pushed along by peristalsis, rhythmiccontractions of muscles in the wall of the canalLE 41-15aEsophagusPharynxOral cavityStomachPyloricsphincterCardiacorificeLiverTongueParotid glandSublingual glandSubmandibular glandSalivaryglandsAscendingportion oflarge intestineGall-bladderPancreasIleumof small intestineRectumAnusAppendixCecumLargeintestineSmallintestineDuodenum ofsmall intestineLE 41-15bEsophagusStomachLiverSalivaryglandsGall-bladderPancreasRectumAnusLargeintestinesSmallintestinesMouthA schematic diagram of thehuman digestive systemThe Oral Cavity, Pharynx, andEsophagus• In the oral cavity, food is lubricated anddigestion begins• Teeth chew food into smaller particlesthat are exposed to salivary amylase,initiating breakdown of glucosepolymers• The region we call our throat is the pharynx, ajunction that opens to both the esophagusand the windpipe (trachea)• The esophagus conducts food from thepharynx down to the stomach by peristalsisLE 41-16_3EpiglottisupBolus of foodEsophagealsphinctercontractedEsophagusTo stomachTo lungsTracheaTonguePharynxGlottisLarynxEsophagealsphincterrelaxedEpiglottisdownGlottis upand closedEpiglottisupEsophagealsphinctercontractedRelaxedmusclesGlottis downand openRelaxedmusclesContractedmusclesStomachThe Stomach• The stomach stores food and secretesgastric juice, which converts a meal to acidchyme• Gastric juice is made up of hydrochloric acidand the enzyme pepsin• Pepsin is secreted as inactive pepsinogen;pepsin is activated when mixed withhydrochloric acid in the stomach• Mucus protects the stomach lining fromgastric juiceLE 41-17EsophagusCardiac orificePyloric sphincterSmallintestineFolds ofepithelialtissueStomachEpitheliumPepsin(active enzyme)PepsinogenHClPepsinogen and HClare secreted into the lumen of the stomach.HCl convertspepsinogen to pepsin.Pepsin then activatesmore pepsinogen,starting a chainreaction. Pepsinbegins the chemicaldigestion of proteins.Parietal cellChief cellChief cellsMucus cellsParietal cellsInterior surface of stomachGastric gland5 µmThe Small Intestine• The small intestine is the longestsection of the alimentary canal• It is the major organ of digestion andabsorptionEnzymatic Action in the SmallIntestine• The first portion of the small intestine is theduodenum, where acid chyme from thestomach mixes with digestive juices from thepancreas, liver, gallbladder, and the smallintestine itselfLE 41-19StomachPancreasLiverGall-bladderDuodenum ofsmall intestineIntestinaljuiceBileAcid chymePancreaticjuice• The pancreas produces proteases,protein-digesting enzymes that areactivated after entering the duodenumLE 41-20PancreasLumen of duodenumMembrane-boundenteropeptidaseInactivetrypsinogenOther inactiveproteasesActiveproteasesTrypsin• The liver produces bile, which aids in digestionand absorption of fats• The epithelial lining of the duodenum, called thebrush border, produces several digestive enzymes• Enzymatic digestion is completed as peristalsismoves the chyme and digestive juices along thesmall intestineLE 41-21Oral
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