Chapter 41. Animal Nutrition. Concept 41.1 An animal s diet must supply chemical energy, organic molecules, and essential nutrients.

Transcription

1 Chapter 41 Animal Nutritin Lecture Outline Overview: The Need t Feed All animals eat ther rganisms dead r alive, whle r by the piece (including parasites). Animal nutritin includes the ingestin, breakdwn, and absrptin f fd. In general, animals fit int ne f three dietary categries. 1. Herbivres, such as cattle, parrtfish, and termites, eat mainly plants r algae. 2. Carnivres, such as sharks, hawks, and spiders, eat ther animals. 3. Omnivres, such as cckraches, bears, and humans, cnsume animal and plant r algal matter. The terms herbivre, carnivre, and mnivre represent the kinds f fd that an animal usually eats, but mst animals are pprtunistic, ccasinally eating fds that are utside their main dietary categry. Fr example, cattle and deer, which are herbivres, ccasinally eat small animals r bird eggs. All animals cnsume bacteria alng with ther types f fd. T survive and reprduce, animals must balance their cnsumptin, strage, and use f fd. Cncept 41.1 An animal s diet must supply chemical energy, rganic mlecules, and essential nutrients. An animal s diet prvides chemical energy that can be cnverted t ATP t pwer living prcesses. In additin t fuel fr ATP prductin, an animal s diet must supply the raw materials needed fr bisynthesis. Animals require a surce f rganic carbn and a surce f rganic nitrgen in rder t cnstruct their wn rganic mlecules. Materials that an animal s cells require but cannt synthesize are called essential nutrients. Essential nutrients, which must be btained frm an animal s diet, include bth minerals and preassembled rganic mlecules. Sme nutrients are essential fr all animals, whereas thers are required fr nly certain species. Overall, an adequate diet must supply chemical energy fr cellular prcesses, rganic mlecules as building blcks fr macrmlecules, and essential nutrients. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 41-1

2 There are fur classes f essential nutrients: essential amin acids, essential fatty acids, vitamins, and minerals. Essential amin acids are thse an animal cannt synthesize. Mst animals require eight amin acids in their diet. A diet that des nt prvide sufficient amunts f ne r mre essential amin acids leads t prtein deficiency, the mst cmmn type f malnutritin. Animal prteins are cmplete, prviding all the essential amin acids in their prper prprtins. Mst plant prteins are incmplete, deficient in ne r mre essential amin acids. Animals can synthesize mst f the fatty acids they need. Essential fatty acids, the nes that animals cannt synthesize, are unsaturated. Vitamins are rganic mlecules with diverse functins that are required in the diet in quantities that are quite small cmpared with the relatively large quantities f essential amin acids and fatty acids that animals need. Althugh vitamins are required in tiny amunts frm abut 0.01 mg t 100 mg per day depending n the vitamin, vitamin deficiency (r verdse in sme cases) can cause serius prblems. S far, 13 vitamins essential t humans have been identified. Vitamins can be gruped int water-sluble vitamins and fat-sluble vitamins, with extremely diverse physilgical functins. One water-sluble vitamin is the B cmplex, which cnsists f several cmpunds that functin as cenzymes in key metablic prcesses. Vitamin C, als water-sluble, is required fr the prductin f cnnective tissue. Excessive amunts f water-sluble vitamins are excreted in urine, and mderate verdses are prbably harmless. The fat-sluble vitamins are A, D, E, and K. They have a wide variety f functins. Vitamin A is incrprated in the visual pigments f the eye. Vitamin D aids in calcium absrptin and bne frmatin. Vitamin E seems t prtect membrane phsphlipids frm xidatin. Vitamin K is required fr bld cltting. Excess amunts f fat-sluble vitamins are nt excreted but are depsited in bdy fat. Overcnsumptin may lead t txic accumulatins f these cmpunds. The subject f vitamin dsage has arused heated scientific and ppular debate. Sme believe that it is sufficient t meet the recmmended daily allwances (RDAs), the nutrient intake prpsed by nutritinists t maintain health. Others argue that the RDAs are set t lw fr sme vitamins, and a fractin f these peple believe, prbably mistakenly, that massive dses f vitamins cnfer health benefits. Debate centers n the ptimal dses f vitamins C and E. Research is nging, but all that can be said with any certainty is that peple wh eat a balanced diet are nt likely t develp symptms f vitamin deficiency. Minerals are simple inrganic nutrients, usually required in small amunts frm less than 1 mg t abut 2,500 mg per day. Mineral requirements vary with animal species. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 41-2

3 Humans and ther vertebrates require relatively large quantities f calcium and phsphrus fr the cnstructin and maintenance f bne. Calcium is als necessary fr the nrmal functining f nerves and muscles. Phsphrus is a cmpnent f the cytchrmes that functin in cellular respiratin. Irn is a cmpnent f the cytchrmes that functin in cellular respiratin and f hemglbin, the xygen-binding prtein f red bld cells. Magnesium, irn, zinc, cpper, manganese, selenium, and mlybdenum are cfactrs built int the structure f certain enzymes. Magnesium, fr example, is present in enzymes that split ATP. Idine is required fr thyrid hrmnes, which regulate metablic rate. Sdium, ptassium, and chlride are imprtant in nerve functin and have a majr influence n the smtic balance between cells and interstitial fluids. Excess cnsumptin f sme minerals can upset the hmestatic balance and cause txic effects. Liver damage due t irn accumulatin affects up t 10% f the ppulatin in regins f Africa that have an irn-rich water supply. Excess cnsumptin f salt (sdium chlride) may cntribute t high bld pressure. The average U.S. citizen eats enugh salt t prvide abut 20 times the required amunt f sdium. When an animal is undernurished, it uses up stred fat and carbhydrates, and the bdy begins breaking dwn its wn prteins fr fuel. Muscles begin t decrease in size, and the brain can becme prtein-deficient. If energy intake remains less than energy expenditure, death will eventually result. Even if a seriusly undernurished persn survives, sme damage may be irreversible. Because a diet f a single staple such as rice r crn can ften prvide sufficient calries, undernurishment is generally cmmn when drught, war, r sme ther crisis severely disrupts the fd supply. Anther cause f undernurishment is anrexia nervsa, an eating disrder assciated with cmpulsive starvatin despite fd availability. The ptential effects f malnurishment include defrmities, disease, and even death. Fr example, cattle, deer, and ther herbivres may develp fragile bnes if they graze in areas where sils and plants are deficient in phsphrus. Sme grazing animals btain the missing nutrients by cnsuming cncentrated surces f salt r ther minerals. Recent experiments with spiders have fund that carnivres can adjust fr dietary deficiencies by switching t prey that restres their nutritinal balance. Humans can suffer frm malnurishment. Peple subsisting n a rice diet may suffer frm vitamin A deficiency, which can cause blindness r death. A genetically engineered strain f Glden Rice synthesizes beta-cartene, a surce f vitamin A. It is difficult t determine an ideal human diet. Humans are genetically diverse and live in varied settings. Ethical cncerns preclude experimenting n the nutritinal needs f children. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 41-3

4 Researchers study genetic defects that disrupt fd uptake, strage, r use. Hemchrmatsis causes a buildup f irn in the absence f excess irn cnsumptin. By studying this disease, scientists have learned abut the genes that regulate irn absrptin. Insights int human nutritin have cme frm epidemilgy, the study f human health and disease at the ppulatin level. Epidemilgists aim t identify nutritinal strategies fr the preventin and cntrl f diseases and disrders. In the 1970s, researchers fund that wmen f lw sciecnmic status are mre likely t have children with neural tube defects, which ccur when tissue fails t enclse the develping brain and spinal crd. Richard Smithells f the University f Leeds fund that flic acid (vitamin B 9) supplements greatly reduce the risk f neural tube defects. In 1998, the FDA began t require that flic acid be added t the enriched grain used t make bread and cereal. The frequency f neural tube defects has been significantly reduced as a result. Cncept 41.2 The main stages f fd prcessing are ingestin, digestin, absrptin, and eliminatin. Fd prcessing by animals can be divided int fur distinct stages: ingestin, digestin, absrptin, and eliminatin. Ingestin, the act f eating, is the first stage f fd prcessing. Fd can be ingested in many liquid and slid frms. Digestin, the secnd stage f fd prcessing, is the prcess f breaking fd dwn int mlecules small enugh fr the bdy t absrb. Fd must be brken dwn because animals cannt directly use the macrmlecules in fd. They are t large t pass thrugh the cell membranes t enter the cells f the animal. In additin, the prteins, carbhydrates, nucleic acids, fats, and phsphlipids in fd are nt identical t thse an animal makes itself. Digestin cleaves macrmlecules int their cmpnent mnmers, which the animal then uses t make its wn mlecules r as fuel fr ATP prductin. Digestin reverses the prcess that a cell uses t link tgether mnmers t frm macrmlecules. Rather than remving a mlecule f water fr each new cvalent bnd frmed, digestin breaks bnds with the additin f water via enzymatic hydrlysis. A variety f hydrlytic enzymes catalyze the digestin f each f the classes f macrmlecules fund in fd. Plysaccharides and disaccharides are split int simple sugars. Fats are digested t glycerl and fatty acids. Prteins are brken dwn int amin acids. Nucleic acids are cleaved int nucletides. Chemical digestin is usually preceded by mechanical fragmentatin f the fd by chewing, fr instance. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 41-4

5 Breaking fd int smaller pieces increases the surface area expsed t digestive juices cntaining hydrlytic enzymes. After the fd is digested, the animal s cells take up small mlecules such as amin acids and simple sugars frm the digestive cmpartment, a prcess called absrptin. During eliminatin, undigested material passes ut f the digestive cmpartment. T avid digesting their wn cells and tissues, mst rganisms carry ut digestin in specialized cmpartments. The simplest digestive cmpartments are fd vacules, rganelles in which hydrlytic enzymes break dwn fd withut digesting the cell s wn cytplasm, a prcess termed intracellular digestin. This prcess begins after a cell has engulfed fd by phagcytsis r pincytsis. Newly frmed fd vacules fuse with lyssmes, which are rganelles cntaining hydrlytic enzymes. A few animals, such as spnges, digest their fd entirely by this mechanism. In mst animals, at least sme hydrlysis ccurs by extracellular digestin, the breakdwn f fd utside cells. Extracellular digestin ccurs within cmpartments that are cntinuus with the utside f the animal s bdy. Thus, rganisms can devur much larger prey than can be ingested by phagcytsis. Many animals with simple bdy plans have digestive sacs with single penings, called gastrvascular cavities. Gastrvascular cavities functin in bth the digestin and distributin f nutrients thrughut the bdy. Fr example, the cnidarians called hydras capture their prey with nematcysts and use tentacles t stuff the prey thrugh the muth int the gastrvascular cavity. The prey is then partially digested by enzymes secreted by specialized gland cells f the gastrdermis. Nutritive muscular cells in the gastrdermis engulf the fd particles. Mst f the actual hydrlysis f macrmlecules ccurs intracellularly. Undigested materials are eliminated thrugh the muth. In cntrast t cnidarians and flatwrms, mst animals have digestive tubes extending between a muth and anus. These digestive tubes are called cmplete digestive tracts, r alimentary canals. Because fd mves in ne directin, the tube can be rganized int specialized regins that carry ut digestin and nutrient absrptin in a stepwise fashin. In additin, animals with alimentary canals can eat mre fd befre the earlier meal is cmpletely digested. Cncept 41.3 Organs specialized fr successive stages f fd prcessing frm the mammalian digestive system. The general principles f fd prcessing are similar fr a diversity f animals, including mammals. We will use the mammalian system as a representative example. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 41-5

6 The mammalian digestive system cnsists f the alimentary canal and varius accessry glands that secrete digestive juices int the canal thrugh ducts. The accessry glands include the salivary glands, the pancreas, the liver, and the gallbladder. Peristalsis, rhythmic waves f cntractin by smth muscles in the walls f the canal, pushes fd alng. Sphincters, muscular ring-like valves, regulate the passage f material between specialized chambers f the canal. The ral cavity, pharynx, and esphagus initiate fd prcessing. Bth physical and chemical digestin f fd begins in the muth, r ral cavity. During chewing, teeth f varius shapes cut, smash, and grind fd, making it easier t swallw and increasing its surface area. The presence f fd in the ral cavity triggers a nervus reflex that causes the salivary glands t deliver saliva thrugh ducts t the ral cavity. Salivatin may ccur in anticipatin f fd because f learned assciatins between eating and the time f day, cking drs, r ther stimuli. Chemical digestin f carbhydrates, a main surce f chemical energy, begins in the ral cavity. Saliva cntains amylase, an enzyme that hydrlyzes starch and glycgen int smaller plysaccharides and the disaccharide maltse. Saliva cntains a slippery glycprtein called mucin, which prtects the sft lining f the muth frm abrasin and lubricates the fd fr easier swallwing. Saliva als cntains buffers that help prevent tth decay by neutralizing acid in the muth. Antibacterial agents in saliva, such as lyszyme, kill micrbes that enter the muth with fd. The tngue tastes fd, manipulates it during chewing, and helps shape the fd int a ball called a blus. During swallwing, the tngue pushes a blus back int the ral cavity and int the pharynx. The pharynx, als called the thrat, is a junctin that pens t bth the esphagus and the trachea (windpipe). When we swallw, the tp f the windpipe mves up s that its pening, the glttis, is blcked by a cartilaginus flap, the epiglttis. This mechanism nrmally ensures that a blus will be guided int the entrance f the esphagus and nt directed dwn the windpipe. If fd r liquid enters and blcks the windpipe, the material can be disldged by vigrus cughing r a frced upward thrust f the diaphragm (the Heimlich maneuver). The esphagus cntains bth striated and smth muscle. The striated muscle at the tp f the esphagus is active during swallwing. In the rest f the esphagus, smth muscle functins in peristalsis, as rhythmic cycles f cntractin mve the blus t the stmach. The stmach stres fd and perfrms preliminary digestin. The stmach is in the upper abdminal cavity, just belw the diaphragm. With accrdin-like flds and a very elastic wall, the stmach can stretch t accmmdate abut 2 L f fd and fluid, string an entire meal. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 41-6

7 The stmach secretes a digestive fluid called gastric juice and mixes this secretin with the fd by the churning actin f the smth muscles in the stmach wall. The mixture f ingested fd and digestive juices is called chyme. Tw cmpnents f gastric juice carry ut chemical digestin in the stmach. One cmpnent f gastric juice is hydrchlric acid (HCl), which disrupts the extracellular matrix that binds cells tgether. With a high cncentratin f hydrchlric acid, the gastric juice has a ph f abut 2 acidic enugh t digest irn nails. This lw ph kills mst bacteria that are swallwed with fd. It als denatures prteins in fd, increasing expsure f their peptide bnds. The secnd cmpnent f gastric juice is pepsin, an enzyme that begins the hydrlysis f prteins. Pepsin, which wrks well in strngly acidic envirnments, is a prtease that breaks peptide bnds adjacent t specific amin acids, prducing smaller plypeptides. Cells in the gastric glands f the stmach prduce the cmpnents f gastric juice. Parietal cells secrete hydrchlric acid in the frm f hydrgen and chlride ins, using an ATP-driven pump. Meanwhile, chief cells release pepsin int the lumen in an inactive frm called pepsingen. HCl in the lumen f the stmach cnverts pepsingen t active pepsin by clipping ff a small prtin f the mlecule t expse its active site. In a psitive-feedback system, activated pepsin activates mre pepsingen mlecules. Because HCl and pepsin frm in the lumen f the stmach, nt within the cells f the gastric glands, the stmach s cells are prtected frm self-digestin. The stmach s secnd line f defense against self-digestin is a cating f mucus, secreted by the epithelial cells, that prtects the stmach lining. Still, the epithelium is cntinuusly erded, and the epithelium is cmpletely replaced by mitsis every three days. Gastric ulcers, lesins in the stmach lining, are caused by the acid-tlerant bacterium Helicbacter pylri. Ulcers are ften treated with antibitics. The discvery that ulcers are caused by a bacterial infectin, nt by stress, earned Barry Marshall and J. Rbin Warren the Nbel Prize in Abut every 20 secnds, the stmach cntents are mixed by the churning actin f smth muscles. As a result f mixing and enzyme actin, what begins in the stmach as a recently swallwed meal becmes nutrient-rich chyme. Mst f the time, the stmach is clsed ff at bth ends. The pening frm the esphagus t the stmach nrmally dilates nly when a blus driven by peristalsis arrives. The ccasinal backflw f acid chyme frm the stmach int the lwer esphagus knwn as acid reflux causes the irritatin f the esphagus called heartburn. The sphincter at the pening frm the stmach t the small intestine helps regulate the passage f chyme int the intestine. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 41-7

8 A squirt at a time, it takes abut 2 t 6 hurs after a meal fr the stmach t empty. The small intestine is the majr rgan f digestin and absrptin. With a length f mre than 6 m in humans, the small intestine is the lngest sectin f the alimentary canal. Mst f the enzymatic hydrlysis f fd macrmlecules and mst f the absrptin f nutrients int the bld ccur in the small intestine. In the dudenum, the first 25 cm r s f the small intestine, chyme frm the stmach mixes with digestive juices frm the pancreas, liver, gallbladder, and gland cells f the intestinal wall. Hrmnes released by the stmach and dudenum cntrl the digestive secretins int the alimentary canal. The pancreas prduces several enzymes and an alkaline slutin rich in bicarbnate that acts as a buffer t neutralize the acidity f chyme frm the stmach. Pancreatic enzymes include trypsin and chymtrypsin, prteases that are secreted int the dudenum in inactive frm. The pancreatic prteases are activated nce they are in the extracellular space within the dudenum. The liver perfrms a wide variety f imprtant functins in the bdy, including the prductin f bile. Bile cntains bile salts that act as detergents that aid in the digestin and absrptin f fats. Bile is stred and cncentrated in the gallbladder. The liver als breaks dwn txins that enter the bdy and helps balance nutrient utilizatin. Bile als cntains pigments that are by-prducts f red bld cell destructin in the liver. These bile pigments are eliminated frm the bdy with the feces. The brush brder f the epithelial lining f the dudenum prduces several digestive enzymes. Sme enzymes are secreted int the lumen, while thers are bund t the surface f the epithelial cells. Enzymatic digestin is cmpleted as peristalsis mves the mixture f chyme and digestive juices alng the small intestine. Mst digestin is cmpleted while the chyme is still in the dudenum. The remaining regins f the small intestine, the jejunum and ileum, functin mainly in the absrptin f nutrients and water. T enter the bdy, nutrients in the lumen must pass the lining f the digestive tract. A few nutrients are absrbed in the stmach and large intestine, but mst absrptin takes place in the small intestine. The enrmus surface area f the small intestine is an adaptatin that greatly increases the rate f nutrient absrptin. The small intestine has a surface area f 300 m 2, rughly the size f a tennis curt. Large circular flds in the lining bear fingerlike prjectins called villi. Each epithelial cell f a villus has many micrscpic appendages called micrvilli that are expsed t the intestinal lumen. The micrvilli are the basis f the term brush brder fr the intestinal epithelium. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 41-8

9 In sme cases, transprt f nutrients acrss the epithelial cells is passive, as mlecules mve dwn their cncentratin gradients frm the lumen f the intestine int the epithelial cells, and then int capillaries. Fructse, a simple sugar, mves by facilitated diffusin dwn its cncentratin gradient frm the lumen f the intestine int the epithelial cells and then int capillaries. Amin acids, small peptides, vitamins, and mst glucse mlecules are pumped against cncentratin gradients by the epithelial cells f the villus. This active transprt allws much mre absrptin f nutrients than wuld be pssible with passive diffusin. Mnglycerides and mst fatty acids are absrbed by epithelial cells and recmbined int triglycerides within the cells. The fats are mixed with chlesterl and cated with special prteins t frm small glbules called chylmicrns. Chylmicrns are transprted by excytsis ut f epithelial cells and int lacteals, lymph vessels at the cre f each villus. The lacteals cnverge int the larger vessels f the lymphatic system, eventually draining int large veins that return bld t the heart. The capillaries and veins that drain nutrients away frm the villi cnverge int the hepatic prtal vein, which leads directly t the liver. Therefre, the liver, which has the metablic versatility t intercnvert varius rganic mlecules, has first access t the amin acids and sugars absrbed after a meal is digested. The liver mdifies and regulates this varied mix befre releasing materials back int the bldstream. The liver helps regulate the levels f glucse in the bld, ensuring that the bld exiting the liver usually has a glucse cncentratin very clse t 90 mg per 100 ml, regardless f the carbhydrate cntent f the meal. The liver als remves txic substances befre the bld circulates bradly. Frm the liver, bld travels t the heart, which pumps the bld and nutrients t all parts f the bdy. Absrptin f water is the majr functin f the large intestine. The large intestine, r cln, is cnnected t the small intestine at a T-shaped junctin where a sphincter cntrls the mvement f materials. One arm f the T is a puch called the cecum. The relatively small cecum f humans has a fingerlike extensin, the appendix, which makes a minr cntributin t bdy defense. The main branch f the human cln is shaped like an upside-dwn U, abut 1.5 m lng. A majr functin f the cln is t recver water that has entered the alimentary canal as the slvent in varius digestive juices. Abut 7 L f fluid are secreted int the lumen f the digestive tract f a persn each day. Mre than 90% f the water is reabsrbed in the small intestine and the cln. Because there is n bilgical mechanism fr the active transprt f water, water absrptin in the cln ccurs by smtically driven bulk flw as ins, particularly sdium, are pumped ut f the lumen. Digestive wastes, the feces, becme mre slid as they are mved alng the cln by peristalsis. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc. 41-9

10 It takes 12 t 24 hurs fr material t travel the length f the rgan. If the lining f the cln is irritated by a viral r bacterial infectin, less water than usual is resrbed, resulting in diarrhea. If t much water is absrbed because peristalsis mves the feces t slwly, the result is cnstipatin. Living in the large intestine is a rich flra f mstly harmless bacteria, which make up apprximately ne-third f the dry weight f feces. One f the mst cmmn inhabitants f the human cln is Escherichia cli, a favrite research rganism. Because E. cli is s cmmn in human digestive systems, its presence in lakes and streams is an indicatr f cntaminatin by untreated sewage. As a by-prduct f their metablism, many cln bacteria generate gases, including methane and hydrgen sulfide. These gases and ingested air are expelled thrugh the anus. Sme bacteria prduce vitamins, including bitin, flic acid, vitamin K, and several B vitamins, which supplement ur dietary intake f vitamins. Feces cntain masses f bacteria and undigested materials, including cellulse. Althugh cellulse fibers have n calric value t humans, their presence in the diet helps mve fd alng the digestive tract. The terminal prtin f the cln is called the rectum, where feces are stred until they can be eliminated. Between the rectum and the anus are tw sphincters, ne invluntary and ne vluntary. Once r mre each day, strng cntractins f the cln create an urge t defecate. Cncept 41.4 Evlutinary adaptatins f vertebrate digestive systems crrelate with diet. The variety f digestive systems crrelates with diet. The digestive systems f mammals and ther vertebrates are variatins n a cmmn plan. The many intriguing varieties f digestive systems are ften assciated with the animal s diet. Dentitin, an animal s assrtment f teeth, is ne example f structural variatin reflecting diet. Mammals evlutinary adaptatin f teeth fr prcessing different kinds f fd is ne f the majr reasns they have been s successful. Nnmammalian vertebrates generally have less specialized dentitin, but there are exceptins. Fr example, pisnus snakes, such as rattlesnakes, have fangs, mdified teeth that inject venm int prey. Sme snakes have hllw fangs, like syringes, while thers drip pisn alng grves in the tth surface. All snakes have anther imprtant anatmical adaptatin fr feeding, the ability t swallw large prey whle. The lwer jaw f a snake is lsely hinged t the skull by an elastic ligament that permits the muth and thrat t pen very wide fr swallwing. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc

11 Large, expandable stmachs are cmmn in carnivres, which may g fr a lng time between meals and, therefre, must eat as much as they can when they d catch prey. Fr example, a 200-kg African lin can cnsume 40 kg f meat in ne meal. The length f the vertebrate digestive system is als crrelated with diet. In general, herbivres and mnivres have lnger alimentary canals relative t their bdy sizes than d carnivres, prviding mre time fr digestin and mre surface areas fr absrptin f nutrients. Vegetatin is mre difficult t digest than meat because it cntains cell walls. A lnger digestive tract allws mre time fr digestin and mre surface area fr absrptin. Symbitic micrrganisms help nurish many vertebrates. Much f the chemical energy in the diet f herbivrus animals is cntained in the cellulse f plant cell walls. Animals d nt prduce enzymes that hydrlyze cellulse, hwever, s many vertebrates (and termites) huse large ppulatins f symbitic bacteria and prtists in special fermentatin chambers in their alimentary canals. These micrbes have enzymes that can digest cellulse t simple sugars that the animal can absrb. The lcatin f symbitic micrbes in herbivres digestive tracts varies depending n the species. The hatzin, an herbivrus bird that lives in Suth American rain frests, has a large, muscular crp (an esphageal puch) that huses symbitic micrbes. Many herbivrus mammals, including hrses, huse symbitic micrbes in a large cecum, the puch where the small and large intestines cnnect. The symbitic bacteria f rabbits and sme rdents live in the large intestine and cecum. Since mst nutrients are absrbed in the small intestine, these herbivres recver nutrients frm fermentatin in the large intestine by cprphagy, eating sme f their feces and passing fd thrugh the alimentary canal a secnd time. The kala als has an enlarged cecum, where symbitic bacteria ferment finely shredded eucalyptus leaves. The mst elabrate adaptatins fr an herbivrus diet have evlved in the ruminants, which include deer, cattle, and sheep. Adaptatins related t digestin are widespread amng multicellular animals. The giant tubewrms that live at deep-sea hydrthermal vents thrive at pressures f 260 atmspheres and temperatures f C. These wrms have n muth r digestive systems. Instead, they rely n symbitic bacteria t generate energy and nutrients frm the carbn dixide, xygen, hydrgen sulfide, and nitrate available at the vents. Fr invertebrates and vertebrates alike, symbisis has evlved as a general strategy fr expanding the surces f nutritin available fr animals. Cncept 41.5 Hmestatic mechanisms cntribute t an animal s energy balance. Energy frm fd balances the expenditure f energy fr metablism, activity, and strage. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc

12 Nearly all ATP generatin is based n the xidatin f rganic fuel mlecules carbhydrates, prteins, and fats in cellular respiratin. The mnmers f any f these substances can be used as fuel, but mst animals burn prteins nly after exhausting their supply f carbhydrates and fats. Fats are especially rich in energy, liberating abut twice the energy liberated frm an equal amunt f carbhydrate r prtein during xidatin. When animals take in mre energy-rich mlecules than they break dwn, the excess is cnverted int strage mlecules such as glycgen. In humans, glycgen is stred in liver and muscle cells. Glycgen synthesis and breakdwn are tightly regulated by the hrmnes insulin and glucagn. Adipse cells are a secndary site f energy strage in the bdy. If glycgen stres are full and calric intake still exceeds calric expenditure, the excess is usually stred as fat. When fewer calries are taken in than are expended perhaps because f sustained heavy exercise r lack f fd fuel is taken ut f strage depts and xidized. The human bdy expends liver glycgen first and then draws n muscle glycgen and fat. Mst healthy peple even if they are nt bese have enugh stred fat t sustain them thrugh several weeks f starvatin. Overnurishment, the cnsumptin f mre fd energy than the bdy needs fr nrmal metablism, causes besity, the excessive accumulatin f fat. Obesity cntributes t a number f health prblems, including type 2 diabetes, cln and breast cancer, and cardivascular disease. Obesity is a cntributing factr in 300,000 deaths per year in the United States. Researchers have discvered several hmestatic mechanisms that help regulate bdy weight. Operating as feedback circuits, these mechansms cntrl the strage and metablism f fat. Several hrmnes regulate lng-term and shrt-term appetite by affecting a satiety center in the brain. Mice with mutatins in the b r db gene eat vraciusly and becme bese. Dug Cleman determined that the b gene is needed t prduce the satiety factr and the db gene is required t respnd t the factr. Clning f the b gene has shwn that it cdes fr the hrmne leptin. The db gene encdes the leptin receptr. Adipse cells prduce leptin. As the amunt f adipse tissue increases, leptin prductin increases. High leptin levels cue the brain t suppress appetite. Cnversely, lss f bdy fat decreases leptin levels in the bld, signaling the brain t increase appetite. It is far frm clear that ur understanding f leptin will lead t effective treatments fr besity. Leptin has cmplex functins and plays a rle in the frmatin f the nervus system. In additin, very few bese peple have defective leptin prductin. In fact, mst bese humans have abnrmally high leptin levels, due t their large amunts f adipse tissue. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc

13 Fr sme reasn, the brain s satiety center des nt respnd t the high leptin levels in many bese peple. Mst humans crave fatty fds. Althugh fat harding is a health liability tday, it may have been advantageus in ur evlutinary past. Our ancestrs n the African savanna were hunter-gatherers wh prbably survived mainly n plant materials, ccasinally supplemented by meat. Natural selectin may have favred thse individuals with a physilgy that induced them t grge n fatty fds n the rare ccasins they were available. Individuals with genes that prmted the strage f fat during feasts may have been mre likely t survive famine. Obesity may be beneficial in certain species. Small seabirds called petrels fly lng distances t find fd that is rich in lipids. By bringing lipid-rich fd t their chicks, the parents minimize the weight f fd that they must carry. Because these fds are lw in prtein, hwever, yung petrels have t cnsume mre calries than they burn in metablism and cnsequently they becme bese. Their fat depsits may help them t survive perids when parents cannt find enugh fd. In sme petrel species, chicks at the end f the grwth perid weigh much mre their parents. They are t heavy t fly and must first starve fr several days. Lecture Outline fr Campbell/Reece Bilgy, 8 th Editin, Pearsn Educatin, Inc