Food presents every chordate with at least two challenges.The first is how to obtain it. Once a chordate has caught, or

gathered its food, it faces a new challenge—how to break thatfood down into small molecules that can be passed to the cellsthat need them. In humans and many other chordates, this isthe job of the digestive system. As food passes through thedigestive system, it gets disassembled, distributing its nutrientvalue to the body along the way.

The human digestive system, like those of other chordates,is built around an alimentary canal—a one-way tube thatpasses through the body. The digestive system includesthe mouth, pharynx, esophagus, stomach, small intestine,and large intestine. Several major accessory structures,including the salivary glands, the pancreas, and the liver,add secretions to the digestive system.

The MouthAs you take a forkful of food into your mouth, the work of thedigestive system begins. The teeth, shown in Figure 38–9, tearand crush the food into a fine paste until it is ready to be swal-lowed. Chewing begins the process of mechanical digestion.Mechanical digestion is the physical breakdown of large piecesof food into smaller pieces. But there is a great deal more to itthan that. As you chew your food, digestive enzymes begin thebreakdown of carbohydrates into smaller molecules. Thisprocess is called chemical digestion. During chemical digestion,large food molecules are broken down into smaller food mole-cules. The function of the digestive system is to helpconvert foods into simpler molecules that can beabsorbed and used by the cells of the body. The organs of

the digestive system are shown in Figure 38–10.

Teeth The teeth are anchored in the bones of the jaw.The surfaces of the teeth are protected by a coating ofmineralized enamel. Teeth do much of the mechanicalwork of digestion by cutting, tearing, and crushing foodinto small fragments.

What do the teeth do?

Key Concepts• What are the organs of the

digestive system?• What is the function of the

digestive system?

Vocabularyamylaseesophagusperistalsisstomachchymesmall intestinepancreaslivervilluslarge intestine

Reading Strategy:Asking Questions Beforeyou read, rewrite the seven blueheads in the section as how,why, or what questions. As youread, write brief answers to yourquestions.

! Figure 38–9 Human teeth include sharpincisors; cuspids and bicuspids, which grasp andtear food; and large, flat molars. InferringHow do human teeth reflect an omnivorous diet?

38–2 The Process of Digestion

978 Chapter 38

1 FOCUSObjectives38.2.1 Identify the organs of the

digestive system.38.2.2 Describe the function of the

digestive system.

Vocabulary PreviewIf students have difficulty pronounc-ing any of the Vocabulary words, itmay interfere with their comprehen-sion. Read each of the words to theclass and have students repeat themafter you. Point out that villus is sin-gular and the plural is villi.

Reading StrategyBefore they read, have students drawa line down the center of a piece ofpaper. On the left side they shouldwrite down the organs of the diges-tive system. Then, as they read thesection, they should record impor-tant details about each organ on theright side of the paper, including theorgan’s location, structure, and function.

2 INSTRUCT

The MouthDemonstrationHelp students understand the differ-ence between the mechanical andchemical digestion that take place inthe mouth by having them observemechanical and chemical processes.Have one student break a raw egginto a bowl and scramble it with afork. Have another student pour thescrambled raw egg into a pan of boil-ing water. Have students watch asthe egg solidifies in the boiling water.Ask: Which process was mechani-cal, and which was chemical?(Breaking and scrambling the raw eggwas mechanical. Cooking the raw eggwas chemical.)

Section 38–2

SECTION RESOURCES

Print:• Laboratory Manual B, Chapter 38 Lab• Teaching Resources, Lesson Plan 38–2,

Adapted Section Summary 38–2, AdaptedWorksheets 38–2, Section Summary 38–2,Worksheets 38–2, Section Review 38–2,Enrichment

• Reading and Study Workbook A, Section 38–2• Adapted Reading and Study Workbook B,

Section 38–2

• Lab Worksheets, Chapter 38 Design anExperiment

Technology:• iText, Section 38–2• Animated Biological Concepts DVD,

40 Human Digestion• Transparencies Plus, Section 38–2

Tim

eSaver

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For: Links on digestion

Visit: www.SciLinks.orgWeb Code: cbn-0382

NSTA

Saliva As the teeth cut and grind the food, the salivary glandssecrete saliva, which helps to moisten the food and make iteasier to chew. The release of saliva is under the control of thenervous system and can be triggered by the scent of food—especially when you are hungry!

Saliva not only helps ease the passage of food through thedigestive system but also begins the process of chemical diges-tion. Saliva contains an enzyme called that breaks thechemical bonds in starches and releases sugars. If you chew on astarchy food like a cracker long enough, it will begin to tastesweet. This sweet taste is a sign that sugar has been releasedfrom starch by the action of amylase. Saliva also containslysozyme, an enzyme that fights infection by digesting the cellwalls of many bacteria that may enter the mouth with food.

amylase

FIGURE 38–10 THE DIGESTIVE SYSTEM

The digestive system includes the mouth, pharynx, esophagus, stomach, smallintestine, and large intestine. Because the pancreas and most of the gallbladder arebehind other organs, their locations are indicated by dotted lines.

Build Science SkillsUsing Models Point out that differ-ent types of teeth have differentmechanical functions: incisors cut,canines tear, and molars crush. Ask:Can you think of tools that per-form similar mechanical functions?(Scissors cut like incisors, tweezers tearlike canines, and mallets crush likemolars.)

DemonstrationDemonstrate how the amylase in saliva chemically breaks down food.Give each student a soda cracker.Have students chew the cracker forfive seconds and record how it tastes.Have them continue chewing thecracker for five minutes and againrecord how it tastes. Ask: How andwhy did the taste of the crackerchange? (The cracker became sweeteras amylase broke down some of thestarches into sugars.)

Use VisualsFigure 38–10 Name each of thedigestive organs. As you name eachorgan, have students locate it in thefigure. Tell students that the liver andpancreas secrete substances that helpbreak down food but that food doesnot actually pass through them.

NSTA

Download a worksheet on digestion for students to complete,and find additional teacher supportfrom NSTA SciLinks.

Answers to . . . The teeth cut, tear, and

crush food into smaller fragments.

Figure 38–9 The different types andfunctions of human teeth make themwell suited for eating the variety offoods in an omnivorous diet.

Inclusion/Special NeedsHelp students master the main points about thedigestive process by creating a flowchart show-ing the organs that food passes through as it isdigested, starting with the mouth and endingwith the large intestine. Students should readthe corresponding subsection in the text todetermine what happens to food as it passesthrough each organ and then illustrate eachstep in the flowchart with a sketch.

English Language LearnersReview the pronunciation and meaning of theVocabulary terms with nonnative speakers andother students who tend to have difficulty withtechnical terminology. Suggest that each stu-dent translate the terms into his or her ownlanguage and write them on index cards, withthe term in English on one side and in his or herown language on the other side.

Digestive and Excretory Systems 979

UNIVERSAL ACCESS

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The EsophagusDuring swallowing, the combined actions of the tongue and throatmuscles push the chewed clump of food, called a bolus, down thethroat. Recall that as you swallow, a flap of connective tissue calledthe epiglottis closes over the opening to the trachea. This actionprevents food from blocking the air passageways to the lungs.

From the throat, the bolus passes through the or food tube, into the stomach. You might think that gravitydraws food down through the esophagus, but this is not correct.The reason food travels through the esophagus into the stomachis that it is moved along by contractions of smooth muscle. Thesecontractions, known as (pehr-uh-STAL-sis), squeezethe food through the esophagus into the stomach. The process ofperistalsis is illustrated in Figure 38–11.

A thick ring of muscle, called the cardiac sphincter, closes theesophagus after food has passed into the stomach and preventsthe contents of the stomach from moving back up into the esoph-agus. Have you ever suffered from “heartburn”? Heartburn is apainful, burning sensation that feels as if it is coming from thecenter of the chest (by your heart), just above the stomach. Thesensation is usually caused by a backflow of stomach acid.Heartburn can be caused by overeating or drinking an excess ofcaffeinated drinks.

The StomachFood from the esophagus empties into a large muscular saccalled the The stomach continues the mechanical andchemical digestion of food. Alternating contractions of thestomach’s three smooth muscle layers thoroughly churn and mixthe food you swallow.

Chemical Digestion The lining of the stomach containsmillions of microscopic gastric glands that release a number ofsubstances into the stomach. Some of these glands produce mucus,a fluid that lubricates and protects the stomach wall. Other glandsproduce hydrochloric acid, which makes the contents of the stom-ach very acidic. The acid activates pepsin, an enzyme that beginsthe digestion of protein and is secreted by a third set of stomachglands. Pepsin works best under the acidic conditions present inthe stomach. The combination of pepsin and hydrochloric acidbegins the complex process of protein digestion. Pepsin breaksproteins into smaller polypeptide fragments. While pepsinrequires the acidic environment of the stomach in order to func-tion, other enzymes such as amalyse are denatured by the stom-ach acid. As a result, chemical digestion of carbohydrates stopswhen food enters the stomach and does not resume until the foodpasses into the small intestine. Not all enzymes that aid in diges-tion are released by the stomach. Other enzymes that help indigestion are shown in Figure 38–12.

What is the role of pepsin?

stomach.

peristalsis

esophagus,Esophagus

Bolus

Musclescontracted

Stomach

! Figure 38–11 Muscles in thewalls of the esophagus contract inwaves. Each wave pushes thechewed clump of food, or bolus, infront of it. Eventually, the bolus ispushed into the stomach.Applying Concepts What kind ofmuscle surrounds the esophagus?

980 Chapter 38

The EsophagusDemonstrationDemonstrate with a simple modelhow peristalsis pushes food throughthe esophagus. Place a marble insideone end of a 25- to 30-cm length offlexible plastic or rubber tubing. Witha squeezing motion of your hands,move the marble down and out theother end of the tube. Ask: If this is amodel of the esophagus, what doesthe tube represent and what doesthe marble represent? (The tube rep-resents the esophagus, and the marblerepresents the bolus of food that is beingswallowed.) How is peristalsis mod-eled? (By the squeezing of your handalong the tube from one end to theother)

The StomachBuild Science SkillsInferring Ask students: How doyou know when you are hungry?(Students probably will say that theirstomach growls or hurts.) Explain thatthese feelings of hunger are con-trolled by a center in thehypothalamus at the base of thebrain, called the hunger center. Thehunger center senses when bloodlevels of nutrients are low, and sendsout nerve impulses that lead to stom-ach contractions. Ask: What do youthink causes the feelings of hungerto stop once you have eaten?(Students may infer that increasing lev-els of nutrients in the blood stimulatethe hunger center to send out nerveimpulses that stop the stomach con-tractions.) You may wish to tellstudents, however, that satiety is tiedto fat intake.

Not just heartburnMost people have experienced heartburn, theburning sensation in the chest that is caused bystomach acids entering the esophagus. In about25 to 35 percent of people, heartburn becomeschronic and signals a more serious disorder, calledgastroesophageal reflux disease, or GERD. In addi-tion to heartburn, symptoms of GERD mayinclude regurgitation and difficulty swallowing.There is no single cause of GERD, but factors such

as defects in the lower esophageal sphincter,slower-than-normal emptying of the stomach,and decreased secretion of bicarbonate by theesophagus may all play a role. Complications ofGERD include esophagitis, or inflammation of theesophagus, and Barrett’s esophagus, a precancer-ous condition in which abnormal cells replacenormal cells in the esophagus. Treatment of GERDincludes lifestyle changes, medications to controlstomach acids, and, in severe cases, surgery.

FACTS AND FIGURES

38–2 (continued)

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Mechanical Digestion As digestion proceeds,stomach muscles contract to churn and mix stom-ach fluids and food, gradually producing a mixtureknown as (KYM). After an hour or two,the pyloric valve, which is located between thestomach and small intestine, opens and chymebegins to flow into the small intestine.

The Small IntestineAs chyme is pushed through the pyloric valve, itenters the duodenum (doo-oh-DEE-num). Theduodenum is the first of three parts of the

and it is where almost all of the diges-tive enzymes enter the intestine. Most of thechemical digestion and absorption of the food youeat occurs in the small intestine. As chyme entersthe duodenum from the stomach, it mixes withenzymes and digestive fluids from the pancreas,the liver, and even the lining of the duodenumitself. The pancreas and liver are shown in Figure 38–13.

Accessory Structures of Digestion Just behind thestomach is the The pancreas is a gland that servesthree important functions. One function is to produce hormonesthat regulate blood sugar levels. Within the digestive system, thepancreas plays two key roles. It produces enzymes that breakdown carbohydrates, proteins, lipids, and nucleic acids. Thepancreas also produces sodium bicarbonate, a base that neutral-izes stomach acid so that these enzymes can be effective. Why isthis neutralization necessary? Recall that enzymes are proteins.Stomach acid can change the shapes of protein molecules. If theshape of an enzyme’s active site does not match the shape of itssubstrate, the enzyme will not be effective.

pancreas.

intestine,small

chyme

Pancreatic duct

Pancreas

Bile ductLiver

Gallbladder

Duodenum

To rest of small intestine

! Figure 38–13 Accessorystructures, including the liverand pancreas, add secretions tothe digestive system. Thepancreas secretes enzymes thathelp break down carbohydrates,proteins, and fats.

Active Site Enzyme Effect on Food

Mouth

Stomach

Small intestine(from pancreas)

Smallintestine

Salivaryamylase

Pepsin

Amylase

Trypsin

Lipase

Maltase,sucrase,lactase

Peptidase

Breaks down starchesinto disaccharides

Breaks down proteinsinto large peptides

Continues the breakdown of starch

Continues the breakdown of protein

Breaks down fat

Breaks down remainingdisaccharides intomonosaccharides

Breaks down dipeptidesinto amino acids

Effects of Digestive Enzymes

" Figure 38–12 Di esti eenzymes break down foods andmake n trients a ailable to thebody. Using Tables andGraphs Where in the body doesthe digestion of carbohydratesbegin?

Use VisualsFigure 38–12 Guide students ininterpreting the information in thetable. Remind them that food passesthrough each of the organs listed inthe table and that the pancreas is agland that secretes digestive enzymesinto the small intestine. Ask: Whichenzymes break down proteins?(Pepsin, trypsin, and peptidase) Whereare these enzymes found? (Pepsin isfound in the stomach; trypsin and pep-tidase are found in the small intestine.)Where does the breakdown ofstarch into simpler carbohydratestake place? (The mouth and smallintestine) Which nutrients doenzymes secreted by the pancreashelp digest? (Starch, protein, and fat)

The Small IntestineUse VisualsFigure 38–13 Point out in Figure38–13 where the liver and pancreasare located. Explain that the role ofthe gallbladder is to store bile pro-duced in the liver. Ask: Where doesthe bile go after it leaves the gall-bladder? (To the small intestine)

Make ConnectionsChemistry Pour 100 mL of 0.5 per-cent hydrochloric acid into a beakerand measure its acidity with pHpaper. Then, stir 2 mL of sodiumbicarbonate solution into the acidand test the pH again. Continueadding small amounts of sodiumbicarbonate as needed until the solu-tion has a neutral pH of 7. Ask:Where in the digestive system issodium bicarbonate produced? (Inthe pancreas and secreted into thesmall intestine) What is the result ofits production? (It neutralizeshydrochloric acid so that it will notbreak down pancreatic digestiveenzymes.)

Digestive and Excretory Systems 981

Watching as the stomach churnsIn 1822, a U.S. army surgeon named WilliamBeaumont was called upon to treat a gunshotwound in the stomach of a Canadian fur trapper.The wound eventually healed, but it left a perma-nent hole to the outside in the man’s stomach.Beaumont saw this as a rare opportunity to studythe role of the stomach in digestion. With hispatient’s reluctant permission, Beaumont inserted

bits of food tied to strings into the stomachthrough the hole. Then, he withdrew them peri-odically to see the extent of digestion. Beaumontalso siphoned off gastric secretions and had theirchemical composition analyzed. He learned thatdigestion is primarily a chemical process and thatgastric secretions consist mostly of hydrochloricacid. These and other results of Beaumont’s inno-vative research still remain valid today.

HISTORY OF SCIENCE

Answers to . . . To break proteins into

smaller polypeptide fragments

Figure 38–11 Smooth muscle

Figure 38–12 In the mouth

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The villi are illustrated in Figure 38–14. The surfaces of thecells of the villi are covered with thousands of fingerlike projec-tions known as microvilli. These folds and projections providean enormous surface area for the absorption of nutrient mole-cules. Slow, wavelike contractions of smooth muscles move thechyme along this surface.

Nutrient molecules are rapidly absorbed into the cells lining thesmall intestine. Most of the products of carbohydrate and proteindigestion are absorbed into the capillaries in the villi. Molecules ofundigested fat and some fatty acids are absorbed by lymph vessels.

By the time food is ready to leave the small intestine, it isbasically nutrient-free. The complex organic molecules have beendigested and absorbed, leaving only water, cellulose, and otherundigestible substances behind.

As the water, cellulose, and other undigestible substancesleave the small intestine and enter the large intestine, they passby a small saclike organ called the appendix. In humans, theappendix appears to do little to promote digestion. In other mam-mals, the appendix is used to store cellulose and other materialsthat the digestive enzymes cannot break down. The only time youmay pay attention to the appendix is when it becomes clogged andinflamed, causing appendicitis. The only remedy for appendicitis isto remove the infected organ by surgery—as quickly as possible.

Capillaries

Epithelial cells

VillusSmall Intestine

Circular folds

Villi

Lymph vessel

Vein

Artery

FIGURE 38–14 THE SMALL INTESTINE

Villi (magnification: 32!)

The lining of the small intestine consists of folds that are covered withtiny projections called villi. Within each villus there is a network of bloodcapillaries and lymph vessels that absorb and carry away nutrients.Applying Concepts How do the folds in the small intestine help inabsorption?

Address MisconceptionsStudents may have the mistakenimpression that the duodenum is themost important part of the smallintestine because the bulk of chemi-cal digestion takes place there. Pointout that about 3 meters of the smallintestine are devoted to absorption,whereas only about 25 centimetersare involved in digestion. Ask: Whatpercentage of the small intestine isinvolved in absorbing nutrients?(About 92 percent) What is the valueof its length? (The small intestine’sgreat length adds more absorbing sur-face area, which is critical to digestionand absorption.)

Use VisualsFigure 38–14 Make sure studentsunderstand how the different parts ofthe figure are related. Call attentionto the many capillaries in each villus,and describe their role in the absorp-tion of nutrients. Also, help studentsrelate the figure to the information inthe text by asking them to completethe following analogy: Villi are tothe small intestine as areto villi. (Microvilli)

DemonstrationGive students an opportunity to feelhow small and densely distributedvilli are. Pass a piece of velvet fabricaround the room, and have studentsrun their hands over the napped sur-face. Tell them that the tinyprojections on the surface of thecloth are similar in size and density tothe villi lining the small intestine.

Digestive and Excretory Systems 983

Beneficial bacteriaThere are enough bacteria in your large intestineto fill a soup can. The relationship between youand the bacteria is mutualistic because both ofyou benefit: The bacteria provide you with vita-mins and help your digestion, while you providethe bacteria with a warm, moist environment and

plenty of nutrients. The environment is also a safeone for the bacteria—unless you take antibioticsfor an infection. Antibiotics kill beneficial as well asharmful bacteria. If too many beneficial bacteriaare killed, you may develop vitamin deficienciesand form light-colored stools, both caused by theabsence of bacteria.

FACTS AND FIGURES

Answers to . . . Bile is a fluid produced

by the liver that dissolves and dispersesthe droplets of fat found in fatty foods.

Figure 38–14 The folds increase the surface area for the absorption ofnutrients.

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The Large IntestineWhen the chyme leaves the small intestine, it enters the large intestine, or colon. The large intestine is shown in Figure 38–15. The primary function of the

is to remove water from the undigestedmaterial that is left. Water is absorbed quickly acrossthe wall of the large intestine, leaving behind the undi-gested materials. Rich colonies of bacteria present in thelarge intestine produce compounds that the body is ableto use, including vitamin K. When large doses of anti-biotics are given to fight an infection, they can destroythese bacteria and a vitamin K deficiency can occur. Theconcentrated waste material that remains after thewater has been removed passes through the rectum andis eliminated from the body.

Digestive System DisordersThe powerful acids released into the stomach sometimesdamage the organ’s own lining, producing a hole in thestomach wall known as a peptic ulcer. For years, physi-cians hypothesized that the primary cause of ulcers wastoo much stomach acid. They prescribed drugs thatsuppressed acid production and recommended bland,easily digested diets. Scientists have since discoveredthat most peptic ulcers are caused by the bacteriumHelicobacter pylori. Doctors now know that many pepticulcers are caused by an infectious disease that can becured. Thanks to powerful antibiotics, cure rates forpeptic ulcers are as high as 90 percent.

Other digestive system disorders include diarrheaand constipation. When something happens that inter-feres with the removal of water by the large intestine,you usually become aware of it right away. If not enoughwater is absorbed, a condition known as diarrhea occurs.If too much water is absorbed from the undigestedmaterials, a condition known as constipation occurs.

large intestine

! Figure 38–15 This barium X-ray showsthe large intestine. Applying ConceptsWhat is the role of the large intestine?

1. Key Concept List theorgans of the digestive systemand give the function of each.

2. Key Concept Explain thefunction of the digestive system.

3. How do mechanical and chemi-cal digestion work together tobreak down foods?

4. How does bile help in the digestion of fats?

5. Critical Thinking InferringWhat can you infer about thediet of an animal that has a largeappendix?

Matter and EnergyHow would the rate ofdigestion be affected ifenzymes were not released bythe various organs andglands? You may wish to referto Chapter 2 for a review ofenzyme action.

38–2 Section Assessment

984 Chapter 38

The Large IntestineMake ConnectionsMathematics Challenge studentsto estimate the surface area of thelarge intestine based on its diameter(6 cm) and length (150 cm), usingthe formula for the area of a cylinder:2!r(r+h). (The surface area is about2800 cm2.) Explain that, because ofvilli, the surface area of the smallintestine is far greater, at about8,000,000 cm2.

Digestive SystemDisordersBuild Science SkillsApplying Concepts Ask students:Why do disorders of the largeintestine often cause diarrhea? (Themain function of the large intestine isto remove water from waste. If thelarge intestine is not working properlybecause of illness, too little water maybe removed, causing diarrhea.)

3 ASSESSEvaluate UnderstandingHave students make a table with theheadings: Mouth, Stomach, SmallIntestine, Pancreas. Have them list theenzymes found in or produced byeach organ and the nutrients that theenzymes help break down.

ReteachDescribe the functions of the diges-tive organs, and have studentsidentify them from their functions.

Answer to . . . Figure 38–15 To remove water fromundigested material

38–2 (continued)

38–2 Section Assessment1. Mouth: begins mechanical digestion, begins

chemical digestion of starch; esophagus:moves food to stomach; stomach: continuesmechanical digestion, begins chemical diges-tion of protein; small intestine: completeschemical digestion of starch and protein,chemical digestion of fats; large intestine:removes water from undigested food

2. To help convert foods into simple moleculesthat can be absorbed and used by cells

3. Mechanical digestion physically breaksdown food into smaller pieces, which makesit easier for enzymes to chemically breakdown large food molecules into smallermolecules.

4. Bile dissolves and disperses fat droplets,making it easier for enzymes to reach andfurther break down fats.

5. The diet probably contains a lot of cellulose.

If your class subscribes to the iText,use it to review the Key Concepts inSection 38–2.

Students should explain that therate of digestion would decrease,because enzymes speed up therate of chemical reactions.

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