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Section 38–2 38–2 The Process of 1 FOCUS Objectives ood presents every chordate with at least two challenges. FThe first is how to obtain it. Once a chordate has caught, or 38.2.1 Identify the organs of the gathered its food, it faces a new challenge—how to break that digestive system. Key Concepts • What are the organs of the food down into small molecules that can be passed to the cells 38.2.2 Describe the function of the digestive system? that need them. In and many other chordates, this is digestive system. • What is the function of the the job of the digestive system. As food passes through the digestive system? digestive system, it gets disassembled, distributing its nutrient Vocabulary value to the body along the way. The digestive system, like those of other chordates, Vocabulary Preview is built around an alimentary canal—a one-way tube that passes through the body. The digestive system includes If students have difficulty pronounc- the , , esophagus, stomach, , ing any of the Vocabulary words, it and . Several major accessory structures, may interfere with their comprehen- small intestine including the salivary glands, the pancreas, and the , sion. Read each of the words to the liver add secretions to the digestive system. class and have students repeat them villus large intestine after you. Point out that villus is sin- The Mouth gular and the plural is villi. Reading Strategy: Asking Questions Before As you take a forkful of food into your mouth, the work of the Reading Strategy you read, rewrite the seven blue digestive system begins. The teeth, shown in Figure 38–9, tear heads in the section as how, and crush the food into a fine paste until it is ready to be swal- Before they read, have students draw why, or what questions. As you lowed. begins the process of mechanical digestion. read, write brief answers to your a line down the center of a piece of Mechanical digestion is the physical breakdown of large pieces questions. paper. On the left side they should of food into smaller pieces. But there is a great deal more to it write down the organs of the diges- than that. As you chew your food, digestive begin the tive system. Then, as they read the breakdown of into smaller molecules. This section, they should record impor- process is called chemical digestion. During chemical digestion, tant details about each on the large food molecules are broken down into smaller food mole- right side of the paper, including the cules. The function of the digestive system is to help organ’s location, structure, and convert foods into simpler molecules that can be function. absorbed and used by the cells of the body. The organs of the digestive system are shown in Figure 38–10.

2 INSTRUCT Teeth The teeth are anchored in the of the jaw. The surfaces of the teeth are protected by a coating of mineralized enamel.Teeth do much of the mechanical The Mouth work of digestion by cutting, tearing, and crushing food Demonstration into small fragments. Help students understand the differ- Molars What do the teeth do? ence between the mechanical and chemical digestion that take place in Bicuspids the mouth by having them observe mechanical and chemical processes. Cuspid ̇ Figure 38–9 Human teeth include sharp Have one student break a raw egg ; cuspids and bicuspids, which grasp and tear food; and large, flat molars. Inferring into a bowl and scramble it with a Incisors fork. Have another student pour the How do human teeth reflect an omnivorous diet? scrambled raw egg into a pan of boil- ing water. Have students watch as the egg solidifies in the boiling water. SECTION RESOURCES Ask: Which process was mechani- cal, and which was chemical? (Breaking and scrambling the raw egg Print: • Lesson Plans, Section 38–2 was mechanical. Cooking the raw egg • Laboratory Manual B, Chapter 38 Lab Technology: was chemical.) • Teaching Resources, Section Review 38–2, • iText, Section 38–2 Enrichment, Chapter 38 Design anSave e r • Animated Biological Concepts Videotape Experiment m i

T Library, 40 Human Digestion • Reading and Study Workbook A, Section 38–2 • Transparencies Plus, Section 38–2 • Adapted Reading and Study Workbook B, Section 38–2

978 Chapter 38 Build Science Skills FIGURE 38–10 THE DIGESTIVE SYSTEM Using Models Point out that differ- ent types of teeth have different The digestive system includes the mouth, pharynx, esophagus, stomach, small mechanical functions: incisors cut, intestine, and large intestine. Because the pancreas and most of the are canines tear, and molars crush. Ask: behind other organs, their locations are indicated by dotted lines. Can you think of tools that per- form similar mechanical functions? (Scissors cut like incisors, tweezers tear like canines, and mallets crush like molars.)

Mouth Demonstration Pharynx Demonstrate how the amylase in chemically breaks down food. Salivary glands Give each student a soda cracker. Have students chew the cracker for five seconds and record how it . Esophagus Have them continue chewing the cracker for five minutes and again record how it tastes. Ask: How and why did the of the cracker change? (The cracker became sweeter as amylase broke down some of the Liver Stomach into .)

Pancreas Gallbladder (behind stomach) Use Visuals (behind liver) Large intestine Figure 38–10 Name each of the digestive organs. As you name each Small intestine organ, have students locate it in the figure. Tell students that the liver and pancreas secrete substances that help break down food but that food does not actually pass through them.

Saliva As the teeth cut and grind the food, the salivary glands secrete saliva, which helps to moisten the food and make it easier to chew. The release of saliva is under the control of the NSTA and can be triggered by the scent of food— For: Links on NSTA digestion especially when you are hungry! Visit: www.SciLinks.org Download a worksheet on Saliva not only helps ease the passage of food through the Web Code: cbn-0382 digestion for students to complete, digestive system but also begins the process of chemical diges- and find additional teacher support tion. Saliva contains an called amylase that breaks the from NSTA SciLinks. chemical bonds in starches and releases sugars. If you chew on a starchy food like a cracker long enough, it will begin to taste sweet. This sweet taste is a sign that has been released from by the action of amylase. Saliva also contains lysozyme, an enzyme that fights by digesting the walls of many that may enter the mouth with food.

Inclusion/Special Needs English Language Learners Help students master the main points about the Review the pronunciation and meaning of the digestive process by creating a flowchart show- Vocabulary terms with nonnative speakers and Answers to . . . ing the organs that food passes through as it is other students who tend to have difficulty with The teeth cut, tear, and digested, starting with the mouth and ending technical terminology. Suggest that each stu- crush food into smaller fragments. with the large intestine. Students should read dent translate the terms into his or her own the corresponding subsection in the text to language and write them on index cards, with Figure 38–9 The different types and determine what happens to food as it passes the term in English on one side and in his or her functions of human teeth make them through each organ and then illustrate each own language on the other side. well suited for the variety of step in the flowchart with a sketch. foods in an omnivorous diet.

Digestive and Excretory Systems 979 38–2 (continued) The Esophagus During , the combined actions of the and throat The Esophagus muscles push the chewed clump of food, called a , down the throat. Recall that as you swallow, a flap of connective tissue called Demonstration the closes over the opening to the . This action Demonstrate with a simple model prevents food from blocking the air passageways to the . how peristalsis pushes food through From the throat, the bolus passes through the esophagus, the esophagus. Place a marble inside Esophagus or food tube, into the stomach.You might think that gravity one end of a 25- to 30-cm length of draws food down through the esophagus, but this is not correct. flexible plastic or rubber tubing. With Bolus The reason food travels through the esophagus into the stomach is that it is moved along by contractions of . These a squeezing motion of your hands, Muscles move the marble down and out the contracted contractions, known as peristalsis (pehr-uh-STAL-sis), squeeze other end of the tube. Ask: If this is a Stomach the food through the esophagus into the stomach. The process of peristalsis is illustrated in model of the esophagus, what does Figure 38–11. A thick ring of muscle, called the cardiac sphincter,closes the the tube represent and what does esophagus after food has passed into the stomach and prevents the marble represent? (The tube rep- the contents of the stomach from moving back up into the esoph- resents the esophagus, and the marble agus. Have you ever suffered from “”? Heartburn is a represents the bolus of food that is being painful, burning sensation that feels as if it is coming from the swallowed.) How is peristalsis mod- center of the chest (by your ), just above the stomach. The eled? (By the squeezing of your hand sensation is usually caused by a backflow of stomach acid. along the tube from one end to the Heartburn can be caused by overeating or drinking an excess of other) caffeinated drinks.

The Stomach The Stomach Build Science Skills Food from the esophagus empties into a large muscular sac called the stomach. The stomach continues the mechanical and Inferring Ask students: How do chemical digestion of food. Alternating contractions of the you know when you are hungry? stomach’s three smooth muscle layers thoroughly churn and mix (Students probably will say that their the food you swallow. stomach growls or hurts.) Explain that ̆ Figure 38–11 Muscles in the these feelings of hunger are con- walls of the esophagus contract in Chemical Digestion The lining of the stomach contains trolled by a center in the waves. Each wave pushes the millions of microscopic that release a number of chewed clump of food, or bolus, in hypothalamus at the base of the substances into the stomach. Some of these glands produce , front of it. Eventually, the bolus is a fluid that lubricates and protects the stomach wall. Other glands brain, called the hunger center. The pushed into the stomach. produce , which makes the contents of the stom- hunger center when blood Applying Concepts What kind of ach very acidic. The acid activates , an enzyme that begins levels of nutrients are low, and sends muscle surrounds the esophagus? the digestion of and is secreted by a third set of stomach out impulses that lead to stom- glands.Pepsin works best under the acidic conditions present in ach contractions. Ask: What do you the stomach. The combination of pepsin and hydrochloric acid think causes the feelings of hunger begins the complex process of protein digestion.Pepsin breaks to stop once you have eaten? into smaller polypeptide fragments. While pepsin (Students may infer that increasing lev- requires the acidic environment of the stomach in order to func- els of nutrients in the blood stimulate tion, other enzymes such as amalyse are denatured by the stom- the hunger center to send out nerve ach acid. As a result,chemical digestion of carbohydrates stops impulses that stop the stomach con- when food enters the stomach and does not resume until the food tractions.) You may wish to tell passes into the small intestine. Not all enzymes that aid in diges- students, however, that satiety is tied tion are released by the stomach. Other enzymes that help in to intake. digestion are shown in Figure 38–12.

What is the role of pepsin?

FACTS AND FIGURES

Not just heartburn as defects in the lower esophageal sphincter, Most people have experienced heartburn, the slower-than-normal emptying of the stomach, burning sensation in the chest that is caused by and decreased secretion of by the stomach acids entering the esophagus. In about esophagus may all play a role. Complications of 25 to 35 percent of people, heartburn becomes GERD include , or inflammation of the chronic and signals a more serious disorder, called esophagus, and Barrett’s esophagus, a precancer- gastroesophageal reflux disease, or GERD. In addi- ous condition in which abnormal cells replace tion to heartburn, symptoms of GERD may normal cells in the esophagus. Treatment of GERD include regurgitation and difficulty swallowing. includes lifestyle changes, to control There is no single cause of GERD, but factors such stomach acids, and, in severe cases, .

980 Chapter 38 Use Visuals Mechanical Digestion As digestion proceeds, Effects of Digestive Enzymes Figure 38–12 Guide students in stomach muscles contract to churn and mix stom- interpreting the information in the ach fluids and food, gradually producing a mixture Active Site Enzyme Effect on Food known as chyme (KYM). After an hour or two, table. Remind them that food passes Mouth Salivary Breaks down starches the pyloric valve, which is located between the through each of the organs listed in amylase into disaccharides stomach and small intestine, opens and chyme the table and that the pancreas is a begins to flow into the small intestine. Stomach Pepsin Breaks down proteins gland that secretes digestive enzymes into large peptides into the small intestine. Ask: Which The Small Intestine Small Amylase Continues the enzymes break down proteins? intestine breakdown of starch (Pepsin, , and peptidase) Where As chyme is pushed through the pyloric valve, it (from are these enzymes found? (Pepsin is Continues the enters the (doo-oh-DEE-num). The pancreas) Trypsin found in the stomach; trypsin and pep- breakdown of protein duodenum is the first of three parts of the small tidase are found in the small intestine.) intestine, and it is where almost all of the diges- Breaks down fat Where does the breakdown of tive enzymes enter the intestine. Most of the Maltase, Breaks down remaining starch into simpler carbohydrates chemical digestion and absorption of the food you Small intestine sucrase, disaccharides into take place? (The mouth and small eat occurs in the small intestine. As chyme enters lactase monosaccharides intestine) Which nutrients do the duodenum from the stomach, it mixes with enzymes secreted by the pancreas enzymes and digestive fluids from the pancreas, Peptidase Breaks down dipeptides help digest? (Starch, protein, and fat) the liver, and even the lining of the duodenum into amino acids itself. The pancreas and liver are shown in Figure 38–13. ̆ Figure 38–12 Digestive enzymes break down foods and The Small Intestine Accessory Structures of Digestion Just behind the make nutrients available to the stomach is the pancreas. The pancreas is a gland that serves body. Using Tables and Use Visuals three important functions. One function is to produce Graphs Where in the body does the digestion of carbohydrates Figure 38–13 Point out in Figure that regulate blood sugar levels.Within the digestive system, the begin? 38–10 where the liver and pancreas pancreas plays two key roles. It produces enzymes that break down carbohydrates, proteins, , and nucleic acids. The are located. Explain that the role of pancreas also produces sodium bicarbonate, a base that neutral- the gallbladder is to store pro- izes stomach acid so that these enzymes can be effective. Why is duced in the liver. Ask: Where does this neutralization necessary? Recall that enzymes are proteins. the bile go after it leaves the gall- Stomach acid can change the shapes of protein molecules. If the bladder? (To the small intestine) shape of an enzyme’s active site does not match the shape of its substrate, the enzyme will not be effective. Make Connections ̇ Figure 38–13 Accessory structures, including the liver Chemistry Pour 100 mL of 0.5 per- and pancreas, add secretions to cent hydrochloric acid into a beaker the digestive system. The and measure its acidity with pH Liver pancreas secretes enzymes that paper. Then, stir 2 mL of sodium help break down carbohydrates, proteins, and . bicarbonate solution into the acid and test the pH again. Continue Pancreas Gallbladder adding small amounts of sodium bicarbonate as needed until the solu- tion has a neutral pH of 7. Ask: Duodenum Where in the digestive system is sodium bicarbonate produced? (In the pancreas and secreted into the small intestine) What is the result of its production? (It neutralizes To rest of small intestine hydrochloric acid so that it will not break down pancreatic digestive enzymes.) HISTORY OF SCIENCE

Watching as the stomach churns bits of food tied to strings into the stomach In 1822, a U.S. army surgeon named William through the hole. Then, he withdrew them peri- Beaumont was called upon to treat a gunshot odically to see the extent of digestion. Beaumont wound in the stomach of a Canadian fur trapper. also siphoned off gastric secretions and had their Answers to . . . The wound eventually healed, but it left a perma- chemical composition analyzed. He learned that nent hole to the outside in the man’s stomach. digestion is primarily a chemical process and that To break proteins into Beaumont saw this as a rare opportunity to study gastric secretions consist mostly of hydrochloric smaller polypeptide fragments the role of the stomach in digestion. With his acid. These and other results of Beaumont’s inno- Figure 38–11 Smooth muscle patient’s reluctant permission, Beaumont inserted vative research still remain valid today. Figure 38–12 In the mouth

Digestive and Excretory Systems 981 ver, 38–2 (continued) Assisting the pancreas is the li a large organ located just above and to the right of the stomach. The liver produces Absorption in the bile, a fluid loaded with lipids and salts. Bile acts like a deter- gent, dissolving and dispersing the droplets of fat found in fatty Small Intestine foods. This action makes it possible for enzymes to reach the smaller fat molecules and break them down. Bile is stored in a small, pouchlike organ called the gallbladder.

What is bile? Objective Students will be able to apply the concept that folding increases surface area. Absorption in the Small Intestine Skills Focus Applying Concepts, The duodenum is much shorter than the remaining parts of the Inferring; Calculating small intestine—the and the , which together Materials 2 paper towels, scissors, average about 6 meters long. By the time chyme enters these 3 cardboard tubes, metric ruler, 30- parts of the small intestine, much of the chemical digestion has mL graduated cylinder, 2 plastic been completed. The chyme is now a rich mixture of medium cups and small nutrient molecules. Time 20 minutes The small intestine is specially adapted for the absorption of nutrients. The folded surfaces of the small intestine are covered Advance Prep Ask students to with fingerlike projections called villi (VIL-eye; singular: villus). bring in cardboard tubes from rolls of paper towels, aluminum foil, or plastic wrap to use for the lab. Strategy You may wish to calcu- late class averages for the data before students answer the ques- tions so that all the students are 5. Predicting Predict working with the same numbers. How do villi help the small intestine absorb nutrients? which model will Expected Outcome Students absorb more water. should observe that the tubes con- 6. Stand each tube in a Materials 2 paper towel sheets, scissors, 3 card- taining folded paper towels retain plastic cup. Slowly board tubes, metric ruler, 30-mL graduated cylinder, pour 30 mL of water more water. 2 plastic cups down the inside of Analyze and Conclude each tube. Remove the tubes. Then, measure and 1. Students’ calculations will vary; Procedure record the quantity of water in each cup. however, the folded model (step 4) 1. Cut cardboard tube 1 lengthwise, and flatten it. Analyze and Conclude will have a far greater surface area CAUTION: Scissors are sharp. Lay paper towel 1. Calculating Use your calculations in steps 2 and than the flat model (step 2). sheet 1 over the flattened cardboard. Cut sheet 1 4 to show which model had more surface area. to the same size as the cardboard tube. 2. The folded paper towel in tube 3 2. Applying Concepts How does surface area affect 2. Determine the area of the flattened sheet with a has more surface area, which the ability to absorb substances? Was your predic- ruler (area = width x length). Record the measure- tion in step 5 correct? enables it to absorb more water. ments. 3. Applying Concepts How do folds and finger- Students may have predicted cor- 3. Roll sheet 1 lengthwise until the sides meet but do like projections affect the area of an absorbing rectly that the folded towel would not overlap. Insert rolled sheet 1 inside tube 2. The surface? How do villi help the intestine absorb tube represents the small intestine, and the sheet absorb more water. nutrients? represents an intestinal lining without villi. 3. Folds and projections increase the 4. Inferring Your kidneys contain about 1 million 4. Fold uncut sheet 2 back and forth in a zigzag area of the surface and its ability to microscopic structures that filter waste products pattern, as for a fan. Determine the area of sheet 2 from your blood. What advantage does this absorb substances. Villi increase the and record the measurement. Roll sheet 2 until the arrangement have over filtering the waste products surface area of the small intestine, sides meet, and insert it in tube 3. The folds repre- out of one large blood vessel? which increases its ability to absorb sent an intestinal lining with villi. nutrients. 4. Dividing the flow of blood among many small structures increases the surface area through which wastes FACTS AND FIGURES can be removed. The gallbladder examined some people, mineral salts in the gallbladder Located just beneath the liver, the gallbladder is harden to form . Some of these may a pear-shaped sack about 9 cm long that con- lodge in the bile duct and block it, causing pain centrates and stores liver bile until it is needed as well as preventing the bile duct from trans- to help digest fats. The gallbladder can store up porting bile and pancreatic enzymes to the to 50 mL of concentrated bile. When the bile is small intestine. Ultrasound is often used to needed for digestion, it travels from the gall- break up the stones so that they can pass out of bladder to the small intestine through the bile the body, although in severe cases removal of duct, which also transports digestive enzymes the gallbladder may be necessary. from the pancreas to the small intestine. In

982 Chapter 38 Address Misconceptions FIGURE 38–14 THE SMALL INTESTINE Students may have the mistaken impression that the duodenum is the The lining of the small intestine consists of folds that are covered with most important part of the small tiny projections called villi. Within each villus there is a network of blood intestine because the bulk of chemi- capillaries and vessels that absorb and carry away nutrients. Applying Concepts How do the folds in the small intestine help in cal digestion takes place there. Point absorption? out that about 3 meters of the small intestine are devoted to absorption, whereas only about 25 centimeters Villi (magnification: 32) are involved in digestion. Ask: What Small Intestine Villus percentage of the small intestine is involved in absorbing nutrients? (About 92 percent) What is the value of its length? (The small intestine’s Epithelial cells great length adds more absorbing sur- face area, which is critical to digestion and absorption.) Villi Capillaries Use Visuals

Lymph vessel Figure 38–14 Make sure students understand how the different parts of the figure are related. Call attention to the many capillaries in each villus, and describe their role in the absorp- tion of nutrients. Also, help students relate the figure to the information in the text by asking them to complete the following analogy: Villi are to The villi are illustrated in Figure 38–14. The surfaces of the cells of the villi are covered with thousands of fingerlike projec- the small intestine as are tions known as microvilli. These folds and projections provide to villi. (Microvilli) an enormous surface area for the absorption of nutrient mole- cules. Slow,wavelike contractions of smooth muscles move the Demonstration chyme along this surface. Give students an opportunity to feel Nutrient molecules are rapidly absorbed into the cells lining the how small and densely distributed small intestine. Most of the products of and protein villi are. Pass a piece of velvet fabric digestion are absorbed into the capillaries in the villi. Molecules of undigested fat and some fatty acids are absorbed by lymph vessels. around the room, and have students By the time food is ready to leave the small intestine, it is run their hands over the napped sur- basically nutrient-free. The complex organic molecules have been face. Tell them that the tiny digested and absorbed, leaving only water, , and other projections on the surface of the undigestible substances behind. cloth are similar in size and density to As the water, cellulose, and other undigestible substances the villi lining the small intestine. leave the small intestine and enter the large intestine, they pass by a small saclike organ called the . In humans, the appendix appears to do little to promote digestion. In other mam- mals, the appendix is used to store cellulose and other materials that the digestive enzymes cannot break down. The only time you may pay attention to the appendix is when it becomes clogged and inflamed, causing . The only remedy for appendicitis is to remove the infected organ by surgery—as quickly as possible.

FACTS AND FIGURES

Beneficial bacteria plenty of nutrients. The environment is also a safe There are enough bacteria in your large intestine one for the bacteria—unless you take antibiotics to fill a soup can. The relationship between you for an infection. Antibiotics kill beneficial as well as Answers to . . . and the bacteria is mutualistic because both of harmful bacteria. If too many beneficial bacteria you benefit: The bacteria provide you with vita- are killed, you may develop vitamin deficiencies Bile is a fluid produced mins and help your digestion, while you provide and form light-colored stools, both caused by the by the liver that dissolves and disperses the bacteria with a warm, moist environment and absence of bacteria. the droplets of fat found in fatty foods. Figure 38–14 The folds increase the surface area for the absorption of nutrients.

Digestive and Excretory Systems 983 38–2 (continued) The Large Intestine When the chyme leaves the small intestine, it enters The Large Intestine the large intestine, or colon. The large intestine is shown in Figure 38–15. The primary function of the Make Connections large intestine is to remove water from the undigested Mathematics Challenge students material that is left.Water is absorbed quickly across to estimate the surface area of the the wall of the large intestine, leaving behind the undi- large intestine based on its diameter gested materials. Rich colonies of bacteria present in the (6 cm) and length (150 cm), using large intestine produce compounds that the body is able the formula for the area of a cylinder: to use, including . When large doses of anti- 2πr(r+h). (The surface area is about biotics are given to fight an infection, they can destroy 2800 cm2.) Explain that, because of these bacteria and a vitamin K deficiency can occur. The villi, the surface area of the small concentrated waste material that remains after the water has been removed passes through the rectum and intestine is far greater, at about is eliminated from the body. 8,000,000 cm2.

Digestive System Digestive System Disorders The powerful acids released into the stomach sometimes Disorders damage the organ’s own lining, producing a hole in the ̆ Figure 38–15 This barium X-ray shows Build Science Skills the large intestine. Applying Concepts stomach wall known as a peptic ulcer.For years, physi- What is the role of the large intestine? cians hypothesized that the primary cause of ulcers was Applying Concepts Ask students: too much stomach acid. They prescribed drugs that Why do disorders of the large suppressed acid production and recommended bland, intestine often cause ? (The easily digested diets. Scientists have since discovered main function of the large intestine is that most peptic ulcers are caused by the bacterium to remove water from waste. If the Helicobacter pylori. Doctors now know that many peptic large intestine is not working properly ulcers are caused by an infectious disease that can be because of illness, too little water may cured. Thanks to powerful antibiotics, cure rates for be removed, causing diarrhea.) peptic ulcers are as high as 90 percent. Other digestive system disorders include diarrhea and . When something happens that inter- feres with the removal of water by the large intestine, 3 ASSESS you usually become aware of it right away. If not enough water is absorbed, a condition known as diarrhea occurs. Evaluate Understanding If too much water is absorbed from the undigested materials, a condition known as constipation occurs. Have students make a table with the headings: Mouth, Stomach, Small Intestine, Pancreas. Have them list the enzymes found in or produced by 38–2 Section Assessment each organ and the nutrients that the enzymes help break down. Enzyme Action How would the rate of 1. Key Concept List the 4. How does bile help in the digestion be affected if Reteach organs of the digestive system digestion of fats? enzymes were not released by and give the function of each. 5. Critical Thinking Inferring Describe the functions of the diges- the various organs and 2. Key Concept Explain the What can you infer about the glands? You may wish to refer tive organs, and have students function of the digestive system. diet of an animal that has a large to Chapter 2 for a review of identify them from their functions. 3. How do mechanical and chemi- appendix? enzyme action. cal digestion work together to break down foods? Students should explain that the rate of digestion would decrease, because enzymes speed up the rate of chemical reactions. 38–2 Section Assessment 1. Mouth: begins mechanical digestion, begins 3. Mechanical digestion physically breaks chemical digestion of starch; esophagus: down food into smaller pieces, which makes moves food to stomach; stomach: continues it easier for enzymes to chemically break If your class subscribes to the iText, mechanical digestion, begins chemical diges- down large food molecules into smaller use it to review the Key Concepts in tion of protein; small intestine: completes molecules. Section 38–2. chemical digestion of starch and protein, 4. Bile dissolves and disperses fat droplets, chemical digestion of fats; large intestine: Answer to . . . making it easier for enzymes to reach and removes water from undigested food further break down fats. Figure 38–15 To remove water from 2. To help convert foods into simple molecules 5. The diet probably contains a lot of cellulose. undigested material that can be absorbed and used by cells 984 Chapter 38