Reading Essentials for Biology Glencoe (794133), страница 97
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What role does pepsinplay in digestion?________________________The inner lining of the stomach contains millions of glands. These glandssecrete a mixture of chemicals calledgastric juice. Gastric juice containspepsin and hydrochloric acid.
Pepsinis an enzyme that begins the chemicaldigestion of proteins in food. Pepsinworks best in an acidic environment.This environment is provided byhydrochloric acid.How is the stomach lining protectedfrom powerful digestive enzymes andstrong acids? The stomach lining secretesmucus. This mucus forms a protectivelayer between the stomach lining and theacidic environment of the stomach.Food stays in the stomach for about two to four hours. Whenfood is ready to leave the stomach, its consistency is similar to theconsistency of tomato soup.
Peristaltic waves become strongerand force small amounts of the liquid out of the stomach and intothe small intestine.________________________________________________426Chapter 35READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.How do chemicals in thestomach break down food?NameDateClassSection35.1Following Digestion of a Meal, continuedThe Small IntestineThe small intestine is a muscular tube about 6 m long.
It iscalled small because it has a narrow diameter. Its diameter is onlyabout 2.5 cm. Food digestion is completed in the small intestine.Muscle contractions continue to help break down the foodmechanically. Carbohydrates and proteins undergo additionalchemical digestion. The pancreas and the liver secrete enzymesthat break down the food substances even further.4. If the small intestine is6 m long, why is it calledsmall?________________________________________________________________________What is the purpose of the duodenum?The first 25 cm of the small intestine is called the duodenum(doo ah DEE num). Most of the enzymes and chemicals thatwork in the duodenum enter it through ducts that collect juicesfrom the pancreas, the liver, and the gallbladder.
Food doesnot pass into these three organs, but they all help in thedigestion process.Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.How does the pancreas help in the digestionprocess?The pancreas is a soft, flattened gland that secretes bothdigestive enzymes and hormones. The enzymes that the pancreassecretes break down carbohydrates, proteins, and fats. Alkalinepancreatic juices also help to neutralize the acidity of the liquidfood in the small intestine. This stops any further actionof pepsin.How does the liver help in the digestion process?The liver is a large, complex organ that has many functions. Itproduces bile. Bile is a chemical substance used in digestion thatbreaks down fats mechanically. Bile breaks large drops of fat intosmaller droplets.
After the liver makes bile, the bile is stored inthe gallbladder. The gallbladder is a small organ located just underthe liver. Bile passes from the gallbladder into the duodenum.How is food absorbed?After it leaves your stomach, liquid food stays in the small intestine for three to five hours. The food moves slowly through thesmall intestine by peristalsis. As digested food moves throughthe small intestine, it passes over thousand of villi.
A villus (plural,villi) is a tiny, fingerlike structure. Villi are projections on theREADING ESSENTIALSChapter 35427NameDateClassSection35.1Following Digestion of a Meal, continuedlining of the small intestine that help absorb digested food.Because villi increase the surface area of the small intestine, theyallow the body to absorb more food from the small intestine.Digested food in the small intestine is in the form of smallmolecules. These small molecules can be absorbed into the cellsof the villi. The food molecules diffuse into the blood vessels ofthe villus and enter the body’s bloodstream. Villi are the linkbetween the digestive system and the circulatory system.The Large IntestineThe material that cannot be digested in the small intestinepasses into the large intestine.
The large intestine is a musculartube that is also called the colon. The large intestine, or colon, isonly about 1.5 m long, but it is about 6.5 cm in diameter. Thelarge intestine is much wider than the small intestine. The appendix is a tubelike extension off the large intestine. It seems to serveno purpose in human digestion.5. What are three purposesof the large intestine?________________________________________________________________________The human body does not waste water. The walls of the largeintestine absorb water and salts from the indigestible material.A more solid material remains in the large intestine. Anaerobicbacteria in the large intestine produce some B vitamins andvitamin K.
Both these vitamins are absorbed as needed by thebody. Other bacteria in the large intestine stop harmful bacteriafrom colonizing. This helps to reduce the risk of infections inthe intestines.How are wastes eliminated?After 18 to 24 hours in the large intestine, indigestible material,now called feces, reaches the rectum. The rectum is the last partof the digestive system. Feces are eliminated from the rectumthrough the anus.428Chapter 35READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What does bacteria in the large intestine do?NameDateClassSection35.1Following Digestion of a Meal, continued◗ After You ReadMini Glossaryamylase: breaks down the starches in food intosmaller moleculesbile: a chemical that helps break down largedrops of fats into smaller dropletsepiglottis (ep uh GLAH tus): a flap of cartilagethat closes over the opening of the respiratory tract during swallowingperistalsis (per uh STAHL sus): a series of smoothmuscle contractions along the walls of thedigestive tractesophagus: a muscular tube that connects themouth to the stomachrectum: the last part of the digestive system;feces are eliminated from the rectumthrough the anusgallbladder: a small organ located just under theliver; bile passes from the gallbladder intothe duodenumsmall intestine: a muscular tube about 6 m longand about 2.5 cm in diameter; food digestion is completed in the small intestinelarge intestine: a muscular tube, also called thecolon, that holds material that cannot bedigested in the small intestinestomach: a muscular, pouchlike enlargement ofthe digestive tractliver: a large, complex organ that produces bilepancreas: a soft, flattened gland that secretesboth digestive enzymes and hormonesCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.pepsin: an enzyme that begins the chemicaldigestion of protein in food; works best inan acidic environmentvillus: a tiny, fingerlike structure that is a projection on the lining of the small intestine;works in the absorption of digested food1.
Read the terms and their definitions in the Mini Glossary above. Use them to create andlabel your own diagram of the digestive system on a separate sheet of paper.2. Complete the sequencing diagram to show the body parts involved in the passage of foodfrom the mouth to the anus.MouthAnusVisit the Glencoe Science Web site at science.glencoe.com to find yourbiology book and learn more about following digestion of a meal.READING ESSENTIALSChapter 35429NameDateClassSection35.2 Nutrition◗ Before You ReadConsider the following two lunches—a tuna salad, an apple, and a glass of milk, or a cheeseburger,french fries, and a soda.
Which would you choose? Which might be the healthier choice? On thelines below, explain your answer to this question.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________◗ Read to LearnCreate a QuizAfter you have read this section, create a quiz based onwhat you have learned. Afteryou have written the quizquestions, be sure to answerthem.1.
What three simple sugarsare absorbed into thebloodstream?________________________________________________Six basic kinds of nutrients can be found in foods. They arecarbohydrates, fats, proteins, minerals, vitamins, and water. Thesenutrients are essential for your body to function properly. Usingthe food pyramid, shown on page 430, to shape your diet can helpyou get the essential nutrients your body needs.How does your body use carbohydrates?Carbohydrates are starches and sugars. They are an importantsource of energy for your body cells. Starches are complex carbohydrates. They are found in bread, cereal, potatoes, rice, corn,beans, and pasta. Sugars are simple carbohydrates. They arefound mainly in fruits such as plums, strawberries, and oranges.During digestion, complex carbohydrates are broken down intosimple sugars, such as glucose, fructose, and galactose.
These simple sugars are absorbed into the bloodstream through the villi ofthe small intestine. These sugar molecules circulate through theblood to fuel body functions. Some sugar is carried to the liverwhere it is stored as glycogen.________________________430Chapter 35READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.The Vital NutrientsNameDateClassSection35.2Nutrition, continuedCellulose, another complex carbohydrate,is found in all plant cell walls.