RAVEN 9/e

CHAPTER 48: The Digestive System

WHERE DOES IT ALL FIT IN?

Chapter 48 builds on the foundations of Chapter 32 and provides detailed information about animal form and function Students should be encouraged to recall the principles of eukaryotic cell structure and evolution associated with the particular features of animal cells. Multicellularity should also be reviewed. The information in chapter 48 does not stand alone and fits in with the all of the chapters on animals. Students should know that animals and other organisms are interrelated and originated from a common ancestor of all living creatures on Earth.

SYNOPSIS

Animals obtain metabolic energy by eating other organisms. They are not collections of simple molecules, but are composed of macromolecules including proteins, fats, and starches. The degradation of these organic molecules is the process of digestion. Very simple organisms, protists and sponges, exhibit intracellular digestion. All other organisms have extracellular digestion where digestive enzymes are released into an internal body cavity. The most primitive invertebrates have a gastrovascular cavity with one opening that serves as mouth and anus. Higher invertebrates and vertebrates possess a one-way digestive tract with separate mouth and anus. Food is often fragmented before it is digested, some animals store ingested food in specialized organs prior to digestion.

Digestion begins in the mouth where teeth aid in the acquisition of food. They are variously shaped for capture, cutting, and grinding. Saliva from three pairs of glands moistens the food and initiates digestion of starches. The process of swallowing begins when food passes into the back of the mouth. It is propelled through the esophagus by peristaltic waves and enters the stomach. A muscular sphincter generally prevents food from reentering the esophagus once it is in the stomach. The inner surface of the stomach is convoluted to accommodate the sporadic ingestion of food, especially by carnivores. The walls of the stomach produce protective mucus in addition to hydrochloric acid and pepsinogen. The production of acid is carefully regulated by gastric hormones to ensure that it can be neutralized sufficiently. Parietal cells produce HCl that provide an acidic environment to keep pepsin active, help denature proteins, and kill ingested bacteria. Passage of chyme into the small intestine is controlled by the pyloric sphincter in response to changes in pH at the beginning of the small intestine.

Most digestion occurs within the first part of the small intestine, the duodenum, as a result of the action of intestinal and pancreatic enzymes. The intestine also produces bicarbonate to neutralize the acid from the stomach. The liver produces bile salts to emulsify fats prior to their digestion. Bile is stored in the gallbladder until it is needed. The remaining divisions of the small intestine, the jejunum, and the ileum, are specialized for absorption. The surface area is increased by finger-like projections called villi. The epithelium of each villus is covered with cytoplasmic extensions, the microvilli. Amino acids and sugars cross the intestinal cell membranes into blood capillaries at the brush border. Lipids are broken down into fatty acids and collected in the lymphatic system. The daily volume of food and water that passes through the gut equals nearly 9 liters, almost all is absorbed in the small intestine. A small amount of liquid is reabsorbed in the large intestine, leaving 50 grams of solids and 100 milliliters of liquid to be excreted. The inner surface of the large intestine lacks villi as its main function is to compact the wastes. Some sodium, vitamin K, and other products of bacterial metabolism are also absorbed here. Exit of wastes from the rectum is regulated by two sphincters; the first is involuntary, the second is voluntarily controlled. Ruminants house cellulose-digesting bacteria in the rumen and essentially digest their food twice. Horses have symbiotic bacteria in the caecum, but digestion of cellulose is less efficient because they only digest their food once.

The digestive system is under a combination of nervous and hormonal control. Sight and smell of food stimulates salivary and gastric secretions. Food in the stomach stimulates secretion of gastrin, causing production of pepsinogen and HCl. Passage of chyme from the stomach to the intestine inhibits stomach contractions. Cholecystokinin is secreted in response to fat in chyme and secretin is released in response to the acidity of chyme. Nutrients absorbed from the small intestine are directed to the liver, which acts as a metabolic reservoir. Under the influence of pancreatic hormones, it absorbs glucose when there is too much in the blood and releases it when there is too little. Glucose is not stored as is, but is converted to glycogen. Vertebrates obtain several vitamins from the food they eat as they are unable to synthesize them. Humans require 13 vitamins, 8 essential amino acids, essential minerals, and various other trace elements.

LEARNING OUTCOMES

48.1 The Types of Digestive Systems

  • Distinguish between incomplete and complete digestive systems.
  • List the components of the vertebrate digestive tract.
  • Describe the tissue layers of the gastrointestinal tract.

48.2 The Mouth and teeth: Food Capture and Bulk Processing

  • Identify adaptive variation in vertebrate tooth shape.
  • Understand the role of the mouth in the digestive process.

48.3 The Esophagus and the Stomach: The Beginning of Digestion

  • Describe how food moves through the esophagus.
  • Explain what digestive processes take place in the stomach.

48.4 The Intestines: Breakdown, Absorption, and Elimination

  • Compare the structures of the small and large intestines.
  • Name the accessory organs and describe their roles.
  • Explain how absorbed nutrients move into the blood or lymph capillaries.

48.5 Variations in Vertebrate Digestive Systems

  • Explain how vertebrates digest cellulose.
  • Describe how rumination works.
  • Discuss convergent evolution at the molecular level in herbivores.

48.6 Neural and Hormonal regulation of the Digestive Tract

  • Explain how the nervous system stimulates the digestive process.
  • Identify the major enterogastrones.

48.7 Accessory Organ Function

  • Describe the liver’s role in maintaining homeostasis.
  • Explain how the pancreas acts to control blood glucose concentration.

48.8 Food Energy, Energy Expenditure, and Essential Nutrients

  • Explain the basal metabolic rate and the effect of exercise.
  • List hormones involved in regulating appetite and body weight.
  • Name the essential nutrients.

COMMON STUDENT MISCONCEPTIONS

There is ample evidence in the educational literature that student misconceptions of information will inhibit the learning of concepts related to the misinformation. The following concepts covered in Chapter 48 are commonly the subject of student misconceptions. This information on “bioliteracy” was collected from faculty and the science education literature.

  • Students believe that all animals have complete digestive systems
  • Students are unaware of the role of pH in digestion
  • Students confuse digestion with absorption
  • Students do not associate the liver and other glands as being digestive system organs
  • Students view the complete digestive system as being on organ
  • Students think that all animals evolved at about the same time
  • Students believe that most animals are vertebrates
  • Students do not equate humans with being animals
  • Students believe that all animals have identical organ system structures

INSTRUCTIONAL STRATEGY PRESENTATION ASSISTANCE

Although no food products are absorbed in the stomach, alcohol and some drugs are absorbed. Alcohol can also be absorbed through the lining of the mouth if it remains there long enough. The presence of small amounts of it in the stomach improves the digestive process by slowing the exit of materials into the small intestine.

Discuss the activity of the liver as a detoxifying organ. The liver is frequently the first body organ damaged by toxins, including alcohol. This is readily evidenced by the damage to the liver of alcoholics due to fatty deposits and cirrhosis.

Discuss the various feeding strategies of herbivores versus carnivores, endotherms versus ectotherms, and how these strategies affect the anatomy of the skull, jaws, and digestive tract.

Discuss the affect of prolonged fasting on muscle.

HIGHER LEVEL ASSESSMENT

Higher level assessment measures a student’s ability to use terms and concepts learned from the lecture and the textbook. A complete understanding of biology content provides students with the tools to synthesize new hypotheses and knowledge using the facts they have learned. The following table provides examples of assessing a student’s ability to apply, analyze, synthesize, and evaluate information from Chapter 48.

Application /
  • Have students explain why certain animals must sun themselves after eating a large meal.
  • Have students explain the role of pH in digestion.
  • Ask students to explain why food can only be absorbed in the digestive system in its simplest molecular form.

Analysis /
  • Have students explain why artificial sweeteners taste like but not provide the body with calories.
  • Have students explain why endotherms have higher caloric needs than exotherms.
  • Ask students compare the relative effectiveness of incomplete and complete digestive systems.

Synthesis /
  • Ask students to design an experiment to test that incomplete digestive system obtain less calories from food than complete digestive systems.
  • Have students come up with commercial applications for animal digestive enzymes.
  • Ask the students come up with an application of the knowledge that food can only be absorbed in its simplest molecular form.

Evaluation /
  • Ask students evaluate the effectiveness and safety of dietary drugs that block the digestion of certain foods.
  • Ask students evaluate the effectiveness and safety of dietary drugs that block the absorption of certain foods.
  • Ask students evaluate the benefits and risks of athletic diets that are composed solely of amino acids and simple sugars.

VISUAL RESOURCES

Obtain photos or scanning electron micrographs showing the various convolutions of the digestive tract. Include the stomach, duodenum, lower small intestine, and the large intestine in the micrographs.

IN-CLASS CONCEPTUAL DEMONSTRATIONS

A. Digestive System Histology

Introduction

This demonstration uses digestive system histology images to help students understand the cellular composition of the digestive system. It is a good demonstration for reinforcing histological form and function.

Materials

  • Computer with Media Player and Internet access
  • LCD hooked up to computer
  • Web browser linked to Boston University histology website at

Procedure & Inquiry

  1. Ask the class what they know about form and function in cell and tissue structure
  2. Load up the Boston University histology website and click on digestive system histology images in order of the progress of food through the system.
  3. Esophagus (H&E)
  4. Mid/esophagus (Lee's stain)
  5. Esophageal/stomach junction I (H&E)
  6. Esophageal/stomach junction II (H&E)
  7. Colon (H&E)
  8. Fundic stomach (H&E)
  9. Pyloric stomach I (H&E)
  10. Pyloric stomach II (PAS/Pb hematoxylin)
  11. Pyloro/duodenal junction I (H&E)
  12. Pyloro/duodenal junction II (H&E)
  13. Duodenum, cells (H&E)
  14. Duodenum, glands, and plica (H&E)
  15. Jejunum I (eosin & toluidine blue)
  16. Ileum I, villi (H&E)
  17. Ileum I, Peyer's patches (H&E)
  18. Jejunum II (PAS/Pb hematoxylin)
  19. Ileum II, Peyer's patches (H&E)
  20. Appendix (H&E)
  21. Colon (H&E)
  22. Colon, anal canal (H&E)
  23. Have students describe the cells and tissue composition.
  24. Then see if the students can confirm the function of the organ based on the cellular structure removed.

LABORATORY IDEAS

A. Comparing Digestive Systems

This activity has students has students investigate the phylogenetic variation in animal digestive systems.

  1. Explain to the students that digestive systems vary based on phylogenetics and on feeding styles.
  2. Then explain that they will be looking for endocrine organs in a fish and frog using a human endocrine system chart as a guide.
  3. Provide students with the following materials:
  4. Dissection diagrams
  5. Slide of flatworm
  6. Preserved specimen of clamworm
  7. Preserved specimen of squid
  8. Preserved specimen of fish
  9. Dissecting equipment
  10. Dissecting microscope
  11. Tell students to note the differences and similarities of the digestive systems between the specimens.
  12. Then have them record any observations about form and function related to the degree of digestive system specialization in the different organisms.

LEARNING THROUGH SERVICE

Service learning is a strategy of teaching, learning and reflective assessment that merges the academic curriculum with meaningful community service. As a teaching methodology, it falls under the category of experiential education. It is a way students can carry out volunteer projects in the community for public agencies, nonprofit agencies, civic groups, charitable organizations, and governmental organizations. It encourages critical thinking and reinforces many of the concepts learned in a course.

  1. Have students do a lesson do a hands-on program on the animal diets for elementary students.
  2. Have students tutor high school students studying animal anatomy and physiology.
  3. Have students volunteer on environmental restoration projects with a local conservation group.
  4. Have students volunteer at the educational center of a zoo or marine park.