Download chapter 51: fueling body activities: digestion

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
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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
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Understand the neural and hormonal regulation of digestion.
Compare the digestive strategies of single cells, primitive invertebrates, and higher animals.
Compare the general structure of teeth of herbivores, carnivores, and omnivores.
Describe the digestive activities that occur in the mouth and indicate how food passes from the
mouth to the stomach.
Describe the digestive activities of and secretions produced by the stomach.
Describe the digestive and absorptive activities of the small intestine, including its secretions.
Explain the importance of convolutions in the digestive tract; identify the regions that are highly
convoluted and the regions that are not convoluted.
Explain the digestive activities associated with the large intestine.
Understand the symbiotic relationships that enhance digestive activities in the vertebrates.
Know the two kinds of exocrine cells present in the pancreas and the function(s) of each of their
secretions.
Describe the metabolic and digestive activities of the liver.
Explain the importance of regulating food intake.
Understand the importance of essential nutrients including vitamins and amino acids.
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.
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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
Analysis
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Have students explain why certain animals must sun themselves after
eating a large meal.
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Have students explain the role of pH in digestion.
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Ask students to explain why food can only be absorbed in the digestive
system in its simplest molecular form.
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Have students explain why artificial sweeteners taste like but not provide
the body with calories.
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Synthesis
Evaluation
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Have students explain why endotherms have higher caloric needs than
exotherms.
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Ask students compare the relative effectiveness of incomplete and
complete digestive systems.
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Ask students to design an experiment to test that incomplete digestive
system obtain less calories from food than complete digestive systems.
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Have students come up with commercial applications for animal digestive
enzymes.
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Ask the students come up with an application of the knowledge that food
can only be absorbed in its simplest molecular form.
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Ask students evaluate the effectiveness and safety of dietary drugs that
block the digestion of certain foods.
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Ask students evaluate the effectiveness and safety of dietary drugs that
block the absorption of certain foods.
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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
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Computer with Media Player and Internet access
LCD hooked up to computer
Web browser linked to Boston University histology website at
http://www.bu.edu/histology/m/t_diges2.htm
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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.
a. Esophagus (H&E)
b. Mid/esophagus (Lee's stain)
c. Esophageal/stomach junction I (H&E)
d. Esophageal/stomach junction II (H&E)
e. Colon (H&E)
f. Fundic stomach (H&E)
g. Pyloric stomach I (H&E)
h. Pyloric stomach II (PAS/Pb hematoxylin)
i. Pyloro/duodenal junction I (H&E)
j. Pyloro/duodenal junction II (H&E)
k. Duodenum, cells (H&E)
l. Duodenum, glands, and plica (H&E)
m. Jejunum I (eosin & toluidine blue)
n. Ileum I, villi (H&E)
o. Ileum I, Peyer's patches (H&E)
p. Jejunum II (PAS/Pb hematoxylin)
q. Ileum II, Peyer's patches (H&E)
r. Appendix (H&E)
s. Colon (H&E)
t. Colon, anal canal (H&E)
3. Have students describe the cells and tissue composition.
4. Then see if the students can confirm the function of the organ based on the cellular
structure removed.
USEFUL INTERNET RESOURCES
1. Colorado State University has a valuable digestive system website for supplementing
lectures with animated diagrams of digestive system function. The site is available at
http://www.vivo.colostate.edu/hbooks/pathphys/digestion/
2. A simple stop and go animation of the passage of food through the human digestive
system can be used to supplement a lecture on digestive system function. The website is
available at: http://www.constipationadvice.co.uk/constipation/digestive_system.html.
3. Faculty and students will find value in websites that simplify animal anatomy and
physiology concepts. The information can be used for projects that educate children and
civic groups about animals. Biology-4-kids is a model website for animal education. The
website can be found at http://www.biology4kids.com/files/systems_digestive.html.
4. Case studies are an effective tool for stimulating interest in a lesson on animals. The
University of Buffalo has a case study called “Sweet Indigestion: A Directed Case Study
on Carbohydrates” which has students investigating the uniqueness of different digestive
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systems on ability to break down and absorb foods. The case study can be found at
http://www.sciencecases.org/carbohydrates/carbohydrates.asp.
LABORATORY IDEAS
A. Comparing Digestive Systems
This activity has students has students investigate the phylogenetic variation in animal
digestive systems.
a. Explain to the students that digestive systems vary based on phylogenetics and on feeding
styles.
b. Then explain that they will be looking for endocrine organs in a fish and frog using a
human endocrine system chart as a guide.
c. Provide students with the following materials:
a. Dissection diagrams
b. Slide of flatworm
c. Preserved specimen of clamworm
d. Preserved specimen of squid
e. Preserved specimen of fish
f. Dissecting equipment
g. Dissecting microscope
d. Tell students to note the differences and similarities of the digestive systems between the
specimens.
e. 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.
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