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26 THE PATH OF FOOD THROUGH THE ANIMAL BODY
EXTENDED LECTURE OUTLINE
Food Energy and Essential Nutrients (p. 554)
26.1
Calories for Energy (p. 554; Figs. 26.1, 26.2, 26.3; Table 26.1)
A. We obtain our energy from the foods we eat.
B. Different classes of nutrients, such as carbohydrates, proteins, and fats, contain the building
blocks necessary for cell growth and renewal.
C. These molecules also have energy within their bonds that cells extract to fuel their own
energy needs.
D. An optimal diet contains the right nutrients in the proper amounts.
1. Carbohydrates contain an average of 4.1 calories per gram, the same as proteins.
2. Fats, on the other hand, have 9.3 calories per gram.
3. Carbohydrates are used mostly for energy, but your body can extract energy from proteins
too, as needed.
E. Proteins and lipids are used to build structural materials in cells, and lipids can also be stored
in fatty tissues for long-term storage of energy.
F. Obesity is a common problem in North America, primarily because of excessive intake of
fats.
G. Fiber in foods is useful to help move intestinal contents along.
H. Low-fiber, high-fat diets are correlated with increased incidence of colon cancer.
I. Essential Substances for Growth
1. Essential nutrients, such as essential amino acids, are those the body cannot manufacture
from raw materials; these nutrients must be included in the diet.
2. Vitamins
a. Vitamins are essential organic nutrients that often act as cofactors for cellular
enzymes.
b. Consuming sufficient quantities of essential vitamins is necessary for good health.
3. Trace Elements
a. Minerals and a number of trace elements are needed for biochemical pathways.
Digestion (p. 558)
26.2
26.3
Types of Digestive Systems (p. 558; Figs. 26.4, 26.5)
A. Heterotrophs are divided into three groups on the basis of their food sources.
1. Animals that only eat plants are herbivores.
2. Animals that rely on meat for a food source are carnivores.
3. Omnivores are those animals that eat both plants and meat.
B. Single-celled animals and sponges digest food intracellularly.
C. Other animals digest food extracellularly, using enzymes within a digestive cavity.
D. Specialization occurs when the digestive tract has a separate mouth and anus, as is seen in all
higher animals.
E. Chemical digestion occurs as enzymes break bonds and reduce food into simple building
blocks that can be absorbed into the body.
F. Animals excrete any food molecules that cannot be absorbed.
Vertebrate Digestive Systems (p. 559; Figs. 26.6, 26.7)
A. The process by which food items are broken down mechanically and chemically and prepared
for use by the cells is called digestion.
B. In humans and other vertebrates, the digestive system consists of a tubular gastrointestinal
tract and accessory organs.
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26.4
26.5
C. In general, carnivores have shorter intestines for their size than do herbivores, which have
long, convoluted intestines as well as chambers where bacteria can digest cellulose.
D. The tubular gastrointestinal tract of vertebrates has four layers.
1. The innermost layer, comprised of epithelium, is the mucosa.
2. The second layer, made up of connective tissue, is the submucosa.
3. Third is a double layer of muscle tissue.
4. An outer serosa layer is made up of connective tissue.
The Mouth and Teeth (p. 560; Figs. 26.8, 26.9, 26.10, 26.11)
A. Digestion begins in the mouth.
B. Vertebrate Teeth
1. Teeth break up larger portions of food.
2. Carnivorous animals have pointed teeth but lack grinding teeth.
3. Omnivores, such as humans, have teeth specialized for eating both plant and animal
matter.
4. Children have 20 teeth and adults have 32 teeth.
C. Processing Food in the Mouth
1. In the mouth, the tongue helps mix the food with saliva from salivary glands.
2. Saliva contains salivary amylase, the first enzyme in digestion that goes to work on
carbohydrates.
3. Salivary secretions are controlled by the nervous system.
D. Swallowing
1. Food is moved by the tongue to the back of the mouth for swallowing.
2. The soft palate is raised, closing off the nasal cavity, and muscles push the food past the
larynx with its protective epiglottis, and into the esophagus.
The Esophagus and Stomach (p. 562; Figs. 26.12, 26.13)
A. Structure and Function of the Esophagus
1. Food is swallowed and passes through a muscular tube called the esophagus on its way to
the stomach.
2. No digestion or absorption occurs in the esophagus; it is merely a passageway.
3. Muscles in the walls of the esophagus contract in waves, called peristalsis, to propel food
along.
4. At the beginning of the stomach is a ring of muscle called a sphincter that opens so food
can move into the stomach.
B. Structure and Function of the Stomach
1. The stomach is a muscular sac that mixes foods with acid and digestive enzymes.
2. It also churns through a series of muscular contractions that function both to mix and
mechanically break up food.
3. Within the epithelial lining of the stomach are several kinds of cells that secrete portions
of the stomach's digestive juices.
4. Parietal cells secrete hydrochloric acid, and chief cells secrete an inactive form of pepsin
that, when activated by the acid in the stomach, begins the breakdown of proteins.
C. Action of Acid
1. The hydrochloric acid functions not only to activate the pepsin but also to kill bacteria
swallowed with food.
2. The mixture of partially digested food and gastric juice is called chyme.
D. Ulcers
1. Parietal cells also secrete the hormone gastrin that regulates the amount of acid
produced.
2. Susceptibility to ulcers increases when the bacterium Helicobacter pylori is present.
E. Leaving the Stomach
1. Chyme leaves the stomach through the pyloric sphincter.
2. Limited absorption occurs in the stomach; only water, aspirin, alcohol, and a few other
substances are absorbed through the stomach wall.
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26.6
26.7
26.8
The Small and Large Intestines (p. 564; Fig. 26.14)
A. Digestion and Absorption: The Small Intestine
1. The small intestine completes the chemical breakdown of food and is the major site of
absorption of nutrients.
2. An accessory organ, the pancreas, produces digestive substances that are added to the
small intestine to facilitate digestion, including digestive enzymes that complete the
digestion of carbohydrates, proteins, and fats, and bicarbonate to neutralize the acidity of
the stomach contents as they pass into the small intestine.
3. The liver produces bile, which contains bile salts that function to emulsify fats.
4. Bile is stored in the gallbladder and released into the small intestine when meals contain
fats.
5. The first portion of the small intestine is the duodenum that receives the stomach contents
and digestive products from the pancreas and liver.
6. Most of the remaining digestion occurs in the duodenum.
7. The next portion of the small intestine is the ileum that has the job of absorbing water and
nutrients from the digested material moving through it.
8. Fingerlike projections on cells called microvilli increase the surface area available for
absorption in the small intestine.
A. Concentration of Solids: The Large Intestine
1. The large intestine, or colon, compacts and stores feces, and reabsorbs water.
2. Feces are propelled toward the rectum, and they are eventually expelled through the anus.
Variations in Vertebrate Digestive Systems (p. 566; Figs. 26.15, 26.16)
A. Most animals lack the enzymes necessary to digest cellulose, although many harbor bacteria
that do the work for the animal host.
B. Cows, deer, and other organisms have rumens that house bacteria where cellulose breakdown
occurs.
C. Still other animals, such as rodents and lagomorphs, practice coprophagy to absorb the
nutrients produced by microorganisms in their digestive tracts.
D. Intestinal microorganisms also produce molecules such as vitamin K that are important to the
well-being of their vertebrate hosts.
Accessory Digestive Organs (p. 568; Figs. 26.17, 26.18)
A. The Pancreas
1. The pancreas produces digestive enzymes and bicarbonate and contributes them to the
small intestine.
2. Enzymes are produced to break down carbohydrates, proteins, and fats into their
component molecules.
3. Bicarbonate solution neutralizes the acidity of chyme leaving the stomach and entering
the small intestine.
B. The Liver and Gallbladder
1. The liver is the largest organ in the body and produces bile to aid in the digestion of fats.
2. Bile is stored in the gallbladder between meals, then released into the small intestine
during the digestive process.
3. Bile functions to emulsify fats.
C. Regulatory Functions of the Liver
1. The liver carries on a number of important metabolic functions that help regulate the
levels of nutrients in the blood.
2. It detoxifies alcohol, stores nutrients, and manufactures proteins, among other functions.
Maintaining the Internal Environment (p. 570)
26.9
Homeostasis (p. 570; Figs. 26.19, 26.20)
A. Homeostasis is the dynamic constancy of the internal environment; body conditions fluctuate
constantly within narrow limits.
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26.10
26.11
B. Regulating Body Temperature
1. The hypothalamus coordinates temperature and homeostasis.
2. Homeothermic vertebrates have physiologic means to adjust body temperature.
3. Ectothermic vertebrates must move to a more desirable temperature within their habitats.
C. Regulating Blood Glucose
1. Excess glucose is stored in the liver as glycogen under the influence of the hormone
insulin from the pancreas.
2. When glucose levels are low in the blood, the pancreas releases the hormone glucagon,
which stimulates the liver to convert glycogen back to glucose.
D. Eliminating Nitrogenous Wastes
1. When animals consume food containing amino acids and nucleic acids, they produce
nitrogenous wastes that must be eliminated from the body.
2. Vertebrates first convert these wastes to ammonia, which is so toxic to cells it must be
excreted in a very dilute urine.
3. In many animals, the ammonia is first converted to its less toxic form, urea, and excreted.
4. Still other animals excrete uric acid, which is an adaptation for water conservation.
Osmoregulatory Organs (p. 572; Figs. 26.21, 26.22, 26.23, 26.24)
A. Animals use various mechanisms for osmoregulation, the regulation of the body’s osmotic
composition, or how much water and salt it contains.
B. Flatworms use protonephridia that branch throughout the body into bulb-like flame cells.
C. Earthworms employ nephridia to obtain fluid from the body cavity through filtration into
nephrostomes.
1. As fluid passes through the tubules of the nephridia, salts are reabsorbed, and urine is
more dilute than body fluids.
D. The excretory organs of insects are called Malpighian tubules, extensions of the digestive
tract.
1. Insects create an excretory fluid by secreting potassium ions into tubules, which draws
water osmotically.
E. Kidneys are the excretory organs of vertebrates, and unlike the Malpighian tubules of insects,
kidneys create a tubular fluid by filtration of the blood under pressure.
Evolution of the Vertebrate Kidney (p. 574; Figs. 26.25, 26.26, 26.27, 26.28, 26.29, 26.30)
A. The kidney is a complex organ made up of thousands of nephrons.
B. Fluids and wastes from the blood are filtered through a glomerulus.
C. Useful nutrients and water are reabsorbed into the bloodstream.
D. Only birds and mammals can reabsorb water from their glomerular filtrate to produce a urine
that is hypertonic to blood.
E. Freshwater Fish
1. Kidneys are thought to have evolved first in the freshwater bony fish.
2. Body fluids of fish are hypertonic to surrounding fresh water, so water tends to enter the
body, and solutes tend to leave.
3. Fish counter this problem by not drinking water, by excreting very dilute urine, and by
reabsorbing ions from the filtrate back into the blood.
F. Marine Bony Fish
1. It is likely that marine bony fish evolved from their freshwater ancestors.
2. To compensate for water loss to the environment from their hypotonic bodies, marine
fish drink lots of seawater, actively excrete excess ions, and the urine they excrete is
hypotonic to their body fluids.
G. Cartilaginous Fish
1. Rather than drinking copious amounts of seawater like the marine bony fish,
cartilaginous fish concentrate blood urea, making their blood isotonic to surrounding
seawater.
H. Amphibians and Reptiles
1. Amphibians, when not in water, spend most of their time in wet places on land.
2. Amphibians produce a very dilute urine and compensate for lost sodium ions by actively
pumping in sodium through their skin from the surrounding water.
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3.
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5.
26.12
Reptiles live in a variety of habitats, wet or dry.
Those that live in aquatic habitats have excretory systems similar to fish and amphibians.
The kidneys of terrestrial reptiles reabsorb much of the salt and water in the nephron
tubules, helping to conserve water and thus blood volume.
I. Mammals and Birds
1. Modifications in the mammals and birds, especially in the loop of Henle, allow these
groups to reabsorb water and produce a hypertonic urine.
2. A long loop of Henle dips deep into the renal medulla and can reabsorb more water.
The Mammalian Kidney (p. 578; Figs. 26.31, 26.32, 26.33)
A. Within the human kidney, the ureter opens into an expanded area, called the renal pelvis.
B. Renal tissue is divided into a renal cortex and a renal medulla.
C. The basic functional unit of the kidney is the nephron, which has three major segments.
1. The first, Bowman's capsule, is a catcher's mitt-shaped portion that wraps around a wad
of blood capillaries called a glomerulus.
2. The fluid portion of the blood is forced out of the glomerulus into Bowman's capsule. The
second portion of the nephron is a hairpin-shaped tube called the loop of Henle.
3. This loop is where reabsorption of nutrients from the plasma occurs.
4. The loop of Henle is lined with active transport channels that capture and save
electrolytes, amino acids, and glucose; without selective reabsorption, these nutrients
would be lost into the urine.
5. The third segment of the nephron is called the collecting duct.
6. Reabsorption of water occurs in the collecting duct when the body needs to conserve
water.
7. Blood vessels surround the loop of Henle and the collecting duct, and these gather up
reabsorbed nutrients and water, carrying them back to the blood.
D. The Kidney at Work
1. Five steps are involved in the formation of urine.
2. Driven by blood pressure, glomerular filtrate is produced as fluids are forced from the
blood in the glomerulus and are collected by Bowman's capsule.
3. The filtrate travels through the loop of Henle where reabsorption of water occurs first,
followed by the selective reabsorption of nutrients.
4. At the end of the loop of Henle, tubular excretion of substances occurs in order to add
extra waste materials to the urine.
5. Finally, further reabsorption of water occurs in the collecting duct if the body is in need
of water.
6. The end product of these processes is urine, which is eventually expelled from the body.
LEARNING OBJECTIVES
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List the major classes of nutrients, their functions in the body, and the number of calories each has per
gram.
Describe the role of essential nutrients in the diet.
Differentiate between the modes of digestion of different kinds of animals.
Discuss the advantages in having a digestive tract that moves food one way through the body.
List the features of the vertebrate digestive system.
Discuss the functions of the parts of the mouth in digestion.
Be able to list the features and functions of the esophagus.
List the types of stomach-lining cells and their products that release digestive juices into the stomach.
Explain what steps of digestion occur in the stomach.
Describe the features of the small intestine.
List the parts and functions of the large intestine.
Compare various vertebrate digestive systems and discuss the variations.
Discuss the functions of the digestive products released by the pancreas and liver.
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Describe the metabolic functions carried out by the liver.
Explain how blood glucose is regulated through the action of pancreatic hormones and the liver.
List the various ways vertebrates employ to flush nitrogenous wastes from their bodies.
Describe how invertebrates excrete wastes.
Explain the changes that occurred in the evolution of the vertebrate kidney.
Describe the basic features of the kidney and its nephrons.
List the five steps involved in the formation of urine.
KEY TERMS
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calories (p. 554) a unit of energy
essential amino acid (p. 555) These amino acids must be supplied by the diet.
trace element (p. 555) mineral required in minute amounts
vitamin (p. 556) an essential organic substance
digestion (p. 558) Digestion refers to the physical and chemical breakdown of foods.
amylase (p. 560) an enzyme that breaks down starches into component sugars
peristalsis (p. 562) muscular movements of food through the alimentary canal
gastrin (p. 563) A hormone produced by gastric glands, gastrin regulates the production of HCl.
duodenum (p. 564) the first portion of the small intestine; pancreatic enzymes join with food here and
chemical digestion continues
villi (p. 564) finger-like projections in the small intestine
colon (p. 564) the longest portion of the large intestine
liver (p. 568) produces bile to emulsify fats in the small intestine
homeostasis (p. 570)
glycogen (p. 571) Glycogen is the starch-like compound the liver uses to store energy.
insulin (p. 571) a hormone produced by the pancreas to lower blood glucose levels
urea (p. 571) a less-toxic form of nitrogenous waste excreted by the body
osmoregulation (p. 572) the regulation of the body’s salt content
kidney (p. 573) an excretory organ found in vertebrates
nephron (p. 574) repeating disposal units of the vertebrate kidney
glomerulus (p. 578) a fine network of capillaries that act as the filtration device in the mammalian
kidney
loop of Henle (p. 578) bent portion of the renal tubule that concentrates urine and conserves water
collecting duct (p. 578) also acts as a water-conservation device when water is limited
LECTURE SUGGESTIONS AND ENRICHMENT TIPS
1.
2.
Kidney Structure. Demonstrate kidney structure to your students. Obtain a beef or pork kidney from
your local meat locker. Pork kidneys are more similar to those of humans. Beef kidneys look like
human kidneys during fetal development before the lobules fuse to form a smooth kidney. Section the
kidney longitudinally, and show students the medulla and cortex regions. Discuss where the nephrons
are found and how urine formation proceeds. Show them the renal sinus and where the ureter leaves the
kidney.
Taste Test for Fat Content. Host a taste test to determine preferences for foods with varying fat
contents. Do we all crave high-fat foods or is it a matter of mind over taste? Set up a taste test for
students involving food having three different fat contents:
Ice creams, yogurts, and/or cheeses: full-fat, low-fat, and no-fat varieties
Sour creams: full-fat, low-fat, and no-fat varieties
Snack chips: full-fat, low-fat, and no-fat varieties
Conduct the test in such a way that students don't know ahead of time whether they are sampling fullfat or no-fat varieties, and compile the results. Most people tend to prefer the taste of the full-fat
varieties, but find the low-fat versions acceptable. The no-fat alternatives seem to leave something to
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3.
be desired. Emphasize, however, that fat content can be reduced during cooking. If additional spices or
other flavorings are used, fat content can be reduced, and the dish is just as enjoyable as its full-fat
counterpart.
Kidney Dialysis. Kidney dialysis, or more accurately, hemodialysis, involves removing waste materials
from the blood by artificial means when kidneys are no longer fully functional. Explain this procedure
to students. Dialysis refers to the separation of larger particles from smaller ones through a membrane
that is selectively permeable. The artificial kidney machine is usually the device that accomplishes
dialysis of human blood. A tube is used to connect the kidney machine to the patient's radial artery.
Blood leaves the artery and enters the machine where it flows to one side of a selectively permeable
membrane made out of cellulose acetate. The other side of the membrane is bathed with an artificial
fluid called dialysate. The dialysate has the same electrolyte concentration of normal plasma. Any
excess electrolytes in the patient's blood move through the selectively permeable membrane, much the
way normal diffusion occurs. Waste materials from the blood, such as urea, also diffuse into the
dialysate. Blood proteins and blood cells are too large to filter across the membrane so they remain in
the blood. Since only about 500 ml of blood is cleansed at one time, a single dialysis session can last
from 4 to 6 hours. Dialysis is normally performed three times weekly. Disadvantages to this type of
dialysis are that it is time-consuming, and anticoagulants must be added to the patient's blood to keep it
flowing. Another form of hemodialysis has been recently developed. Continuous ambulatory peritoneal
dialysis employs the peritoneum as the selectively permeable membrane. A catheter connects the
individual's peritoneal cavity with a bag of dialysis solution. The solution enters the peritoneal cavity
by gravity, and exchange occurs between the blood and dialysate. When finished, the fluid is removed
from the peritoneal cavity and discarded. The advantage of this method is that it allows the person to
complete the dialysis process at home during sleeping hours. A drawback is the danger of infection.
CHANGES TO THE NEW EDITION
Refer to the Johnson instructor web site at http://www.mhhe.com/biosci/genbio/tlw4 for a complete list of
changes to this edition.
CRITICAL THINKING QUESTIONS
1.
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3.
What kind of diet did early humans evolve with? How is that related to what we eat today?
How does having a very simple digestive system limit the size of an animal?
Freshwater and marine bony fishes have evolved means to compensate for hypertonic and hypotonic
body fluids, respectively. Explain why humans cannot drink seawater and survive for long.
FILMS/MEDIA SUGGESTIONS
(Telephone and fax numbers and/or web sites for the sources of the following materials are listed in the
Appendix.)
Digestion. The structures and functions of the digestive tract are explained as this video explores how
dietary fat is digested and metabolized by the liver and becomes deposited. Healthy diets and how body
shape affects overall health are discussed. 1995. 20 minutes.
Films for the Humanities and Sciences, #BVL5986
The Digestive System. This video explores the functioning of the digestive system from mouth to large
intestine. Part of the Human Body Systems Series. 17 min.
CLEAVUE/eav, WW6VH 4265
The Food Machine. Live-action video using internal cameras in a journey through the digestive system
highlight this video from The Body Atlas video series. 25 minutes.
CLEAVUE/eav, WW6VH 2210
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