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CHAPTER 36
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Date
Digestive and Excretory Systems
SECTION
1 Nutrition
KEY IDEAS
As you read this section, keep these questions in mind:
• How do our bodies use energy from food?
• What nutrients provide energy for cellular activity?
• What other types of nutrients are required in our diets?
• Why is it important to be physically active?
READING TOOLBOX
Summarize in Pairs Read
this section quietly to
yourself. Then, talk about
the material with a partner.
Together, try to figure out
the parts that you didn’t
understand.
READING CHECK
1. Explain What happens to
the extra energy that results
when a person consumes
more calories than they use?
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Compare With a partner or
in a small group, talk about
how carbohydrates and fats
are the same and how they
are different.
How Do Foods Supply Energy?
Foods contain nutrients. A nutrient is a substance
that the body requires for energy, growth, repair, and
maintenance. Some nutrients have chemical bonds that
store energy. When the body breaks these chemical bonds,
energy is released from the nutrients. The body uses this
energy to function. Even when you are not moving, your
body needs energy to breathe, pump blood, and grow.
The energy stored in nutrients is measured in units
called calories. Nutritionists measure food energy in
Calories (with a capital C ). One Calorie is equal to
1,000 calories. When you consume more calories than you
use, your body stores the extra energy as fat. As a result,
you will gain weight. When you consume fewer calories
than you use, your body breaks down its fat stores for
energy. As a result, you will lose weight.
What Nutrients Provide Energy?
Carbohydrates, proteins, and fats are three nutrients
that the body requires in large amounts. These nutrients,
described below, contain many chemical bonds that can
be broken to release energy. They supply most of the
energy the body needs. They also provide the body with
building materials for growth, repair, and maintenance.
Nutrient
Functions
Food sources
Carbohydrates
(made of sugars)
used for energy
honey, potatoes, breads, cereal
grains
Proteins
(made of amino
acids)
used as enzymes
and antibodies,
make up muscles
eggs, milk, fish, poultry, and beef
Fats
(made of fatty
acids)
store energy, provide butter, cream, oils
insulation, make cell
membranes, dissolve
certain vitamins
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Nutrition continued
What Other Nutrients Does the Body Require?
The body also requires nutrients that do not provide
energy. These nutrients include water, vitamins, and
minerals. They help the body function. The body requires
smaller amounts of vitamins and minerals than of
carbohydrates, proteins, fats, and water.
WATER
You can survive longer without food than without water.
The body uses water to regulate temperature and transport
substances such as gases, nutrients, and wastes.
READING CHECK
2. List What are three
nutrients the body requires
that do not provide energy?
VITAMINS
Vitamins are organic substances that are found in
many foods. There are many different kinds of vitamins.
Each vitamin plays a different role in metabolism. For
example, vitamin D helps the body absorb calcium.
Vitamins are classified into two groups: fat soluble
and water soluble. Vitamins A, D, E, and K dissolve in
fats. They are stored in body fat and are toxic in excess
amounts. Vitamins B and C dissolve in water. Therefore,
excess amounts of these vitamins are excreted.
Background
Recall that metabolism is
the sum of all chemical
processes that occur in an
organism.
MINERALS
A mineral is an inorganic compound that must be
supplied in the diet. A few common minerals and their
functions are described in the table below.
Mineral
Function
Food sources
Sodium
maintains water balance, helps
nerves function
table salt, processed foods
Potassium
maintains water balance, helps
nerves and muscles function
meats, fruits, vegetables,
beans
Calcium
helps form healthy bones and
teeth, helps blood clot
milk, dark-green leafy
vegetables
Iron
helps in bone growth, transports red meat, whole grains,
oxygen in blood
peas, eggs
Iodine
helps regulate thyroid hormone
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3. Predict What would
happen if there were no iron
in the blood?
iodized salt, seafood
Why Is Physical Activity Important?
Good nutrition must be balanced with regular physical
activity to maintain a healthy body. Physical activity can
help the body maintain a healthy amount of body fat.
Excess body fat can increase a person’s risk for many
diseases, such as heart disease and Type II diabetes.
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Section 1 Review
SECTION VOCABULARY
calorie the amount of energy needed to raise
the temperature of 1 g of water 1 ºC; the
Calorie used to indicate the energy content
of food is a kilocalorie
mineral a class of nutrients that are chemical
elements that are needed for certain body
processes
nutrient a substance or compound that provides
nourishment (or food) or raw materials
needed for life processes
vitamin an organic compound that participates
in biochemical reactions and that builds
various molecules in the body; some vitamins
are called coenzymes and activate specific
enzymes
1. List What are four roles of nutrients in the body?
2. Identify What three nutrients provide most of the energy the body needs?
3. Explain How is energy released from nutrients?
4. Describe What are two ways that the body uses water?
5. Explain Which vitamins are not stored in body fat and why?
6. Describe What two things must be balanced to maintain a healthy body?
7. Explain When does a person’s body store fat?
8. Describe Why is excess body fat unhealthy?
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Digestive and Excretory Systems
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CHAPTER 36
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Digestive and Excretory Systems
SECTION
2 Digestion
KEY IDEAS
As you read this section, keep these questions in mind:
• How is food broken down into a form that the body can use?
• Where does digestion begin?
• How does the body absorb nutrients?
• What happens to the material that the body cannot use?
How Does the Body Break Down Food?
The process of breaking down foods into molecules
that the body can use is called digestion. As shown
below, the digestive system consists of a long, winding
tube that food passes through from the mouth to the
esophagus, stomach, small intestine, and large intestine.
The digestive system also includes the liver and
pancreas. Food does not pass through these organs.
Instead, they aid digestion by delivering fluids to the
digestive tract through tubes called ducts.
READING TOOLBOX
Organize As you read this
section, make a chart that
describes the function of
each part of the digestive
system.
READING CHECK
1. Explain How do the
liver and pancreas aid in
digestion?
Mouth
Esophagus
Liver
Stomach
Pancreas
Small intestine
Large intestine
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The digestive system breaks down food into individual molecules that can
be absorbed in the bloodstream.
2. Identify What structure
does food pass through from
the mouth to the stomach?
MECHANICAL DIGESTION
In mechanical digestion, food is physically broken
into smaller pieces, but the molecules of food do not
change. This digestion includes chewing food with the
mouth and churning food in the stomach and small
intestine.
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Digestion continued
CHEMICAL DIGESTION
READING CHECK
3. Describe What does
chemical digestion of food
produce?
The process by which chemical bonds in food are
broken is called chemical digestion. Chemical digestion
uses enzymes to produce new molecules that the body can
use. Carbohydrates are broken down into simple sugars,
proteins into amino acids, and fats into fatty acids.
What Are the Steps in Digestion?
Both mechanical digestion and chemical digestion
start in the mouth. Teeth break food into smaller pieces.
The tongue mixes the food with a watery solution called
saliva. Saliva contains amylases, which are enzymes that
begin the chemical digestion of starches.
When the food is swallowed, it passes from the mouth
into the esophagus. The esophagus is a long tube that
connects the back of the mouth to the stomach. As
shown below, smooth muscles in the lower portion of
the esophagus contract in waves called peristalsis.
Peristalsis helps move food through the digestive tract.
Esophagus
Wave of
contraction
Smooth muscles in the esophagus contract in waves
to squeeze food down to the stomach. No digestion
occurs in the esophagus.
Food
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4. Explain What
does peristalsis in the
esophagus do?
Stomach
STOMACH
READING CHECK
5. Describe How does
mechanical digestion occur
in the stomach?
Mechanical digestion of food continues in the
stomach. The stomach is a muscular, saclike organ that
churns food to break it into smaller pieces. The stomach
also begins the chemical digestion of proteins.
Cells of the stomach lining secrete gastric juice,
which contains hydrochloric acid and an enzyme called
pepsin. Pepsin breaks down proteins into smaller chains
of amino acids. It works only in an acidic environment.
A mucous coating protects the lining of the stomach from
the acid.
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Digestion continued
SMALL INTESTINE
Partially digested food passes from the stomach into
the first portion of the small intestine, the duodenum.
In the duodenum, digestive enzymes, shown below,
complete the breakdown of nutrients into molecules
that are small enough for the body to absorb.
Enzyme
Substrate
Digested products
Amylase
starch
disaccharides
Trypsin
proteins
peptides
Lipase
fat
fatty acids, glycerol
Maltase, sucrase, lactase disaccharides
monosaccharides
Peptidase
amino acids
peptides
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6. Apply Concepts What
type of digestion occurs in
the small intestine?
LIVER AND PANCREAS
The liver and the pancreas play important roles in the
digestive system and in other body systems. The pancreas
secretes several enzymes into the small intestine. The liver
secretes bile. Bile is a greenish fluid that is stored in the
gallbladder. A duct carries bile from the gallbladder to the
small intestine. Bile breaks down fat into tiny droplets that
can be digested by enzymes called lipases.
READING CHECK
7. Describe What is the
function of bile?
Liver
Stomach
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Gallbladder
Duodenum
(small intestine)
8. Identify Where do
digestive enzymes from
the pancreas go?
Pancreas
The liver and pancreas contribute to the complete digestion of nutrients in
the duodenum of the small intestine.
How Does the Body Absorb Nutrients?
The body absorbs nutrients after digestion is complete.
Absorption of nutrients takes place mainly in the small
intestine after the duodenum. The small intestine is a
very long, coiled organ. The lining of the small intestine is
covered in structures that allow nutrients to pass into the
bloodstream.
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Digestion continued
VILLI
READING CHECK
9. Identify What structures
cover villi?
Villi (singular, villus) are fingerlike extensions that
cover the lining of the small intestine. Each villus is
covered with smaller extensions called microvilli.
Together, the villi and microvilli greatly increase the
surface area available for the absorption of nutrients.
Sugars and amino acids enter the bloodstream through
capillaries in the villi. Fatty acids and glycerol are
absorbed by lymphatic vessels in the villi and eventually
enter the bloodstream. Blood carries these digested
nutrients to cells throughout the body.
Microvilli
Capillaries
Villus
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10. Identify Name two
structures that are located
inside villi.
Small
intestine
Lymphatic Inner lining
of small
vessels
intestine
Villi expand the surface area of the small intestine to allow more nutrients
to be absorbed by the blood.
What Happens to Undigested Materials?
Almost all the fluids and nutrients in food are
absorbed by the small intestine. Materials that are not
absorbed pass into the large intestine.
LARGE INTESTINE
READING CHECK
11. Describe What are two
functions of bacteria that live
in the large intestine?
The large intestine has a larger diameter than the small
intestine and is not coiled. Many bacteria live in a region
of the large intestine called the colon. These bacteria
make vitamins that are not easily obtained from foods.
They also help change undigested materials into solid
waste, or feces. Peristalsis moves feces to the final section of the large intestine, called the rectum. Feces exit
the body through the anus.
The large intestine absorbs minerals and water. Failure
of the large intestine to balance water absorption can lead
to diarrhea (watery feces) or constipation (hard feces).
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Digestive and Excretory Systems
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Section 2 Review
SECTION VOCABULARY
digestion the breaking down of food into chemical
substances that can be used for energy
esophagus a long, straight tube that connects
the pharynx to the stomach
pepsin an enzyme that is found in gastric juices
and that helps break down proteins into
smaller molecules
peristalsis the series of rhythmic muscular
contractions that move food through the
digestive tract
villus one of the many tiny projections from
the cells in the lining of the small intestine;
increases the surface area of the lining for
absorption
1. Explain What is the purpose of the digestive system?
2. Describe How does mechanical digestion occur in the mouth?
3. Describe How does saliva contribute to digestion?
4. Identify What are the components of gastric juice and how do they function?
5. Explain How do amino acids from digested proteins enter the bloodstream?
6. Identify What enters the large intestine from the small intestine?
7. Predict How would a person be affected if the large intestine absorbed too much
water from undigested materials?
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Digestive and Excretory Systems