Lesson 2 | The Digestive System



Name Date Class

LESSON 2

The Digestive System

A. Functions of the Digestive System

1. All that a person eats goes through four steps—ingestion,

digestion, absorption, and elimination.

2. is the act of eating, or putting food in the mouth.

3. is the mechanical and chemical breakdown of food into

small particles and molecules that your body can absorb and use.

4. occurs when the cells of the digestive system take in

small molecules of digested food.

5. is the removal of undigested food and other wastes

from the body.

B. Types of Digestion

1. In , food is physically broken into smaller pieces.

2. In , chemical reactions break down pieces of food into

smaller molecules.

3. are proteins that help break down large

molecules into smaller molecules. They also speed up, or catalyze, the rate of

reactions. do not change in

the reactions.

4. The digestive system produces enzymes that are specialized to help break down

each type of food .

C. Organs of the Digestive System

1. The digestive system is made of two parts: the and the

other organs that help the body break down and absorb food.

2. Mechanical digestion of food begins in the .

3. contains an enzyme that helps break down

carbohydrates.

4. The is a muscular tube that connects the mouth to the

stomach.

5. Food moves through the esophagus and the rest of the digestive tract by waves of

muscle contractions, called .

Digestion and Excretion 29

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Lesson Outline continued

6. The is a large, hollow organ that stores food and aids in

chemical digestion.

a. The stomach contains a(n) fluid called gastric juice.

As food mixes with gastric juice in the stomach, it forms a thin, watery liquid

called .

b. Gastric juice contains , an enzyme that helps break

down protein.

7. Chemical digestion of proteins, carbohydrates, nucleic acids, and fats takes place in

the first part of the small intestine, called the .

8. The folds of the small intestine are covered with fingerlike projections

called .

9. move from the small intestine into the blood through

small blood vessels in the villi.

10. The pancreas, , and gall bladder produce substances that

enter the small intestine and help with chemical digestion.

11. As food travels through the large intestine, is absorbed.

Materials that pass through the large intestine are the

products of digestion.

12. in the intestines digest food. They also produce

important and amino acids.

D. The Digestive System and Homeostasis

1. Nutrients from the digestive system are necessary for other body systems to

maintain .

2. Blood in the circulatory system carries nutrients from digestion to all other body

systems, providing these other systems with materials that

contain .

30 Digestion and Excretion

ame Date Class

LESSON 2

Lactose Intolerance

For nearly all people, the first food that

enters the digestive system is milk, whether

it is from a formula or a mother’s breasts.

Some people, however, are lactose

intolerant, meaning they cannot properly

digest the lactose in milk. Lactose is a kind

of sugar. Normally, it is broken down into

simpler sugars by the enzyme lactase,

which is present in the small intestine.

People who are lactose intolerant do not

produce enough lactase to digest milk.

When they drink milk, they get the

following symptoms: diarrhea, nausea,

stomach cramps, bloating, gas.

Aging and Lactase

It’s rare for an infant to be born lactose

intolerant. Babies generally produce more

lactase than older children and adults.

Lactase production decreases with age.

That’s why a person’s chances of become

lactose intolerant increase with age.

Lactose intolerance can also be a

temporary condition caused by an illness

or trauma to the small intestine. In most

cases, the condition clears up on its own

within weeks.

Getting Essential Nutrients

Milk is an important source of calcium

and vitamins D and A. People who are

lactose intolerant can still get these

nutrients by making careful dietary choices.

The few infants who are lactose intolerant

can be given lactose-free formula. Older

children and adults can substitute soy or

rice milk for cow’s milk. They can also get

calcium from foods such as sardines,

broccoli, and oranges.

Doctors point out that the degree of

lactose intolerance varies from person to

person. Someone who has difficulty

digesting a whole glass of milk might be

able to sip milk throughout the day. People

might also be able to tolerate other dairy

products that contain less lactose, such as

certain cheeses or yogurt.

Applying Critical-Thinking Skills

Directions: Answer each question.

1. Infer Premature infants have a greater risk of lactose intolerance than babies who are

born full term. What does this tell you about lactase production in the womb?

2. Analyze A food diary is a kind of journal that lists the foods someone eats each day.

How might a food diary help a person who has lactose intolerance?

42 Digestion and Excretion

Name Date Class

LESSON 2

Where does your food go?

You come home after school and smell the delicious aroma of your favorite dish

simmering in the oven. Your mouth begins to water. Your body prepares for digestion by

producing the moisture in your mouth, known as saliva.

System Diagram

The diagram below shows the digestive system. In this activity, you will trace the path of

your favorite food through the digestive system.

• Identify the four steps that food goes through after it enters your mouth.

• Label each organ of the digestive system. Describe its function.

• Explain what happens to food as it passes through various organs in the digestive

system. Use the terms mechanical digestion, chemical digestion, enzymes, peristalsis, chyme,

villi, and bile in your explanation.

Digestion and Excretion 43

Name Date Class

LESSON 2

The Digestive System

Multiple Choice

Directions: On the line before each question, write the letter of the correct answer.

1. Which function is characteristic of the digestive system?

A. relays signals between cells

B. regulates chemical reactions in the body

C. helps break down food into nutrients the body can use

2. Which is a protein that helps break down food molecules?

A. villi

B. enzyme

C. peristalsis

Matching

Directions: On the line before each definition, write the letter of the term that matches it correctly. Each term is

used only once.

3. breaks down food physically

4. receives undigested waste

5. tube that connects the mouth to the stomach

6. structures in the small intestine that release

nutrients into blood

7. most nutrients from food are absorbed here

8. acidic liquid that helps break down food in the

stomach

9. starts chemical digestion of food in the mouth

10. muscle contractions that move food through the

digestive system

A. large intestine

B. esophagus

C. mechanical digestion

D. small intestine

E. villi

F. peristalsis

G. gastric juice

H. saliva

44 Digestion and Excretion

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Lesson Outline

Enrichment

Challenge

Lesson Quiz A

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