The Digestive System - MR. KELLECI'S SCIENCE CLASS

Digestion and Excretion

CHAPTER 13 LESSON 2

The Digestive System

What do you think? Read the two statements below and decide

whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you've read this lesson, reread the statements and see if you have changed your mind.

Before

Statement

After

3. Digestion begins in the mouth.

4. Energy from food stays in the digestive system.

Key Concepts

? What does the digestive system do?

? How do the parts of the digestive system work together?

? How does the digestive system interact with other systems?

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Functions of the Digestive System

What did you eat for breakfast today? Do you ever think about what happens to the food you eat?

Someone might have told you to take small bites and chew your food thoroughly. The size of the chewed food particles can affect how quickly your food is digested.

As soon as the food enters your mouth, it begins its journey through your digestive system. All the food you eat goes through a process with four steps--ingestion, digestion, absorption, and elimination. All four steps happen in the organs and tissues of the digestive system in the following order.

? Food is ingested. Ingestion is the act of eating, or putting food in your mouth.

? Food is digested. Digestion is the mechanical and chemical breakdown of food into small particles and molecules that your body can absorb and use.

? Nutrients and water in the food are absorbed, or taken in, by cells. Absorption occurs when the cells of the digestive system take in small molecules of digested food.

? Undigested food is eliminated. Elimination is the removal of undigested food and other wastes from your body.

Reading Essentials

Building Vocabulary As you read, circle all the words you do not understand. Highlight the part of the text that helps you define these words.

Key Concept Check 1. Describe What does the digestive system do?

Digestion and Excretion 211

REVIEW VOCABULARY

chemical reaction a process in which a compound is formed or broken down

Reading Check 2. Define What are enzymes?

Reading Check 3. Explain What happens to an enzyme after it breaks down a food molecule?

Types of Digestion

Before your body can absorb nutrients from food, the food must be broken into small molecules by digestion. There are two types of digestion--mechanical and chemical. In mechanical digestion, food is physically broken into smaller pieces. Mechanical digestion happens when you chew, mash, and grind food with your teeth and tongue. Smaller pieces of food are easier to swallow and have more surface area than larger pieces. This helps with chemical digestion. In chemical digestion, chemical reactions break down pieces of food into small molecules.

Enzymes

Chemical digestion cannot occur without substances called enzymes (EN zimez). Enzymes are proteins that help break down larger molecules into smaller molecules. Enzymes also speed up, or catalyze, the rate of chemical reactions. Without enzymes, some chemical reactions would be too slow or would not occur at all. There are many kinds of enzymes. Each one is specialized to help break down a specific molecule at a specific location.

The Role of Enzymes in Digestion

Nutrients in food are made of different molecules, such as carbohydrates, proteins, and fats. Many of these molecules are too large for your body to use. But, because these large molecules are made of long chains of smaller molecules joined together, they can be broken down into smaller pieces.

The digestive system produces enzymes that are specialized to help break down each type of food molecule. For example, the enzyme amylase helps break down carbohydrates. The enzymes pepsin and papain help break down proteins. Fats are broken down with the help of the enzyme lipase. The figure below shows how an enzyme helps break down food molecules into smaller pieces.

Notice in the figure that the food molecule breaks apart, but the enzyme does not change. Therefore, the enzyme can immediately be used to break down another food molecule.

Food particle

Enzyme

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Visual Check 4. Identify Circle the food molecule that is in the process of being broken down.

212 Digestion and Excretion

Step 1 An enzyme attaches to a food particle.

Step 2 The enzyme speeds up a chemical reaction that breaks down the food particle.

Step 3 The enzyme releases the broken-down food particle.

Reading Essentials

Organs of the Digestive System

For your body to use the nutrients in the foods you eat, the nutrients must pass through your digestive system. There are two parts to your digestive system: the digestive tract and the other organs that help the body break down and absorb food. These organs, shown at the bottom of this page, include the tongue, salivary (SA luh ver ee) glands, liver, gallbladder, and pancreas.

The digestive track extends from the mouth to the anus. It has different organs that are connected by tubelike structures. Each of these organs is specialized for a certain function.

The Mouth

Mechanical digestion of food begins in your mouth. Your teeth and tongue mechanically digest food as you chew. But even before chewing begins, your salivary (SA luh ver ee) glands produce saliva (suh LI vuh) at the thought of food. They produce more than 1 L of saliva every day. Saliva contains an enzyme that helps break down carbohydrates. It also contains substances that neutralize acidic foods and a slippery substance that makes food easier to swallow.

The Esophagus

When you swallow a bite of food, it enters your esophagus (ih SAH fuh gus). The esophagus is a muscular tube that connects the mouth to the stomach. Food moves through the esophagus and the rest of the digestive tract by waves of muscle contractions called peristalsis (per uh STAHL sus). Peristalsis is similar to squeezing the bottom of a toothpaste tube. This forces toothpaste to the top of the tube. Muscles in the esophagus contract and relax. This action pushes partially digested food down the esophagus and into the stomach.

Tongue Esophagus

Salivary glands

Create a shutterfold book to illustrate the organs of the digestive system and their functions.

Mouth Esophagus Stomach Sinmteasltl ine Linatregsetine

Reading Check 5. Define What is the esophagus?

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Liver

Gallbladder Large

intestine Rectum

Reading Essentials

Peristalsis

Stomach Pancreas Small intestine

Visual Check 6. Identify Trace with a highlighter the organ that connects the mouth to the stomach.

Digestion and Excretion 213

Reading Check 7. Describe Why is the stomach's ability to store food beneficial?

Visual Check 8. Identify Where does food go after it leaves the stomach?

The Stomach

Partially digested food leaves the esophagus and enters the stomach. The stomach, as shown below, is a large, hollow organ. One function of the stomach is to store food. This allows you to go many hours between meals. The stomach is like a balloon. It can stretch when filled. An adult stomach can hold about 2 L of food and liquid.

The stomach also helps with chemical digestion. The walls of the stomach are extremely folded. These folds enable the stomach to expand and hold large amounts of food. The cells in these folds produce chemicals that help break down proteins.

The stomach contains an acidic fluid called gastric juice. Gastric juice makes the stomach acidic. Acid helps break down some of the structures that hold plant and animal cells together, such as the cells in meat, lettuce, and tomatoes. Gastric juice also contains pepsin. Pepsin is an enzyme that helps break down the proteins in foods into amino acids. Food and gastric juices mix as muscles in the stomach contract through peristalsis. As food mixes with gastric juice in the stomach, it forms a thin, watery liquid called chyme (KIME).

1 Food moves through the esophagus to

the stomach.

2 Stomach muscles contract through peristalsis and mix

food with gastric juices.

4 Food leaves the stomach and enters

the small intestine.

3 The folds in the stomach enable it to expand to hold food.

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

214 Digestion and Excretion

Reading Essentials

The Small Intestine

Chemical digestion begins in the mouth and the stomach. But most chemical digestion occurs in the small intestine. The small intestine is a long tube that is connected to the stomach. Chemical digestion and nutrient absorption take place in the small intestine. The small intestine is named for its small diameter--about 2.5 cm. The small intestine is about 7 m long.

Chemical digestion of proteins, carbohydrates, nucleic acids, and fats takes place in the first part of the small intestine, called the duodenum (doo uh DEE num). The remainder of the small intestine absorbs nutrients from food. Like the stomach, the wall of the intestine has many folds. The folds of the small intestine are covered with fingerlike projections called villi (VIH li) (singular, villus). Each villus contains small blood vessels. Nutrients in the small intestine diffuse into the blood through these blood vessels. You can see the small vessels in a villus in the figure below.

Locate the pancreas and liver in the figure below. These organs produce substances that enter the small intestine and help with chemical digestion. The pancreas produces the enzyme amylase and a substance that neutralizes stomach acid. The enzyme amylase helps break down carbohydrates. The liver produces bile. Bile makes it easier to digest fats. The gallbladder stores bile until it is needed in the small intestine.

Key Concept Check 9. Name What organs work together to help with chemical digestion?

Visual Check 10. Identify the structures in the folds of the small intestine.

Liver Gallbladder

Pancreas Small intestine

Folds in small intestine covered with villi

Blood vessels

Lymph vessel

One villus

Copyright ? Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.

Reading Essentials

Digestion and Excretion 215

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