Food and Digestion

Percentages A percentage (%) is a ratio that compares a number to 100. For example, 30% means 30 out of 100.

Suppose that a person eats a total of 2,000 calories in one day. Of those calories, 300 come from protein. Follow these steps to calculate the percentage of calories that come from protein.1. Write the comparison as a fraction:

2. Multiply the fraction by 100% to express it as a percentage:

- Food and Energy

Food and Digestion - The Digestive Process Begins

Functions of the Digestive System

The digestive system has three main functions. First, it breaks down food into molecules the body can use. Then, the molecules are absorbed into the blood and carried throughout the body. Finally, wastes are eliminated from the body.

Food and Digestion - The Digestive Process Begins

The Mouth

Both mechanical and chemical digestion begin in the mouth.

Food and Digestion - The Digestive Process Begins

The Mouth

The shape of an enzyme molecule is specific to the shape of the food molecule it breaks down. Here an enzyme breaks down a starch into sugars.

Food and Digestion

Protein Digestion

A scientist performed an experiment to determine the amount of time needed to digest protein. He placed small pieces of hard-boiled egg white (a protein) in a test tube containing hydrochloric acid, water, and the enzyme pepsin. He measured the rate at which the egg white was digested over a 24-hour period. His data are recorded in the graph.

- The Digestive Process Begins

Food and Digestion

Protein Digestion

Percentage of egg white digested

Reading Graphs:

What do the values on the y-axis represent?

- The Digestive Process Begins

Food and Digestion

Protein Digestion

About 14 hours

Interpreting Data:

After about how many hours would you estimate that half of the protein was digested?

- The Digestive Process Begins

Food and Digestion

Protein Digestion

About 70%

Interpreting Data:

How much digestion occurred in 16 hours?

- The Digestive Process Begins

Food and Digestion

Protein Digestion

From 13 to 16 hours

Drawing Conclusions:

During which four-hour period did the most digestion take place?

- The Digestive Process Begins

Food and Digestion - The Digestive Process Begins

The Stomach

Most mechanical digestion and some chemical digestion occur in the stomach.

Food and Digestion

What You Know

What You Learned

Using Prior KnowledgeBefore you read, look at the section headings and visuals to see what this section is about. Then write what you know about the digestive system in a graphic organizer like the one below. As you read, write what you learn.

1. Food is digested in the stomach.

1. Digestion begins in the mouth.

- The Digestive Process Begins

Food and Digestion

Digestion

Click the Video button to watch a movie about digestion.

- The Digestive Process Begins

Food and Digestion

Links on Digestion

Click the SciLinks button for links on digestion.

- The Digestive Process Begins

Food and Digestion - Final Digestion and Absorption

The Small Intestine

The small intestine is the part of the digestive system where most chemical digestion takes place.

Food and Digestion - Final Digestion and Absorption

The Small Intestine

Tiny finger-shaped projections called villi line the inside of the small intestine. Villi absorb nutrient molecules. The molecules pass from the villi into blood vessels.

Food and Digestion

Main Idea

Detail Detail Detail

Identifying Main IdeasAs you read the section “The Small Intestine,” write the main idea in a graphic organizer like the one below. Then write three supporting details that further explain the main idea.

Chemical digestion takes place in the small intestine.

Food mixes with enzymes and secretions.

Starches, proteins, and fats are digested chemically.

Enzymes and secretions are produced by the small intestine, liver, and pancreas.

- Final Digestion and Absorption

Food and Digestion

More on the Digestive System

Click the PHSchool.com button for an activity aboutthe digestive system.

- Final Digestion and Absorption

Food and Digestion

Graphic Organizer

Esophagus pushes food from mouth to stomach.

In the mouth, the teeth break food into smaller pieces and saliva begins to break down starches.

Large intestine absorbs water and eliminates waste.

In the stomach, food is churned and mixed with digestive juices that break down protein.

In the small intestine, almost all chemical digestion and absorption occurs.

Food and Digestion

End of Section:Graphic Organizer