1. Percentages - Food and Energy
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:

2. Functions of the Digestive System
- 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.

3. The Mouth - The Digestive Process Begins
Both mechanical and chemical digestion begin in the mouth.

4. The Mouth - The Digestive Process Begins
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.

5. Protein Digestion - The Digestive Process Begins
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.

6. Protein Digestion - The Digestive Process Begins Reading Graphs:
What do the values on the y-axis represent?
Percentage of egg white digested

7. Protein Digestion - The Digestive Process Begins Interpreting Data:
After about how many hours would you estimate that half of the protein was digested?
About 14 hours

8. Protein Digestion - The Digestive Process Begins Interpreting Data:
How much digestion occurred in 16 hours?
About 70%

9. Protein Digestion - The Digestive Process Begins Drawing Conclusions:
During which four-hour period did the most digestion take place?
From 13 to 16 hours

10. The Stomach - The Digestive Process Begins
Most mechanical digestion and some chemical digestion occur in the stomach.

11. Using Prior Knowledge - The Digestive Process Begins
Before 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.
What You Know
Food is digested in the stomach.
What You Learned
Digestion begins in the mouth.

12. Click the Video button to watch a movie about digestion.
- The Digestive Process Begins
Digestion
Click the Video button to watch a movie about digestion.

13. Click the SciLinks button for links on digestion.
- The Digestive Process Begins
Links on Digestion
Click the SciLinks button for links on digestion.

14. The Small Intestine - Final Digestion and Absorption
The small intestine is the part of the digestive system where most chemical digestion takes place.

15. The Small Intestine - Final Digestion and Absorption
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.

16. Identifying Main Ideas
- Final Digestion and Absorption
Identifying Main Ideas
As 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.
Main Idea
Chemical digestion takes place in the small intestine.
Detail
Detail
Detail
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.

17. More on the Digestive System
- Final Digestion and Absorption
More on the Digestive System
Click the PHSchool.com button for an activity about the digestive system.

18. Graphic Organizer
In the mouth, the teeth break food into smaller pieces and saliva begins to break down starches.
Esophagus pushes food from mouth to stomach.
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.
Large intestine absorbs water and eliminates waste.

19. End of Section: Graphic Organizer