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13.38: Digestion

Difficulty Level: At Grade Created by: CK-12
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What's the first step in the digestion process?

It all starts with the mouth. Food goes in, you chew it up, swallow it, then what happens? The process of turning that food into energy and proteins and other things necessary for life begins. But it all starts with the mouth.

The Start of Digestion: Mouth to Stomach

Does the sight or aroma of your favorite food make your mouth water? When this happens, you are getting ready for digestion.


The mouth is the first digestive organ that food enters. The sight, smell, or taste of food stimulates the release of digestive enzymes by salivary glands inside the mouth. The major salivary enzyme is amylase. It begins the chemical digestion of carbohydrates by breaking down starch into sugar.

The following interactive animation demonstrates the chewing and swallowing process.

The mouth also begins the process of mechanical digestion. Sharp teeth in the front of the mouth cut or tear food when you bite into it (see Figure below). Broad teeth in the back of the mouth grind food when you chew. Food is easier to chew because it is moistened by saliva from the salivary glands. The tongue helps mix the food with saliva and also helps you swallow. After you swallow, the chewed food passes into the pharynx.

Teeth are important for mechanical digestion.


From the pharynx, the food moves into the esophagus. The esophagus is a long, narrow tube that passes food from the pharynx to the stomach by peristalsis. The esophagus has no other digestive functions. At the end of the esophagus, a muscle called a sphincter controls the entrance to the stomach. The sphincter opens to let food into the stomach and then closes again to prevent food from passing back into the esophagus.


The stomach is a sac-like organ in which food is further digested both mechanically and chemically. (To see an animation of how the stomach digests food, go to the link below.) Churning movements of the stomach’s thick, muscular walls complete the mechanical breakdown of food. The churning movements also mix food with digestive fluids secreted by the stomach. One of these fluids is hydrochloric acid. It kills bacteria in food and gives the stomach the low pH needed by digestive enzymes that work in the stomach. The main enzyme is pepsin, which chemically digests protein.

See http://www.youtube.com/watch?v=URHBBE3RKEs&feature=related for additional information.

The stomach stores the partly digested food until the small intestine is ready to receive it. When the small intestine is empty, a sphincter opens to allow the partially digested food to enter the small intestine.

The following interactive animation demonstrates the processes that occur in the stomach.


  • Digestion consists of mechanical and chemical digestion.
  • Mechanical digestion occurs in the mouth and stomach.
  • Chemical digestion occurs mainly in the small intestine.
  • The pancreas and liver secrete fluids that aid in digestion.


Use this resource to answer the questions that follow.

  • http://www.hippocampus.org/Biology \begin{align*}\rightarrow\end{align*} Biology for AP* \begin{align*}\rightarrow\end{align*} Search: The Human Digestive System
  1. Diagram the digestive structures of the mouth and throat.
  2. What is the role of the tongue?
  3. How does food move down the esophagus?
  4. Diagram the stomach, labeling important features of this organ.
  5. What is gastric juice? What are its components?
  6. Why does the stomach move in series of rhythmic contractions?
  7. What happens to the cells lining the stomach?

Practice II


1. Define mechanical and chemical digestion.

2. Describe functions of the stomach.

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The major salivary enzyme in the mouth; breaks down starch into sugar.

chemical digestion

Chemical breakdown of large, complex food molecules into smaller, simpler nutrient molecules that can be absorbed by the blood.


Long, narrow digestive organ that passes food from the pharynx to the stomach.

mechanical digestion

Physical breakdown of chunks of food into smaller pieces by organs of the digestive system.

salivary glands

Glands inside the mouth that release digestive enzymes; stimulated by the sight, smell, or taste of food.


Sac-like organ of the digestive system between the esophagus and small intestine in which both mechanical and chemical digestion take place.

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At Grade

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Date Created:
Feb 24, 2012
Last Modified:
Aug 28, 2016
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