Physical & Chemical Changes in the Digestive System

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  • 0:00 Digestive System
  • 1:09 Digestion in the Mouth
  • 2:26 Digestion in the Stomach
  • 3:23 Digestion in the Small…
  • 4:39 Lesson Summary
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Lesson Transcript
Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

Expert Contributor
Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson, we'll be learning about the changes that take place in your digestive system when you eat. We'll cover both the physical and chemical changes as they happen in different organs, like your mouth, stomach and small intestine.

Digestive System

Right now, you're probably having a snack and something to drink as you work. Brain food is important! But how does that food actually get from your plate to your brain? The answer is your digestive system, a collection of organs that break down food into smaller components that your cells can use.

There are two main ways that digestion happens: physical and chemical. In physical digestion, your body mechanically breaks down food, grinding or smashing it into smaller pieces. However, during chemical digestion, enzymes, or tiny proteins, alter the structure of the food, which is made up of lots of atoms linked together.

When your body employs chemical digestion, the bonds between certain atoms are broken, liberating smaller molecules to be taken to your cells. Although your digestive system has additional organs that perform crucial functions, like the esophagus for transporting food, the large intestine for absorbing water and the rectum for storing waste, we'll focus on three organs that are also important for the breakdown of food: the mouth, stomach and small intestine.

Digestion in the Mouth

The mouth is the first site of digestion. Even though you might think of the mouth as just a gateway to the rest of the workers in the digestive system, it actually participates in both the mechanical and chemical processes. As you might guess, your teeth head up mechanical digestion. They grind, or macerate, your food into a ball called a bolus, which is transported through the esophagus to the stomach. Your tongue also helps push the food around, forming the bolus.

Now, all that saliva in your mouth isn't just to lubricate the process, it actually contains an enzyme called salivary amylase, which mediates chemical digestion. The salivary amylase breaks down carbohydrates, like the molecules in bread, into individual sugar molecules. Your cells can't take in large carbohydrates, like starch, but they can easily use sugars. So, one goal of the digestive system is to break down large carbohydrates into sugars.

One key characteristic of chemical digestion is that it is more specific than mechanical digestion. Typically, enzymes only work on one substance; for example, salivary amylase only works on carbohydrates. Mechanical digestion, such as maceration, however, will break all food down evenly.

Digestion in the Stomach

From the mouth, the food goes to your stomach. Your stomach is a giant pit of acid, a strong chemical that easily breaks the bonds of any substance. The acid eats away at the bonds in proteins and carbohydrates, forming a soupy liquid of acid and food called chyme. As this is happening, enzymes like pepsin break down proteins, while other enzymes break down carbohydrates.

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Additional Activities

A Model of Digestion

In this lesson you learned about the two types of digestion, physical and chemical. In this activity, we are going to model both types of digestion in a "stomach" and compare what happens. Models are useful when we want to learn about something that is too large, too small, or inaccessible to us in the lab. In this activity, we'll use a model stomach to see how each type of digestion plays a role in it's function.


  1. Start by getting two plastic zipper bags. These are your model stomachs. Label one bag chemical and one bag physical.
  2. Next add one slice of bread to each bag.
  3. Next, in one bag pour 1/4 cup of vinegar and in the other bag pour nothing.
  4. Next, use your hands to mash up the bread in each bag. Then, answer the analysis questions.


  1. What did the vinegar in the first bag represent in the stomach?
  2. What did the motion of your hands represent in the stomach?
  3. Which bag had a more complete "digestion" of the bread and why do you think that?
  4. Why do you think the stomach needs to use both physical and chemical digestion?

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