Digestive System - The Role of Enzymes

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SWBAT to explain the role of enzymes in the digestion process.

Big Idea

Digestion begins in the mouth through both mechanical and chemical digestion.


10 minutes

This lesson is taught after students have created a paper model of the digestive system. There are a variety of possible models for the digestive system, the one that I have decided to use in the past comes from Part 1 of Activity 5 from the Wonderwise Parasite Sleuth activity book (free download).

One of the reasons I like this particular model is that students are able to observe the tremendous length of the small intestine and gain an appreciation to how well our bodies are designed in both structure and function.  This model is used as a reference point for all subsequent digestive system lessons including this one.  

To engage students in lesson I conduct a formative assessment probe.  This is one of many terrific probes found in a series of books published by the NSTA*. 

The purpose of the assessment probe is to see if students understand the purpose of the digestive system (break down food so it can be used by cells). This is a first step toward my goal - students will create an argument supported by evidence later during lesson. (MS-LS1-3 -Use argument supported by evidence for how the body is a system of interacting subsystems composed of groups of cells.)

The best answer is Kassandra, " I think the main function of the digestive system is to break down food into molecules that can be absorbed by cells."  The main function of the digestive system is to break down food in a manner that cells will be able to use to absorb and make energy.

* Uncovering Student Ideas in Science (Volume 4)- 25 NEW Formative Assessment Probes by Page Keeley and Joyce Tugel (2009).


20 minutes

To begin to explore the role of both mechanical and chemical digestion in the whole digestion process we start in the mouth.  It is in the mouth where food begins to be broken down into a form that will eventually be absorbed by cells to release energy.  

Three vocabulary words that I introduce here are enzymesmechanical digestion, and chemical digestion. 

Chemical digestion refers the to the breakdown of food in the mouth, stomach and intestines through the use of acids and enzymes

Mechanical digestion refers to  processes such as chewing, swallowing and the muscular movements that move food through the digestive tract, it supports chemical digestion by physically breaking whole foods into smaller pieces to facilitate chemical breakdown.

Teacher Note:  Students add these vocabulary words to their interactive notebook in their glossary page of their notebook and in addition these words are added to our Word Wall.

Chemical digestion and enzymes are my areas of focus for this lesson since for the majority of my students this is the first time they have been introduced to these important science concepts.

To begin to study the role of enzymes in chemical digestion I have students complete the Saltines and Spit activity.  This activity serves as a model (SP2 - Developing and Using Models) to describe unobservable mechanisms.

The procedure is found below (Students are required to follow a multi-step procedure when conducting this experiment (RST.6-8.3).

1) Hand each pair of students a cracker and place a drop of iodine which will results in the cracker turning blue-black. (i.e. may need to explain that iodine is a starch indicator)

2) Hand each students a new clean cracker and have them chewing it (NOT SWALLOW) and have students record taste observations for at least 5 minutes.

As students sit and record observations for at least 5 minutes you may want to play the following video which introduces the function of enzymes in a fun, accessible way. 

3)  Have students spit the chewed cracker into dish and add iodine, which will result in a blue-black color.  (There is not enough salivary amylase to break down starch, therefore you still have a blue-black color. Due to this we proceed to step 4.) (See video below of this step)

4)  One student per pair spits about a teaspoon of saliva into an empty cup.

5)  Each pair of students receives 2 clean test tubes, 2 small pieces of cracker, and wax paper.

6)  Wrap each small piece of cracker in wax paper and crush repeatedly.

7)  Place each crushed cracker bit into separate test tubes.

8)  Place 5ml of water in one test tube. Place 5ml of saliva in the other test tube.

9)  Mix tubes thoroughly and allow to sit at room temperature for at least 5 minutes.

10) Add one drop of iodine to each tube mix.

Students record their observations as they do the lab. (As you circulate, make sure to check on this. Sometimes they race ahead and need a reminder of the purpose of the lab - to gather data.)


10 minutes

In the next section we learn about the science behind what students observed during the explore activity.  I accomplish this through a power point presentation and a close reading of an informational text.

1)  The Amylase powerpoint presentation visually explains what an enzyme does, in this case salivary amylase.  To check for student understanding of amylase function I have students answer questions on slides 8 and 9 and then do a Think-Pair-Share.  These questions specifically assess understanding that amylase breaks down starch when there is a high proportion of amylase to starch but its effectiveness decreases as this ratio is decreased. (i.e. increase in starch relative to amount of amylase).

2)  In addition to the powerpoint, students read the "What's Going On?" section of Saltines and Spit.  The literacy strategy that I use is with this reading is Writing and Drawing in the Margins.  

Writing in the margins engages readers in the reading task and allows them to document their thinking while reading. Both writing and drawing in the margins engages students in actively thinking about the texts they read. The power of this strategy is not the actual act of writing and drawing in the margins; instead, it is the thinking processes that students must undergo in order to produce the ideas. 

The specific strategy I use of the six is Visualize, since it's imperative that students visualize the scientific explanation for the appearance of the blue-black color.  

When students are visualizing they ask:  

  • What does this look like?

  • How can I draw this concept/ idea?

  • What visual and/ or symbol best represents this idea? 


10 minutes

In this section of the lesson I visit an interactive website about the digestive system (created by John Kitses).  Students choose from a variety of foods and observe (in animation) how the body digests different foods.  

In particular I want students to observe that digestion begins at the mouth through both mechanical and chemical digestion.  In addition, the simulation shows examples of enzymes both in the mouth and stomach during digestion process.

The objective of this activity is to get students thinking about the what, where and how of the digestion process.   


5 minutes

In the last section of lesson I evaluate our progress toward the learning objective by completing an Exit Slip.  

1)  The exit slip requires students to write an argument with supporting evidence and reasoning (SP7 -Use an oral and written argument supported by evidence to support or refute an explanation or a model for a phenomenon/W.7.1 - Write arguments to support claims with clear reasons and relevant evidence.)

I want students to make the claim that digestion actually begins in the mouth, how that occurs (through a combination of mechanical and chemical digestion).  The evidence that can be used could come from the Saltine and Spit activity/text as well as the digestion simulation website.  The reasoning should include the observation that amylase breaks down starch  - this demonstrates chemical digestion in the mouth -- and the teeth break and grind the food in the mouth, exposing more of it to amylase, and making it small enough to go down the gullet.