Yeast & Bananas Investigation

24 teachers like this lesson
Print Lesson


SWBAT explain the role of decomposers in an ecosystem.

Big Idea

In this lesson, students will be continue learning about decomposers, including mold and yeast, which are both types of fungi. Then, students will complete an investigation using bananas and yeast!

Lesson Overview

Inquiry Based Instructional Model

To intertwine scientific knowledge and practices and to empower students to learn through exploration, it is essential for scientific inquiry to be embedded in science education. While there are many types of inquiry-based models, one model that I've grown to appreciate and use is called the FERA Learning Cycle, developed by the National Science Resources Center (NSRC):

1. Focus

2. Explore

3. Reflect

4. Apply

A framework for implementation can be found here

I absolutely love how the Center for Inquiry Science at the Institute for Systems Biology explains that this is "not a locked-step method" but "rather a cyclical process," meaning that some lessons may start off at the focus phase while others may begin at the explore phase. 

Finally, an amazing article found at, How Inquiry-Based Learning Works with STEMvery clearly outlines how inquiry based learning "paves the way for effective learning in science" and supports College and Career Readiness, particularly in the area of STEM career choices. 

Unit Explanation

In this unit, students will first develop an understanding of the biotic and abiotic factors within ecosystems, the characteristics and classification of living organisms, and how plants and animals obtain and use energy to fulfill their needs. 

Then, students will delve deeper into the NGSS standards by examining the interdependent relationships within an ecosystem by studying movement of matter between producers, consumers, and decomposers by creating models of food chains and food webs.  

At the end of this unit, students will study ways that individual communities can use science ideas to protect the Earth's resources and environment.  

Summary of Lesson

Today, I will open the lesson by adding onto our class decomposer poster from yesterday. Students will then begin an investigation involving the decomposition process of banana slices with and without yeast. 

Next Generation Science Standards  

This lesson will support the following NGSS Standard:

5-LS2-1.    Develop a model to describe the movement of matter among plants, animals, decomposers, and the environment.  

Scientific & Engineering Practices

For this lesson, students are engaged in the following Science & Engineering Practice:

Science & Engineering Practice 3: Planning and Carrying out Investigations

Students will plan and conduct an investigation with bananas and yeast. We will also review discuss the importance of controlled variables during the investigation process. 

Crosscutting Concepts

To relate ideas across disciplinary content, during this lesson I focus on the following Crosscutting Concept

Crosscutting Concept 2: Cause and Effect

Students will identify and test relationships. If yeast is a decomposer, then what will happen if we place yeast on one banana slice and not the other? By examining this model, students will then be able to make sense of cause and effect relationships between decomposers and larger ecosystems.  

Disciplinary Core Ideas

In addition, this lesson also aligns with the following Disciplinary Core Ideas

LS2.A:  Interdependent Relationships in Ecosystems

The food of almost any kind of animal can be §traced back to plants. Organisms are related in food webs in which some animals eat plants for food and other animals eat the animals that eat plants. Some organisms, such as fungi and bacteria, break down dead organisms (both plants or plants parts and animals) and therefore operate as “decomposers.” Decomposition eventually restores (recycles) some materials back to the soil. Organisms can survive only in environments in which their particular needs are met. A healthy ecosystem is one in which multiple species of different types are each able to meet their needs in a relatively stable web of life. Newly introduced species can damage the balance of an ecosystem. (5-LS2-1)

LS2.B:  Cycles of Matter and Energy Transfer in Ecosystems

Matter cycles between the air and soil and among plants, animals, and microbes as these organisms live and die. Organisms obtain gases, and water, from the environment, and release waste matter (gas, liquid, or solid) back into the environment. (5-LS2-1)

Choosing Science Teams

With science, it is often difficult to find a balance between providing students with as many hands-on experiences as possible, having plenty of science materials, and offering students a collaborative setting to solve problems. Any time groups have four or more students, the opportunities for individual students to speak and take part in the exploration process decreases. With groups of two, I often struggle to find enough science materials to go around. So this year, I chose to place students in teams of three! Picking science teams is always easy as I already have students placed in desk groups based upon behavior, abilities, and communication skills. Each desk group has about six kids, so I simply divide this larger group in half. 

Gathering Supplies & Assigning Roles

To encourage a smooth running classroom, I ask students to decide who is a 1, 2, or 3 in their groups of three students (without talking). In no time, each student has a number in the air. I'll then ask the "threes" to get certain supplies, "ones" to grab their computers, and "twos" to hand out papers (or whatever is needed for the lesson). This management strategy has proven to be effective when cleaning up and returning supplies as well!  


20 minutes

Lesson Introduction & Goal 

I introduce today's learning goal: I can explain the role of decomposers in an ecosystem. Referring to the Decomposers Poster Before, I explain: Yesterday, we began discussing the importance of decomposers in ecosystems. We now know that decomposers make nutrients available to plants because they recycle the nutrients from dead organisms back to living organisms. Today, we're going to take a closer look at a specific type of decomposer, called yeast. Let's take a moment to review the decomposition process first. 

Review & Engage

I continue referring the Decomposers Poster: Yesterday, we talked about the decomposition process and how large and small organisms work together to break down dead matter. We have large scavengers, such as a hawk, that eats dead organisms. Then, we have small scavengers, such as a beetle, that break matter down into even smaller pieces. Finally, there are decomposers that break matter down into soluble chemical nutrients. This means that the nutrients are so small that they can be dissolved in water. As a result, plants can absorb water along with the nutrients dissolved in the water. 

Prior to the lesson, I printed and cut out ten of the Scavengers and Decomposers Cards found at the following site. Each card is numbered, so I purposefully cut the numbers off so that students focus on the organisms on the cards instead of finding a pattern with the numbers! I made sure to print a variety of cards: Large Scavengers (crow, turkey vulture), Small Scavengers (termite, sow bug, night crawler, ant), and Decomposers (bacteria, mushroom, mold, actinomycetes). 

At this time, I pass out a card to each group and ask them to discuss the best-fit category for each organism... a large scavenger, small scavenger, or a decomposer. It doesn't take long for students to decide!

One by one, each group sends a group member up to the poster to tape their organism in the correct category: Adding Pictures to Poster. Meanwhile, the rest of the group explains why they chose that category. For example, one student explains, "The mushroom is a decomposer because it breaks matter down into chemical nutrients."

Types of Decomposers

Building upon student knowledge of decomposers, we continue completing the poster, Decomposer Poster After, by discussing types of bacteria and fungi. We first discuss bacteria, going over the fact that bacteria can either be aerobic (requires oxygen) or anaerobic (do not require oxygen). Next, we move on to fungi: first we talk about mushrooms, then mold, and finally yeast. It's important to end on yeast as this provides a nice sag-way into today's investigation! 

Balloon Demonstration

Today, I want to end by discussing the following balloon demonstration with students. I decide to set it up now so that the balloon has time to inflate while students are working.  

I place warm water, a 1/2 tsp of yeast, and a 1/2 tsp of sugar inside a clear bottle. Next, I place a balloon over the top and ask students to turn and share: What do you think is going to happen? Many students predict that the balloon is going to fill up with various amounts of air (ranging from a little to a lot), but they aren't sure why. We'll continue discussing this demonstration later on! 


30 minutes


Before today's lesson, I empty the materials out of Team Boxes and replace them with the following materials for each of my ten teams:


Getting Ready

After teams gather their investigation supplies (#1 students got the bananas, #2 students got the team boxes, and #3 students got the yeast), I explain: Today, we are going to use the Scientific Method to investigate how yeast, a common decomposer, works. As you know, yeast is a decomposer because it is a type of fungus. As with all decomposers, yeast helps break down organisms into chemical nutrients. 

Scientific Method

At this time, I also invite students to get out their science journals to write out a question, prediction, and procedure for this investigation: Investigation in Student Journal. My students are still learning how to write out a procedure, so I choose to have the whole class write one out together. As students suggest what to write next, I model this in my journal: Teacher Model.

Referring to the The Scientific Method posters on the wall, I ask: What is the first step in the Scientific Method? (Ask a question)

Investigative Question

Holding up the bags and bananas, I continue: What could we do do investigate the effect of yeast on a banana? (Place a banana slice in each bag. Then add yeast to one bag and not the other.)

What would your investigative question be? One student suggests, "Which banana decomposes faster... the one with yeast or the one without yeast?" I then model how to rewrite this question to fit the following questions model: What effect does (the changed variable) have on the (measured variable)? (What effect does yeast have on the decomposition of bananas?) 

Making a Prediction

Following the the Scientific Method, we then move on to writing a prediction. Students then share their predictions out loud. One student shares, "I predict that the banana with yeast will decompose faster." 

Controlled Variables & Writing a Procedure

Remember, during science investigations, we want to control all the variables, but one. If we only change one variable, we can actually tell why one banana decomposes differently than the other. In this investigation, what do we want to be different (or changed) between the two bags? (one bag has yeast, the other does not) As we write the procedure, begin thinking about the variables we want to keep the same. 

What do you think we should do first? One student says, "Put one banana slice in one bag and the other banana slice in the other bag." Then another student suggests, "They need to be the same size banana slice." Students agree to write: 1. Put the same size banana pieces in each bag. 

We then discuss the next few steps, including labeling, adding yeast, sealing the bags tightly, and making observations. 


At this point, students follow their procedures with their teams. (Here's an example of a Banana with Yeast looks like.) This is a quick and easy project for students to complete. 

Monitoring Student Understanding

Once students begin working, I conference with every group. My goal is to support students by asking guiding questions (listed below). I also want to encourage students to engage in Science & Engineering Practice 7: Engaging in Argument from Evidence

  1. What have you noticed? 
  2. Why do you suppose ____? 
  3. What have you found so far? 
  4. Has your thinking changed? 
  5. What evidence do you have? 
  6. Why is it important to date each bag?
  7. What do you think is going to happen? 

Student Conferences

During this conference, Students Completing the Investigation, the students are doing a great job completing the steps of the procedure. As students work together, I encourage them to include all group members and to make sure it's okay if they compete the next step. These collaboration skills are not only important in science, but in life! I also ask these students, "Why do you think it's important to get all the air out of the bag?" I love how one student makes a connection with yesterday's video that showed decomposing fruit over time. She explains how the grapes were in the center of the fruit and were bunched together. She then reasons that the grapes would have less exposure to air, which may have slowed the decomposition process. 

Clean Up

After students finish, they place their banana bags back in their team boxes and return them to the counter. Several days later, I'll check on the bananas and I'll be thankful that we placed them in the plastic shoe boxes. Boy did they smell! 

Reflect & Apply

10 minutes

Balloon Demonstration

I end today's lesson by calling attention to the Yeast & Balloon Demonstration that we started at the beginning of the lesson. The balloon is now extended straight up and has filled with a gas. I ask students to explain why they think has happened. 

After some time, one student shares, "The balloon is filling up with gas." I respond: You're right! As yeast decomposes the teaspoon of sugar, carbon dioxide is produced. This is the same gas that plants need to produce sugar using the sun's energy and it's the same gas that is found in soda! What do you think might happen with your banana and yeast bag? Students comment, "It might get bigger too as it fills with carbon dioxide." (As a side note, this is exactly what happens to the banana/yeast bags! They fill with air as the yeast breaks down sugar into ethanol and carbon dioxide.)