Cellular Respiration - Problem Solving

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Students will be able to use a lab set-up to understand the carbon transfer from snails and elodea.

Big Idea

Snails and Elodea, together with light, make a system that cycles matter through the system.

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Purpose of Lesson:

This is a two-day lesson that gives the students a chance to set up a complicated lab on cellular respiration and photosynthesis. This lab can be done as a real lab using snails and elodea, or it can be done as a virtual lab. In the lesson plans I'll write it using the virtual lab, however it is easy enough to get real snails and elodea from a pet store and have students perform the lab. This "real" lab takes 2 days because of the time needed for the snail and elodea system to work. The virtual lab generally takes either a block period or two days as well because of the depth of the discussion needed for the students to understand the results. I share some specific tips about pacing this lesson in my reflection.

If I am using the virtual lab and in a two-regular period class configuration, I set it up as a demonstration on the projector instead of each student at their own computer. This means that the students need to complete a lab report on their own. In some ways this is my favorite method of doing this lab because it allows me to control the pacing.  

When I am using actual snails and elodea, I confess that on the second day I go around to the test tubes and make sure they end up the right color by blowing carbon dioxide into them. This is necessary in case the snails die.   The frailty of the snails is another reason why I prefer to use the virtual set up!  If you choose to use live animals, I urge you to consider the NSTA position statement on use of live animals in the classroom and have a plan for the snails afterward instruction.  I was able to take them home to my home aquarium where they are still alive.   

Ready. Set. Engage! (Day 1)

5 minutes

Learning Goal: Discover how transfers through photosynthesis and cellular respiration. 

Opening Question: How does carbon cycle from the air, through plants and animals, and back to the air?

Students record their opening question on their learning goal sheet and are ready to start class 3 min after the bell has rung.  I reward students who get started early with ROCK STAR SCIENTIST tickets.  

Follow the links to learn more about the beginning of class strategies and ROCK STAR scientist tickets.   


5 minutes

This lab has a great hook to get kids engaged in the learning. I give the students a cup of water with a few drops of bromothymol blue indicator in it.  It is important to make sure that students understand that Bromothymol Blue is an indicator and will change colors to indicate a change in carbon dioxide.  This explanation ensures that the experiment will stay about science and not "Magical color change" Then I ask them to blow, using a straw, into the cups. The water changes from blue to green to yellow.

I share with the students that the bromothymol blue indicates that carbon dioxide is present in the cup. The more carbon dioxide -- the more yellow the substance becomes.  At the end of the engagement activity, I let the students know that we are going to use the bromothymol blue to indicate when carbon dioxide is being created through cellular respiration.

Understand and Make a Plan

30 minutes

The Virtual Lab is a well set up walk though which pushes the students to follow a scientific set-up and even has the students write out a lab report which can be printed at the end of the lab. The first step in the virtual lab that the students do is to explore the lab. They have to click on the objects in the lab and then objects are marked off the check list.  

The students then open the procedure which gives them access to the Lab Notebook. The students are prompted to write a hypothesis explaining how carbon dioxide cycles in aquarium water through snails and Elodea. In my class, I post anchor charts of photosynthesis and cellular respiration to help the students with this question. Anchor charts are a great way to create self- sufficient learners in the classroom.  They can rely on the charts for directions instead of me and this leaves me free to work with students that are having more serious problems.  

The lab notebook will prompt the students to write about how they will measure the level of COand to identify the independent variable and the control. In my classroom the students are not ready to do this on their own. We go through this part of the lab together.  

We also decide how to set up the lab together. This is a great opportunity to discuss control and variables. Test tubes 1-4 go in the light; test tubes 5-8 go in the dark. 

Tube 1- Water only

Tube 2- Water and elodea only

Tube 3- Water and snail only

Tube 4- Elodea, water, and snail 

Tube 5- Water only

Tube 6- Water and elodea only

Tube 7- Water and snail only

Tube 8- Elodea water, and snail 


Since this is a high school lab and we are using it in a 7th grade classroom, students need support planning the lab set-up. My next step is to make a chart for the class to determine whether photosynthesis or cellular respiration will be happening, or both, or neither. This is an important step with 7th graders because they need to THINK about what is happening in the cup before they are able to predict what will happen with the bromothymol blue.

This video explains how I use the Virtual Lab in a 7th grade classroom. 

Unable to display content. Adobe Flash is required.  

Carry Out Plan (Day 1)

15 minutes

The virtual lab prompts the students to set up the lab according to their design.  I really like this part of the lab because it helps the students feel more involved in the lab set-up.  The students also predict what color the test tubes will be after 24 hours.  In the virtual lab, all they have to do is click the button and the 24 hours goes by.  They will then be able to see and record the results of the lab.

Below is a screencast of what it looks like to set up the actual lab and what the results look like.

Unable to display content. Adobe Flash is required.

Evaluate / Revise the Answer (Day 2)

20 minutes

It is now time for the students to understand WHY they got their results. To help them with this problem I have the students explain each test tube result. This is a tricky conversation to have with students. The way that I have found it works the best is to start with the test tubes in the light and go through each one, facilitating a structured discussion and nudging them to come to a shared agreement. 

For example, when we start with test tube 1 I say to the kids, "Why is this test tube green?  What does green represent?" The students then have a quick discussion using their sentence starters and then I answer the question.  I make sure to have the anchor chart with the sentence starters on the board for easy student access.

I usually choose not to poll the class for these answers because I don't want to spread misconceptions.  

Once we have worked all the way through the test tubes, the students are now ready to answer the conclusion and analysis questions. 


3 minutes

Closing Statement: "We did a lab to understand how carbon cycles through living systems.  We found that the most carbon dioxide was present when the plant was able to photosynthesize and there was no snail to use up the Carbon."

Closing Question:  "What did you learn about how carbon cycles through living systems in this lab?"

Closure depends greatly on timing and is not as easy to plan in advance as opening.  You can find more information about how I manage closure here.