A Hands-On Activity to Explore the Energy Cycle between Photosynthesis and Cellular Respiration

This experiment has got to be my favorite hands-on activity to explore the energy cycle between photosynthesis and cellular respiration...two vital processes that keep life on Earth going. The chemical reactions may be complex, but we, as science teachers, can make them visible to our students with a simple experiment using snails, a plant, and a few other basic supplies. Here's how it works:

What You Need to Explore the Cycle of Energy

1. Snails

2. Elodea

3. Vials with lids

4. Bromthymol blue indicator solution

5. pH strips

6. Light source

7. Water

8. Straws
   

Background Information 

I like to have students think about what should happen in the hands-on activity and then investigate to prove it! Therefore, they need to know a little about photosynthesis and cellular respiration. Think about a 5E lesson. To engage and explore, the students participate in a quick respiration investigation using water, Bromthymol Blue indicator solution, flasks, and straws. The activity is simple. They add approximately 50 mL of water to a 250 mL Erlenmeyer flask and around 15 drops of Bromthymol Blue indicator solution Next, they blow bubbles into the solution by taking a deep breath and then exhaling through the straw. Have your students repeat the exhaling procedure at least five times to ensure a color change. As a result, the solution changes color, indicating the pH of the solution has also changed. Note: Set the solution aside to use later as the water in your vials.  

At this point, the students can research the color change or have a quick class discussion. As a result of respiration, we breathe out carbon dioxide, the product of cellular respiration. Carbon dioxide interacts with the water in the flask to form carbonic acid, thus lowering the pH of the solution and resulting in a color change. Next, get them to think about how we breathe in oxygen and where that oxygen comes from (photosynthesis). Ultimately, you want to connect the cycle of energy between the processes of cellular respiration and photosynthesis. Once your students have made the connection, discuss the reactants and products of these two processes, how they are opposites, and therefore a cycle. Now, you are ready to let them investigate!  

Let Them Investigate with a Hands-On Activity to Explore the Energy Cycle

I have students prepare the following using the water with the Bromthymol Blue indicator solution from the engagement/exploration activity (because it contains a carbon source for the plants to use): 

1. Vial 1-Water only

2. Vial 2-Water and Elodea

3. Vial 3-Water, Elodea, and one snail

4. Vial 4-Water, Elodea, and three snails
   

Next, the students place the cap on the vials and place the vials in an area of a light source to react. Don't forget to ask them to make their predictions, too! The next day, students make observations of the vials and note color changes and, therefore, pH. For some, the color change will be slight. Thus, we also measure the pH using pH strips. Students should see a lighter color in Vial 4 with three snails, indicating more carbon dioxide production, a greater concentration of carbonic acid, and a lower pH. In conclusion, your students will see the Elodea making oxygen during photosynthesis, the snails using the oxygen to produce carbon dioxide in cellular respiration, and the plants using the carbon dioxide during photosynthesis to start the energy cycle again.

Frequently Asked Questions

What if we don't have plastic vials with lids? You can use test tubes and stoppers.

What if we don't have Bromthymol Blue indicator solution? You can use pH strips to measure the pH changes. Don't forget to measure the initial pH and the pH after reacting overnight. 

What if I cannot get access to Elodea in my state? Many states do not ship Elodea (Egeria densa). However, they will send Elodea canadensis. Another option is to substitute algae or harvest and use plant material from a local lake, river, or stream.

What if we do not have the Bromthymol Blue solution from the engagement/exploration activity? You will need to prepare a solution with a source of carbon dioxide. You can prepare the solution by adding a small pinch of baking soda to 250 mL of water. Or, blow through a straw into the water without the Bromthymol Blue indicator solution.   

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