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Global Atmospheric Change

Author(s): Nancy Moreno, Barbara Tharp and Judith Dresden

Fuel for Living Things

Some living things, especially plants and algae, are able to build all the materials they need from very simple substances. Using energy from light, carbon dioxide and water, these organisms, known collectively as producers, are able to make carbohydrates, which serve as fuel and raw material for the processes of life. All other organisms (consumers) rely on producers for food. Food provides energy and needed raw materials.

When organisms consume food, it is broken down to release energy and to obtain building blocks for other molecules. During this process, oxygen is consumed and some carbon is given off as carbon dioxide. This can be compared to the burning of fuels, which also uses oxygen and releases carbon dioxide. When something burns, most of the energy released is given off as heat. Inside living things, some of the energy is used to maintain the body and conduct reactions necessary for life.

All organisms (with a few exceptions) release carbon dioxide when they use food. In mammals, the released carbon dioxide is carried through the bloodstream to the lungs, where it is given off when we breathe out (exhale).

In this activity, students observe how carbon dioxide gas is given off by yeast cells, when the cells use sugar as food. Red cabbage “juice” will serve as an indicator for the presence of carbon dioxide. Cabbage “juice” turns bright pink in the presence of acids, such as the carbonic acid produced by dissolved carbon dioxide in water.

When sugar is used for energy inside living things, CO2 is released. This is comparable to what happens when fuels are burned for energy.

Students can observe how they exhale CO2 by blowing vigorously with a straw into the cup of indicator solution for 5–10 minutes.

Session 1: Making the indicator (can be done in advance)

  1. Have Materials Managers collect the materials for their groups.

  2. Have students place the sliced red cabbage in the plastic bags, along with 1/2–1 cup warm water, and seal the bags tightly. Direct students to take turns gently rubbing the cabbage inside the bags until the water becomes dark purple (usually about 10–15 minutes). This is the indicator solution.

Session 2: Demonstration of cabbage juice indicator

  1. Tell students that they will be using an indicator to look for the presence of an acid. If students are not familiar with things that are acidic, list some common examples, such as lemon juice and vinegar. Explain that the indicator will be used to test for the presence of carbon dioxide (CO2), which becomes a weak acid in water.

  2. Pour some indicator liquid into a clear cup. Ask, What color is the liquid? What do you think will happen if I put something acidic into the water? Add a few drops of vinegar to the solution until it turns pink. You also may show how the indicator solution reacts to bases by adding about 1/2 teaspoon (or more) of baking soda (the solution will turn pale blue or green).

  3. Explain to students that they will be using the indicator to test for the presence of carbon dioxide (CO2), a gas that is given off when living things use food for energy.

Session 3: Conducting the investigation

  1. Talk about yeast with students. Ask, Did you know that yeast is a living thing? Explain that yeast is a living, microscopic single-celled organism. Under the right conditions, yeast begins to grow and multiply. 

  2. Direct the students to label two cups as “no food” and “food.” Have them add about 1/2 cup of warm water and 1/2 teaspoon of yeast to each cup. Ask, Do you think the yeast cells have very much to eat in the cup now? Help students understand that all living things need food to survive and grow. Ask, What do you think will happen if we add yeast food to one of the cups? Have students record their predictions. 

  3. Have one person in each group add one teaspoon of sugar to one cup. He or she should swirl or stir the contents of the cup gently. 

  4. Direct the groups to set the cups side-by-side and to observe both cups at 5–10 minute intervals. The yeast in the cup with sugar will begin to produce CO2 (making the liquid foamy) after a short period of time. Students should stir the cups (with separate stirrers) each time they make their observations. 

  5. After 30–45 minutes, instruct students to pour small, equal amounts of cabbage “juice” into both cups and to stir the mixture. Ask them to observe the colors. Have students record their observations. (The cup with sugar will be pinker in color than the other cup.)

  6. Ask, What happened when you fed the yeast? Point out that the gas given off by the yeast is the same as that given off when wood, coal or oil is burned. Help students understand that the yeast cells were using the sugar as a source of energy.

  7. Assess student understanding by having the members of each group write a paragraph describing its investigation and results.

Funded by the following grant(s)

National Institute of Environmental Health Sciences, NIH

National Institute of Environmental Health Sciences, NIH

My Health My World: National Dissemination
Grant Number: 5R25ES009259
The Environment as a Context for Opportunities in Schools
Grant Number: 5R25ES010698, R25ES06932

Houston Endowment Inc.

Foundations for the Future: Capitalizing on Technology to Promote Equity, Access and Quality in Elementary Science Education