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Fuel for Living Things

Author(s): Nancy P. Moreno, PhD, Barbara Z. Tharp, MS, and Judith Dresden, MS.

Let's Talk About It

This activity allows students to observe how carbon dioxide gas is released by yeast cells when they use sugar as food. The cabbage juice turned pink when yeast was given sugar. This is because yeast produced carbon dioxide, a weak acid when it used the sugar for food. 

Ask, What happened when you fed the yeast sugar? Students should observe foaming in the container where the sugar was present. What color did the cabbage juice turn after you added it to the ‘FOOD’ cup? Students should observe a slight color change: from a purple color to a slightly pink color.

Remind students that yeast is a microscopic living organism and that living organisms need food to grow and reproduce. They use energy and oxygen, and release carbon dioxide as a waste product. Help students understand that the yeast cells used the sugar as a source of energy. 

Ask, How did we detect if the yeast released carbon dioxide? Why did the cabbage juice turn pink after adding it to the “FOOD” cup? Tell students that the cabbage juice was an indicator that helped up to detect the carbon dioxide produced by the yeast after it was fed sugar. The indicator solution turned pink when the presence of carbon dioxide caused the water to turn acidic. Point out that the gas given off by the yeast (carbon dioxide) is the same gas given off when wood, coal or oil is burned. Also note that the “NO FOOD” cup showed no foaming when sugar was added and no color change when the indicator was added.


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