Natural Selection Image Map

Activity 1 


The purpose of this activity is to display the movement of carbon from sugar to air. We do this by performing the bread making exercise at


2 cups white bread flour
1 cup whole wheat flour
3 tablespoons powdered milk
3 tablespoons sugar
1 teaspoon salt
1 package Rapid Rise yeast
3 tablespoons vegetable oil
1 cup hot water (125 to 130 degrees F)
Extra flour to use during the kneading process

Oven to bake at 350 degrees

Student Worksheet


After reading about exchange cycles, you should be aware that carbon is constantly re-cycled within the environment.  Carbon recycling occurs when the 6-carbon glucose molecule is broken down into two 3-carbon molecules (pyruvic acid) by living organisms (including yeast).  In the aerobic (oxygen present) respiratory process of animals, the pyruvic acid molecules enter the Krebs Cycle, and the carbons are released in the form of carbon dioxide (see the Energy unit in Cells Are Us).  When there is no oxygen available, the pyruvic acid molecules do not enter the Krebs Cycle.  Instead, they enter the process of fermentation.  Fermentation breaks pyruvic acid into a 2-carbon ethanol molecule and one carbon dioxide molecule.

Perform the exercise at the Web site above, according to any modifications by your teacher. Then, answer the questions on your worksheet.

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