Ms. Green's biology students were studying cellular respiration. One simple organism, yeast, can be used to test the rate of fermentation using different variables. Each group mixed 2 grams of yeast with warm water and placed the mixture into one of five test tubes. The variables can be seen on the graph of student data. Test tube five held only yeast and warm water. The groups used the height of the carbon dioxide bubbles produced in a test tube to determine which group had the highest rate of fermentation.