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Name: Mary
Status: Student
Age: 17
Location: N/A
Country: N/A
Date: September 2004

we recently completed a lab in biology class wherein we took molasses and yeast and put it into 10 different test tubes with varying water concentrations. 24 hours later we checked to see how much carbon dioxide was produced in the form of bubbles. the results were varied. some tubes had very little co2 production whereas others had more. however we repeated the experiment again, this time substituting the molasses with glucose, fructose, and sucrose.

however, this time there was absolutely no co2 production. so my question is why did this change occur? i can't find any information anywhere else that will help me to solve this issue.

When you say the results were varied, do you mean that some tubes had no bubbles and some had lots? Was there a varying amount of molasses in the tubes, and did you see proportional amounts of bubbles depending on the amount of molasses? Or did the tubes contain the same amount of molasses but varying amounts of yeast? Or did all the tubes have the same of both and you got varying results? Did you see no growth with any of the other sugars or with just some? If you saw no growth at all with any of the sugars I would suspect your technique somehow. You need to be more specific as to the questions asked. Ask yourself this: What do the yeast use the molasses for? What do the bubbles represent?


It sounds like maybe your yeast culture was dead. Did you heat the water to dissolve the sugars? If so, maybe the water was too warm. This experience makes a good case for doing a control experiment along with your experimental variables. In other words, if you had also set up a tube with molasses, just like the one that worked in the first experiment, then you would have lots more information now. Other things I would look at would be the amounts of sugar you used, compared to the amount of sugar in the molasses. If you go about it systematically, I'm sure you will figure out what's going on.

Paul Mahoney, PhD

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