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Hi Biology Gurus! I am perplexed by a lab I am trying to prepare for my students to do. I want to show them that a living thing (yeast) can generate a voltage. I read about an activity that does this by putting the yeast into lime jello (http://www.instructables.com/id/Lime-Jello-Yeast-and-Carbon-Paper-Fuel-Cell/ and http://thinkbiologically.com/wp-content/uploads/2012/03/Microbial-Fuel-Cells.pdf) I can’t figure out what made the top carbon paper electrode different from the bottom one, though some of the comments to the activity suggested that it was the greater exposure of the top electrode to the air that caused it to take on the identity of the cathode.
As it turns out, you can eliminate the carbon paper sandwich altogether and still get a voltage reading. I need some help understanding the science here. Thanks in advance for any advice! :) Kalee
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Jello Experiment:
Mixed together:
2 cups boiling water
1/2 cup sugar
1 packet Knox gelatin
After the mixture cooled to 90 degrees, added 1 packet of yeast and stirred for a few minutes to dissolve. Poured into two plastic tupperware containers and left these in the fridge for about 3 hours. After 3 hours, much of the yeast appeared to have settled to the bottom of the containers, because the bottom of the gel looked grainy.
We used a multimeter to measure the voltage generated by the yeast. (http://www.amazon.com/DT830B-Digital-Voltmeter-Ammeter-Multimeter/dp/B005KGCI0Y/Yellow multimeter, red probe in the bottom hole, black probe in the middle hole, diaI turned to the 2000m.) Initially we followed the instructions on the Instructables website, sandwiching the gel between two pieces of carbon paper and inserting the probes so that they touched both the carbon paper and the gel. This configuration got a reading of about 30 millivolts.
But then, we noticed something odd. It did not appear to matter whether the multimeter probes were touching the carbon paper. In fact, it did not appear to matter *where* the probes were inserted: the reading was the same regardless. We got the second tupperware container of gel out of the fridge and tested the multimeter probes in different spots without removing the gel from the container and without including any carbon paper.
Tupperware container of gel, both probes are touching the plastic bottom of the container in different spots: initial voltage 25 millivolts.
At first the voltage appeared to one down with time, but then it started increasing steadily with time.
The second time we got a reading of -33 millivolts when we were touching the bottom of the jello container with both electrodes.
We switched the probes at the multimeter to get a positive reading.
Breathing on it increased the voltage (~45 millivolts). We think that was due to the temperature increase.
As it sat around at room temperature, the temperature of the jello increased, and this also increased the voltage. Jiggling the probes around also increased the voltage (due to heat generated by the jiggling?). We are at 88 millivolts now.
My nephew Eli blew a warm hairdryer on it. It did not increase while the hairdryer was blowing, but it went up to 116 millivolts quickly after the hairdryer was turned off. The increased airflow may have impeded the voltage increase, while the hairdryer was on, and perhaps there was a thermal delay after the hairdryer was turned off.
Both probes have to be on the bottom on the container to read the high voltage. Perhaps this is because a lot of the yeast settled to the bottom on the container. In any case, if one probe was near the top and another probe was touching the bottom, the reading was low (less than 10) or zero. If one probe was halfway down into the jello and the other was on the bottom of the container, the reading settled at around 30 millivolts.
After about 10 minutes, the voltage had dropped even with both probes on the bottom of the container. We are back at about 88 millivolts now. It does not matter how close the probes are to each other horizontally in the jello. As long as both probes are touching the bottom of the container the reading is the same. However, if the probes are actually touching each other, the reading is zero. Taking the probes out and putting them in again sometimes causes the voltage to switch to the negative. But, generally the absolute value of the voltage is approximately constant.
One experiment that we did not do that we should have done was to touch the probes to the top and bottom of the carbon paper, not the jello. (Hopefully that would not give any voltage reading.)
Conclusion: Anaerobic yeast metabolism of sugar definitely does generate a voltage. The remaining open question is, why is the potential difference being generated across the bottom of a symmetric container? Why is one side of the container different from any other?
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