Laboratory Experiment 5: Cell Respiration for AP Biology
This experiment focuses on cellular respiration. A brief review of the major concepts of cellular repiration might help you understand this experiment, which attempts to examine the rate at which respiration occurs.
For a quick review, refer to the following concepts:
This experiment points out three ways to measure respiration:
- Oxygen consumption: how much O2 is actually consumed.
- Carbon dioxide production: how much CO2 is actually produced.
- Energy released during respiration: how much energy is released.
This particular experiment examines germinating peas by measuring the volume of gas that surrounds the peas at certain intervals in an effort to determine the rate of respiration. Two gases contribute to the volume around the pea: O2 and CO2. How can we use the amount of oxygen consumed during respiration as our measuring point if CO2 is present as well? Something needs to be done with the CO2 released during respiration. Otherwise we would not get a true representation of how much the volume is changing as a result of KEY IDEA oxygen consumption. The CO2 would skew the numbers by making it appear as if less O2 were being consumed.
The CO2 problem can be handled by adding potassium hydroxide, which reacts with CO2 to produce K2CO3. This reaction allows us to limit the number of variables that could be affecting the volume around our beloved peas to
- Change in the volume of oxygen.
- Change in the temperature (PV = nRT ).
- Change in pressure of the surrounding atmosphere.
Aerobic respiration requires and uses oxygen. So, one would expect the volume around the pea to decline as respiration occurs. The reactions of interest for this experiment occur in a tubelike device known as a respirometer. To calculate the change in volume that occurs with these peas, one first has to measure the initial volume around the peas. A control group must then be set up that consists of peas that are not currently germinating and will have a rate of respiration lower than that of germinating seeds. This will give the experimenter a baseline with which to compare the respiration rate of the germinating seeds. Since temperature and pressure are also able to affect the volume around the peas, it is important to set up another control group that can calculate the change in volume that is due to temperature and pressure as opposed to respiration. Any changes in this control group should be subtracted from the changes found in the germinating seeds to determine how much of the\ volume change is actually due to oxygen consumption and respiration.
Just a side thought—can you imagine how awkward it could have been if one of Mendel's lab partners had decided to run this experiment way back then? I can see it now: Mendel walks into the lab and asks, "Has anyone seen my peas? After 7 long years … I've nearly completed my research. Just need to tally up that last generation of peas. … Very exciting. … Hmm. … I thought my peas were sitting here on this desk by my respirometer."
Anyway, the significant points from this experiment are:
- Germinating seeds consume more oxygen than do nongerminating seeds. This makes sense, because they have more reactions going on.
- Seeds germinating at a lower temperature consume less oxygen than do seeds germinating at a higher temperature.
- You can determine how much oxygen is consumed by watching how much water is drawn into the pipettes as the experiment proceeds. (Refer to your classroom lab manual if you are confused by the pipette portion of this lab.) This water is drawn in as a result of the drop in pressure caused by the consumption of oxygen during respiration.
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