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CBL Interfacing

Breathing: A Germinating Pea vs An Earthworm

Teacher Tips

Length of Lab: 2 X 40-45 minute periods.

Safety Precautions: The lab involves working with potassium hydroxide, a base. Goggles should be worn at all times and students should be reminded how to handle acids and bases.

1. Remind students to fully insert the I/O cable in the CBL and TI 82.

2. Although we suggest doing the lab in a temperature controlled water bath, it works fine if the respirometers and barometers are lying on their side on the desk at room temperature. This facilitates the set up and decreases the set up time. The respirometers should be placed on paper towels to insulate them from the cold counter top.

If the students choose to do the extended experiment, then temperature controlled water baths are essential.

3. The control and experimental respirometers should be plugged into the barometers at the same time just before starting the experiment. The starting pressures should therefore be the same.


While the volume of both the control and experimental respirometers should be the same, in practice, the students may not insert the rubber stoppers exactly equally. The starting pressures will still be the same...as long as they insert the rubber stoppers


attaching the respirometers to the barometers. However, by inserting the stoppers in differently, the starting volume will be slightly different and thus a second variable is introduced.

4. One problem which we occasionally encounter is that the earthworms block the tube going to the respirometers. Students should observe their respirometers and the readings on the CBL. If the readings become erratic because their pesky earthworms start blocking the tube, the students may have to start over.

The vials can be reused. The students should turn the CBL and TI 82 off. Unplug the respirometers, remove the stoppers for a moment to allow the pressure to equalize with the atmosphere. Place the stoppers back in the vial, reprogram the TI 82, plug in the respirometers to the barometers and restart the experiment.

5. We used medium sized earthworms purchased from Carolina Biological Supply Co.

6. The new Vernier CHEMBIO program may be used for this experiment.

Here is some sample data, using peas. The slope of the line is 3.2 X 10 E-5. It is very small value, indicating the corrected pressure change, per gram of pea, per second. A scatter plot was made and a best fit line is superimposed on top.

Answer To Questions:

1. Based on a comparison of slopes, how does the rate of respiration of the germinating peas, compare to the rate of respiration of the earthworms?

This answer will vary depending on the stage of pea development and the condition of earthworms.

2. In your own words, explain why the following procedures were performed:

A. Setting up respirometers with no earthworms or peas in it.

Pressure changes may occur because of environmental effects which have nothing to do with the respiration rate of the biological specimens. For example, perhaps the pressure changed because of a temperature change. The vials with glass beads serve as controls.

B. Determining the mass of the earthworms and peas.

We need to have a basis for comparing different organisms, or the same organisms at different temperatures. We can do this, if we know the pressure change per gram of tissue. Therefore, we must determine the mass of the earthworms and peas.

3. What activities of these organisms accounts for the different slopes which you observed? Try to be intuitive.

Answers will vary. Students may mention metabolic activities involved in growth, movement, digestion and others.

4. Both the pea plant and earthworm are respirers. If plants and animals both use oxygen, how is the oxygen in our atmosphere replenished?

Plants also photosynthesize, and they produce more oxygen during the day than they use in both the day and night. Animals, breath the excess oxygen.

5. If set up our pea respirometer the day the peas first began to germinate, and left it set up for about a week, how would the following plot appear? Sketch and justify your prediction. There are a number of reasonable responses here, depending on what factors you think of!

There are several answers the students may come up with. First, the pressure would continually decrease for a period of time. Once the peas produced foliage leaves could begin to photosynthesize significantly. This could cause the pressure to rise. However, with KOH in the vial, some of the carbon dioxide would be consumed as it is produced. This would reduce the amount of photosynthesis which could occur. More important than predicting the exact appearance of the graph is having the student consider all of the factors which may affect pressure.

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