Breathing: A Germinating Pea vs An Earthworm
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
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
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
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
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
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.