Related Issues and Concerns
One of the rationales for this lesson is to have
students active in the entire scientific process. They need to be
given a specific concept or problem to explore and, with teacher guidance,
provided the opportunity to design their own investigation whenever possible.
Demonstrations of common procedures can quickly familiarize students with
some necessary techniques, but students can only develop an understanding
for the practical side of the scientific process and ownership of their
own learning process when the entire lab, from design to conclusion, comes
from them. Cookbook style procedures may allow students to learn
laboratory techniques, but they do not foster the evolution of critical
thinking skills and appreciation for the rigor of the scientific method.
Lab is more than conducting cookbook procedures and answering questions
about the results, and therefore, the primary student
lab model presented here is a hands-on, minds-on inquiry one.
Two teaching methods have been designed. In
the first one, the teacher can compile a list of students' responses to
pre-lab question 12 and provide a list
of possible hypotheses for class discussion and analysis. He or she
would demonstrate the basic protocol
needed to perform any potential investigations, and then either the
class or individual groups determine the feasibility of the various
proposed investigations and design and perform their own definitive labs
with teacher guidance. This approach could be used with all levels
of biology classes.
The other approach is simply to present the class
with a lab assignment sheet and have
them take full responsibility for all facets of their experiment including
hypothesis formation, protocol design, etc. The teacher makes the
materials available, but students are expected to do their own preliminary
research and are encouraged to go through the real trial and error process
of scientific inquiry. This approach would be appropriate for an
AP level class or an independent research project.
Regardless of which approach a teacher
takes, there are some practical concerns that must be addressed no matter
what. These are:
germinated and sprouting seeds are fairly hardy organisms but it is important
to keep them moist and hydrated to maintain their metabolic activity.
Sandwiching samples between paper towel that has been soaked and squeezed
out is a good way to prevent dehydration, and plant samples should be kept
damp this way whenever they are not being used.
KOH can be highly caustic and care should be taken in its distribution.
A centralized location for students to use is recommended, and latex gloves
should be made available for those particularly sensitive its effects.
Be sure students are aware that if they get any on themselves, they need
to rinse the area immediately with ample running water. All students
should wear goggles.
care should be used when adding or removing samples from respirometers
to avoid contact and contamination with the KOH. Using forceps is
KOH reacts naturally with the CO2 in the atmosphere; so it is
important to use as fresh a solution of KOH as possible and to keep whatever
container it is in as tightly sealed as possible. Old KOH will be
cloudy from the K2CO3 that has precipitated out and
should not be used.
Respirometers only work if they are tightly sealed; be sure students double-check
all locations where gas might enter or exit to be sure it cannot.
It is absolutely vital to control for the volume of the samples and keep
the temperature constant. The technique used here rests on the ideal
gas law (PV=nRT); changes in the amount of oxygen (n) result in corresponding,
testable changes in P and/or V.
to control for volume, use water-displacement of the plant samples and
match the measured volume(s) with comparable volumes of glass beads or
some other inert, dense, non-absorbent substance.
to control for temperature, respirometers need to be contained in water
baths that were allowed to stabilize for at least 24 hours, and the respirometers
should be permitted to equilibrate to the bath temperature for 10 minutes
for all methods of testing, the time frame for the model experimental system
is 20 minutes following equilibration.
while the lab protocol presented here has been written
for use with CBL probes, not everyone has access to this technology.
The alternative is to use a water-displacement method such as the one described
in the AP Biology Lab Manual (see diagram 1)
or create a water-displacement apparatus by attaching the respirometers
to a second 1-ml/0.01 ml graduated pipette with latex tubing. Insert
the second pipette into a 250-ml beaker containing water, and as the volume
in the respirometer changes due to oxygen consumption, water is drawn up
into the second pipette, permitting students to measure the rate of change.
when using CBLs, it is possible to perform any potential experiment the
teacher or the students design with only one probe. If one has only
one, be sure to test the control respirometer first and the the variable
respirometer second, but if two biology gas sensor probes are available,
then run the control in channel 1 and any variable in channel 2.
whether one uses CBL probes or water-displacement methods, what is actually
being measured is the rate of pressure or volume changes; hence, units
are either mm Hg/sec or ml gas/sec.
One will need to help students understand that changes in mm Hg or ml of
gas correspond to units of oxygen being consumed, and a simple way to do
this is to have students create an "artificial" unit that has a one-to-one
correspondence with the pressure or volume unit (e.g. a change of 1 mm
Hg of pressure or 1 ml of volume would represent 1 unit of oxygen
used). It is possible, though, to use the ideal gas law to determine
the actual mass of oxygen consumed, and this calculation might be appropriate
for more advanced students to perform.
in order to compare actual rates of respiration between different plant
parts, it is important to incorporate the amount of actual tissue mass
respiring. Therefore, the final units of measurement should be units
of oxygen/sec/mg of tissue.