Before having students tackle the problems presented
in The Student Lab, the teacher will
need to build respirometers (see figure 1).
To do this, take a large metal washer and attach it to the bottom of a
125-ml Erlenmeyer flask using silicon cement (this is necessary to ensure
that the respirometers remain submerged in their water baths during an
experiment). Next, insert a 1-ml/0.01-ml graduated pipette through
a single-hole #4 rubber stopper and glue it in place permanently, again
with silicon cement (be sure to seal both sides of the stopper to avoid
any leaks [see figure 2]). Placing
the stopper in the flask completes the respirometer.
With the rest of the materials, the teacher should take time to show students how to set up the basic apparatus and run it. To prepare the respirometers for use, place a 2 cm layer of absorbent cotton in the bottom of the flasks on the day the experiment will be performed and use an eyedropper to saturate the cotton with fresh 15% KOH. Cover this layer with 2-3 cm of non-absorbent cotton to protect the living plant samples from the KOH, and then ready the test sample for placement inside the flask
Any plant sample should be controlled for volume in all experiments (and for mass when comparing different species). To do so, use the water displacement method by filling a 100-ml graduated cylinder to the 50 ml mark and then dropping in the plant tissue and recording the difference between 50 and the new reading. Be sure the entire sample is completely submerged before taking the reading. Remove the plant sample then and set it aside on a moist paper towel and repeat this procedure with glass beads (or some other inert, dense, non-absorbent substance), being sure that there are enough beads to match the exact volume of the plant sample. If one is using partial tissues, one may need to put a combination of beads and plant sample in the graduated cylinder to achieve the appropriate volume, but the process is the same regardless.
Once both the respirometers, a test sample, and the control are ready, carefully place the plant tissue being tested in one prepared flask and the glass beads in another. Seal each flask with a pipette-stopper and take the assembled respirometers and attach the measuring device (either a gas pressure sensor or a water-displacement device) to the open tip of the pipette (see figure 3). Next, place the flasks into 600-ml beakers containing room temperature tap water (allowed to sit overnight) and equilibrate them for 8-10 minutes.
Students are now ready to start their lab activity(ies). Use the techniques appropriate for the measuring device chosen to collect the data.