Using a respirometer to measure the rate of uptake of oxygen
The organisms to be investigated are placed in one tube, and non-living material
of the same mass in the other tube. Soda lime is placed in each tube, to absorb
all carbon dioxide. Cotton wool prevents contact of the soda lime with the
organisms.
Coloured fluid is poured into the reservoir of each manometer and allowed to
flow into the capillary tube. It is essential that there are no air bubbles. You
must end up with exactly the same quantity of fluid in the two manometers.
Two rubber bungs are now taken, fitted with tubes as shown in the diagram.
Close the spring clips. Attach the manometers to the bent glasstubing, ensuring
an airtight connection. Next, place the bungs into the tops of the tubes.
Open the spring clips. (This allows the pressure throughout the apparatus to
equilibrate with atmospheric pressure.) Note the level of the manometer fluid in
each tube. Close the clips. Each minute, record the level of the fluid in each
tube.
As the organisms respire, they take oxygen from the air around them and give
out carbon dioxide. The removal of oxygen from the air inside the tube reduces
the volume and pressure, causing the manometer fluid to move towards the
organisms. The carbon dioxide given out is absorbed by the
soda lime. The distance moved by the fluid is therefore affected only by the
oxygen taken up and not by the carbon dioxide given out.
You would not expect the manometer fluid in the tube with no organisms to
move, but it may do so because of temperature changes. This allows you to
control for this variable, by subtracting the distance moved by the fluid in the
control manometer from the distance moved in the experimental
manometer (connected to the Living organisms), to give you an adjusted
distance moved.
Calculate the mean (adjusted) distance moved by the manometer fluid per
minute. If you know the diameter of the capillary tube, you can convert the
distance moved to a volume:
This gives you a value for the volume of oxygen absorbed by the organisms per
minute.
Using a respirometer to investigate the effect of temperature on the
rate of respiration
The respirometer can be placed in water baths at different temperatures. You
can use the same respirometer for the whole experiment. Or you could have
different ones for each temperature. (In each case, there are difficulties with
controlling some variables – you might like to think about what these are.) At
each temperature, you need a control respirometer with no organisms in it.
If you are simply comparing the rates of respiration at different temperatures,
then you do not need to convert the distance moved by the manometer fluid to
a volume. You could just plot distance moved on the y-axis of your graph and
time on the x-axis.
The rate of respiration is represented by the gradient of the graph.
Using a respirometer to measure RQ
For this, we need to know both how much oxygen is taken in, and how much
carbon dioxide is given out.
Set up two respirometers as shown above. However, the second respirometer
should also contain the same mass of live maggots (or whatever organism you
are investigating) but should not contain soda lime. You could put some inert
material into the tube (for example, the beads) instead of soda lime. The mass
and volume of the inert material should be the same as the mass and volume of
the soda lime.
This second tube is therefore just like the first one except that it does not
contain soda lime. The carbon dioxide given out by the respiring organisms is
therefore not absorbed.
The difference between the distance moved by the manometer fluid in the experimental tube and the distance moved in the control tube is therefore due
to the carbon dioxide given out.
