AP LABS.
Dr. L. Rueda
CUHS
Cellular Respiration
• The equation for cellular respiration is
• glucose + oxygen + ADP = carbon dioxide
+ water + ATP
• ATP is generated in
the mitochondria
during cellular
respiration. It powers
most cellular
processes that require
energy; one example is
muscle contraction.
Design of the Experiment
• How can the rate of cellular respiration be
measured? When you study the equation for
cellular respiration, you will see that there
are at least three ways:
– Measure the amount of glucose consumed.
– Measure the amount of oxygen consumed.
– Measure the amount of carbon dioxide produced.
• In this experiment, we are going to measure
the amount of oxygen consumed.
• You can construct a respirometer by putting
any small organism in a vial with a pipette
attached. This example uses a cricket; in the
laboratory experiment, you will use peas.
Remember, cellular respiration occurs in the
cells of both animals and plants!
•
As O2 is used up, the
pressure of gases
inside the respirometer
decreases. This causes
water to enter the
pipette
• The CO2 that is produced combines with
KOH to form a solid precipitate, K2CO3.
• Notice that as the gas volume inside the vial
decreases, the pressure of water outside the
vial forces water into the pipette. Because
the amount of water that enters the pipette is
directly proportional to the amount of
oxygen consumed by the cricket, measuring
the water volume in the pipette allows you
to measure the rate of respiration
How to Read a Pipette
• Water in a pipette
adheres to the side of
the tube and forms a
curved surface called a
meniscus. By
common practice, all
readings are made at
the bottom of the
meniscus. The units
for this pipette are
milliliters (ml).
Assembling the Respirometer
• In this experiment you will compare the rate
of respiration in peas that are germinating to
the rate in peas that are dormant (dry peas).
You will make the comparison at two
different temperatures: 10oC and 25oC. In
addition, you will compare these rates to a
nonmetabolizing control
• It is important that the three vials contain an
equal volume of contents. You do this by
adding glass beads to the vial with the
dormant peas, since the dry peas take up
less space than an equal quantity of
germinating peas.
• Note: Because you are measuring the rate of
respiration at two different temperatures,
prepare two sets of three vials.
• Lab Hints
– You will need to use a layer of nonabsorbent cotton
between the KOH and the peas.
– The stopper must be firmly inserted for an air-tight seal.
Check that no peas or beads block the opening to the
pipette.
– To seal it we’ll use petrolum jelly around the stopper.
– To read the pipette more easily we’ll add a drop of food
coloring to the tip just before placing the respirometer in
the water.
– Let the respirometers equilibrate for several minutes in
their respective waterbaths. This will minimize volume
changes due to change in air temperature
• Gas volume is related to the temperature of
the gas. According to the gas law (
V=nRT/P) , a change in temperature will
cause a direct change in volume. Because
the temperature in the respirometers may
vary during the course of the experiment,
you must correct for differences in volume
that are due to temperature fluctuation
rather than rate of respiration. To do this,
subtract any difference in the movement of
water into the vial with glass beads from the
experimental vials held at the same
temperature. Record the result as the
corrected difference
Analysis of the Results
• After you have collected data for the
amount of oxygen consumed over time by
germinating and nongerminating peas at
two different temperatures, you can
compare the rates of respiration. Let's
review how to calculate rate.
• Rate = slope of the line, or
• In this case, y is the change in volume, and
x is the change in time (10 min).
• What would be the
rate of oxygen
consumption if the
respirometer readings
were as shown here?
Answer
• 0.016 ml/min