Biol 160: Lab 4
Name: __________________________
Respiration and Photosynthesis in Plants
OBJECTIVES
In this laboratory exploration, you will

Use an oxygen probe to measure the oxygen concentration in a chamber.

Use the inferences about the amount of oxygen in the chamber to make conclusions about
whether the plant is respiring or photosynthesizing more in the light.

Reinforce concepts about respiration and photosynthesis.
PREPARATION
Before coming to class, it is very important that you read this handout. After reading the handout, fill out
the “Worksheet” below. ALSO fill out hypotheses for Tables 1 and 2.
INTRODUCTION
♦ Recall that plant cells, like animal cells, use their mitochondria to produce energy in the form of ATP.
This process is known as cellular respiration. We can measure rates of respiration in several ways, all of
which come from the basic equation of cellular respiration:
C6H12O6 + 6O2  6 H2O + 6 CO2 + energy
(glucose)
(oxygen)
(water)
(carbon dioxide)
Thus, if we wanted to know how much respiration was occurring in an organism, we could measure any
of the following:
 Rates of the disappearance (consumption) of glucose
 Rates of the disappearance (consumption) of oxygen
 Rates of the production of water
 Rates of the production of carbon dioxide
 Rates of the production of energy
Because we are measuring respiration in living organisms, it is not easy to measure the consumption of
glucose or the production of water molecules or energy. Also, remember that some of the energy
produced is captured as ATP (and some is lost as heat). If we want to measure respiration, the easiest
things to measure, is either the consumption of oxygen or the production of carbon dioxide. In this
laboratory exploration, we will concentrate on the consumption or production of these two gases.
♦ As animals, we acquire the sugars we use in our mitochondria by ingestion (eating!). Most plants,
however, produce their own sugars using carbon dioxide and sunlight in a process known as
photosynthesis. We can measure rates of photosynthesis in several ways, all of which come from the
basic equation of photosynthesis:
6 H2O + 6 CO2 + sunlight energy  C6H12O6 + 6O2
(water)
(carbon dioxide)
(glucose)
(oxygen)
Thus, if we wanted to know how much photosynthesis was occurring in an organism, we could measure
any of the following:
 Rates of the appearance (production) of glucose
 Rates of the appearance (production) of oxygen
 Rates of the disappearance (consumption) of water
 Rates of the disappearance (consumption) of carbon dioxide
 Rates of the consumption of light energy
Rev. 10/2011
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Respiration and Photosynthesis in Plants
Of these possibilities, again the easiest to measure is the appearance of oxygen or the
disappearance of carbon dioxide. For this lab exploration, we will measure the concentration of both
these gases to estimate the rate of photosynthesis and respiration. Remember, however, that plants are
made of cells and so they must also undergo cellular respiration. Therefore, we will be measuring both
respiration and photosynthesis by measuring changes in oxygen levels under light conditions. Based on
what you know about photosynthesis and respiration, which would you expect to be more prevalent in a
plant under light conditions compared to dark conditions? What would you expect to happen to oxygen
levels in light compared to dark conditions? These are questions you will address as part of this
exploration.
♦ Discuss these questions with your lab partners and fill-in the worksheet that follows.
Pre-Lab Worksheet
a. Do plants have mitochondria? ____________
b. Why do plants do cellular respiration? To produce ___________________.
c. Do plants do cellular respiration in light? _________
d. Do plants do cellular respiration in the dark? _________
e. Do plants photosynthesize in the light? _________
f. Do plants photosynthesize in the dark? _________
g. Why do plants photosynthesize? To produce _____________________.
h. What are two possible ways a plant might use the product you identified in “g”?
___________________________________________________________________________
i. Given your answer to the previous questions, what would happen to a plant cell that
did not photosynthesize and why?
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
j. Consider a plant in optimal light conditions (during daylight hours) and answer the
following three questions.
Is it possible for this plant to consistently do more cellular respiration than photosynthesis?
Why or why not? ________________________________________________________________
_________________________________________________________________________________
_________________________________________________________________________________
_________________________________________________________________________________
Is it possible for this plant to consistently do the same amount of photosynthesis and
respiration? (Beware! The sun will set eventually, leaving our plant in the dark!) Why or
why not? _______________________________________________________________________
_________________________________________________________________________________
_________________________________________________________________________________
_________________________________________________________________________________
Is it possible for this plant to do more photosynthesis than respiration under light
conditions? Why or why not?
_________________________________________________________________
_________________________________________________________________________________
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Respiration and Photosynthesis in Plants
_________________________________________________________________________________
_________________________________________________________________________________
k. So, given your answer to the previous questions (j), what do you predict will happen
to the oxygen concentration in the light, and why?
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
i. What do you predict will happen to the carbon dioxide concentration in the light and
why?
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
♦ NOW: Propose and enter into Tables 1 and 2, below, the appropriate hypotheses and predictions for
our experiments, testing whether photosynthesis and/or respiration occur in a plant in light and/or dark
conditions. Remember that a hypothesis is a testable, tentative explanation of what will occur in your
experiment. In this case, your hypothesis should explain what you think will happen to the rates of
photosynthesis and respiration under each condition (light vs dark). In contrast, a prediction is much
more specific; it describes what you will see or measure in your experiment if your hypothesis is supported.
Table 1: Hypothesis and Prediction for Plant in Light
Hypothesis:
Prediction:
Table 2: Hypothesis and Prediction for Plant in Dark
Hypothesis:
Prediction:
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Respiration and Photosynthesis in Plants
MATERIALS (per group)
LabQuest
BioChamber 2000
Calibration Stick
Vernier O2 Gas Sensor
Spinach
Vernier CO2 Gas Sensor
Ring Light Apparatus
PROCEDURE
1. Work in groups of 4. To work efficiently, split up the work!
2. Connect the white connectors from the CO2 and O2 sensors to the top of the LabQuest. Make sure the
switch on the CO2 Gas Sensor is set to low (0-10,000 ppm) setting. The O2 sensor must remain upright
during the entire experiment!
3. Change the units to ppt by tapping Sensors at the top of the screen, then Change Units -> CO2 Gas
Sensor -> ppt. Repeat this process to select ppt as the units for the O2 Gas Sensor.
4. Only the O2 Sensor will need to be calibrated for this experiment. To do this tap Sensors -> Calibrate ->
Oxygen Gas -> Calibrate Now. Have your partner use the Calibration Stick to hold down the CAL button
on the O2 Sensor. While this is happening enter 0.0 for the Reading 1 Known Value and tap Keep. Your
partner can release the CAL button the probe. Enter 20.8 for the Reading 2 Known Value and tap Keep
then OK. Your O2 probe is now calibrated.
5. Tap on the data collection box on the right side of the home screen. Change the rate to 15 and the
length to 15 minutes.
6. Weight out 40 grams of spinach and place them into the BioChamber 2000. Insert the CO 2 and O2 Gas
Sensors into the top of the respiration chamber. Make sure they fit snuggly so no air can escape.
7. Turn the ring light on and place the BioChamber 2000 in the center of the light. The lamp should be on
for 5 minutes prior to beginning data collection. After 5 minutes start data collection by pressing the Start
button on the LabQuest.
8. After 15 minutes the data collection will end. Turn off the light and sketch copies of the CO 2 and O2 vs.
time graphs.
9. Perform a linear regression to calculate the rate of respiration / photosynthesis for the CO 2 Gas Sensor.
a. Choose Curve Fit from the Analyze menu and select CO2 Gas.
b. Select Linear as the Fit Equation. The linear-regression statistics are displayed to the right of
the graph for the equation in the form y = mx + b. X
c. Record the absolute value of the slope, m, as the rate of respiration / photosynthesis for
the CO2 Gas Sensor in Table 1.
d. Select OK.
10. Repeat 9a-d for the O2 graph.
11. Place the BioChamber 2000, probes and the LabQuest handheld into a dark cabinet for 5 minutes to
equalize. After 5 minutes start data collection by pressing the Start button on the LabQuest. When
prompted, select Discard to start a fresh graph.
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Respiration and Photosynthesis in Plants
12. Data will be collected for 15 minutes and stop automatically. Take the apparatus out of the cabinet
and sketch both CO2 and O2 graphs.
13. Repeat steps 9 and 10 to determine the rate (slope) of photosynthesis.
14. Remove the plant leaves from the respiration chamber and return them to the container in the front
of the classroom. Clean and dry the BioChamber 2000.
15. Turn off the LabQuest by holding down the power button and tap Discard to completely turn off the
machine. Neatly wrap up all cords and return the CO2 probe and the BioChamber to the lab box. Return
the O2 probe to the probe rack at the front of the classroom. Plug the LabQuest back into its charging
dock and make sure the stylus lanyard is not blocking the power connector.
GRAPHS
Darkness
O2 Gas vs. Time
CO2 Gas vs. Time
O2 Gas vs. Time
CO2 Gas vs. Time
Light
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Respiration and Photosynthesis in Plants
DATA
Table 3
Leaves
O2 rate of
production/consumption
(ppt/s)
CO2 rate of
production/consumption
(ppt/s)
In the dark
In the light
INTERPRETING THE DATA AND DRAWING CONCLUSIONS
Revisit the hypotheses and predictions you made in Tables 1 and 2. Using your data, answer the following
questions:
1. Do you support or reject your hypothesis explaining the rates of photosynthesis and respiration
occurring in the light? Explain your reasoning. If you reject your hypothesis, write a corrected
hypothesis below.
2. Do you support or reject your hypothesis explaining the rates of photosynthesis and respiration
occurring in the dark? Explain your reasoning. If you reject your hypothesis, write a corrected
hypothesis below.
Additional Questions:
3. Were either of the rate values for CO2 a positive number? If so, what is the biological significance of
this?
4. Were either of the rate values for O2 a negative number? If so, what is the biological significance of
this?
5. Do you have evidence that cellular respiration occurred in leaves? Explain.
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Respiration and Photosynthesis in Plants
6. Do you have evidence that photosynthesis occurred in leaves? Explain.
7. List three factors that might influence the rate of oxygen production or consumption in leaves. Explain
how you think each will affect the rate.
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