Biol 160: Lab 4
Name: __________________________
Respiration and Photosynthesis in Plants
OBJECTIVES
In this laboratory exploration, you will

Use a dissolved oxygen probe to measure the oxygen concentration of water.

Use the inferences about the amount of oxygen in the water 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 of oxygen.
♦ 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
Rev. 1/2009
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Respiration and Photosynthesis in Plants




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
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 appearance of
oxygen to estimate the rate of photosynthesis. 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? _______________________________________________________________________
_________________________________________________________________________________
_________________________________________________________________________________
_________________________________________________________________________________
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Respiration and Photosynthesis in Plants
Is it possible for this plant to do more photosynthesis than respiration under light
conditions? Why or why not?
_________________________________________________________________
_________________________________________________________________________________
_________________________________________________________________________________
_________________________________________________________________________________
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?
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
♦ 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)
4 beakers
1 O2 probe
well water
2+ sprigs of Elodea
wax pencil
scale
1 weigh boat
PROCEDURE
1. Work in groups of 4. To work efficiently, split up the work!
2. Obtain and label 4-250 ml beakers. Using a wax pencil, label them “Light-Control”,
“Light-Experimental”, “Dark-Control”, and “Dark-Experimental”. Also label all beakers
with your group name.
3. Obtain an O2 meter, and turn it on to warm up.
4. Fill each beaker with well water (about ½ full).
5. Obtain 2-4 large sprigs of Elodea (or other aquatic plant). You will eventually divide your
sprigs between the “Light-Experimental” and “Dark-Experimental” beakers. Obtain
enough plant to fill the water in the test tube. Pat the plants dry with a paper towel,
weigh them and record the data in Table 1.
6. Place 1-2 sprigs in beaker “Light-Experimental”, and the other sprigs in beaker “DarkExperimental”. The sprigs should be submerged (under water).
7. Follow the directions provided on the O2 meter, placing it into the beaker “Light-Control”
and gently swirl briefly to allow water to move past the probe’s tip. When the reading
stabilizes, record the O2 value in Table 3. Repeat this process for your “LightExperimental” tube.
8. Place the probe into beaker “Dark-Control” and gently swirl briefly to allow water to
move past the probe’s tip. When the reading stabilizes, record the O2 value in Table 3.
Repeat this process for your “Dark-Experimental” beaker.
9. When all readings have been taken, rinse the O2 probe with distilled water, replace the
cap, and turn off the meter.
10. Place beakers “Light-Control” and “Light-Experimental” under light conditions in the light
rack. Leave the beakers for 40 minutes.
11. Place beakers “Dark-Control” and “Dark-Experimental” under dark conditions in the
cabinet at the front of the lab. Leave them for 40 minutes.
12. After 35 minutes, turn on your O2 meter, so that it has time to warm up.
13. After 40 minutes, measure O2 for each of the 4 beakers.
below.
Record the data in Table 3
14. Clean up by returning the well water and Elodea to the Elodea container, and putting
your beakers, wax pencil, squirt bottle and the O2 meter back at the front of the room.
Wipe up any spilled water, and throw away paper towels and the weigh boat.
15. Before leaving the lab, be sure to record both your own data and class data in the table
below.
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Respiration and Photosynthesis in Plants
PROCESSING THE DATA
♦ Calculate the change in O2,  O2, for your tubes. Do this by subtracting the starting O2 from
the ending O2. Record your results in Table 3.
♦ Calculate the Corrected  O2. You must correct for any changes that occurred to the O2 of
the water that were NOT the direct result of photosynthesis or respiration by Elodea. Do this
by subtracting the  O2 of the control tube from that of the Elodea tube. Record this
“Corrected  O2” in Table 3.
♦ Calculate the Corrected  O2/g. Do this by dividing the “Corrected  O2” for the Eldoea tube
by its weight, in grams. Record in Table 3.
♦ Report your findings to the class. Report only your Corrected  O2/g on the table on the wipe
board at the front of the classroom. Neatly copy all class data from the wipe board to Table
4.
♦ Calculate the “Average Corrected  O2/g” from the class data for both Light and Dark Elodea
beakers. For each condition (Light or Dark), find the average by adding all class data
(across the table) and then dividing by the number of groups.
DATA
Table 3: Data from respiration/photosynthesis experiment with Elodea
beaker
treatmen
t
“LightControl”
control
“LightExperimental”
“DarkControl”
“DarkExperimental”
weight
(g)
starting O2
ending O2
Corrected
 O2
 O2
Corrected O2
g
Elodea
control
Elodea
Table 4. Corrected  O2/g from the different class groups and class average.
Corrected  O2/g
Condition
Group
1
Group
2
Group
3
Group
4
Group
5
Group
6
Group
7
Group
8
Average
ElodeaLight
ElodeaDark
GRAPH THE AVERAGE CLASS DATA (“Average Corrected  O2/g”) for BOTH the
light and dark data. USE GRAPH PAPER (or a computer). Recall that, in all sciences, we
almost always place the independent variable (the one you selected and manipulated) on
the horizontal, or X-axis, and the dependent variable (the one that depends on your
manipulations) on the vertical or Y-axis. Be sure to correctly label the x- and y-axes. Attach
your labeled graph to this handout when you turn in your lab. If you need help generating
your graph, please ask me! You cannot receive full credit for the lab without a labeled
graph.
INTERPRETING THE DATA AND DRAWING CONCLUSIONS
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Respiration and Photosynthesis in Plants
Revisit the hypotheses and predictions you made in Tables 1 and 2. Using the data from the
class average (Table 4), 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:
3a. Circle the correct word(s) from italicized choices in the sentence below.
Consider the plant kept in the dark. As long as there was no light available, the change
in O2 we observed reflects the amount of photosynthesis/ respiration/ photosynthesis and
respiration occurring in the plant.
3b. Circle the correct word(s) from italicized choices in the sentence below.
Consider the plant kept in the light. As long as there was adequate light available, the
change in O2 we observed reflects the amount of photosynthesis/ respiration/
photosynthesis and respiration occurring in the plant.
3c. Considering your answers to parts “a” and “b” above, calculate what the corrected 
O2/g would have been if we could have prevented the plant from respiring in the “light”
conditions (i.e., in the absence of respiration, so that the plant was only photosynthesizing).
4. Why do we divide  O2 by the weight of the organism?
5. What was the purpose of the empty beakers in your experiment? If the O2 changed in these
tubes, what might have caused these changes? Write down two possibilities!
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Respiration and Photosynthesis in Plants
6. So… do plants both respire and photosynthesize in the light (yes or no)?
7. How did your group data (Corrected  O2/g) compare to the class data (Average
Corrected  O2/g)? Was it identical? In general, why might it be beneficial to report the
“Averaged Data” from many experimental trials versus data from just one single experiment?
TO TURN IN: Turn in this packet, making sure to fill in all tables, answer the questions,
and attach your graph.
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Respiration and Photosynthesis in Plants
Figure 1.
Variable measured
(
)
units of measure
(Descriptive title)
(
Variable measured
)
units of measure
10% sucrose
40% sucrose
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