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Seeing
the
by Pamela Drouin, David J. Welty,
Daniel Repeta, Cheryl A. EngleBelknap, Catherine Cramer,
Kim Frashure,and Robert Chen.
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he most important biochemical reactions for life in the ocean and
on Earth are cellular respiration and photosynthesis. These two
reactions play a central role in the carbon cycle. The ocean-based
carbon cycle is highly relevant to today’s students because of its key
role in global warming. The Earth’s atmosphere maintains the temperature of
the Earth within a relatively narrow range that can support life. The atmosphere is made up of gases such as carbon dioxide, methane, water vapor, and
others that allow radiant energy to pass through, but prevent heat loss back
into space. As the composition of the atmosphere changes due to more carbon
dioxide emissions produced by humans generating power, the insulating capac-
Pamela Drouin is a seventh-grade physical science teacher at Hastings Middle School in
Fairhaven, Massachusetts. David J. Welty ([email protected]) is a science teacher and
a 6–12 science and technology academic coordinator for the Fairhaven Public Schools in
Fairhaven, Massachusetts. Daniel Repeta is a senior scientist in the Marine Chemistry and
Geochemistry Department at the Woods Hole Oceanographic Institution in Woods Hole, Massachusetts. Cheryl A. Engle-Belknap is an instructional curriculum specialist for the Atlantis
Charter School of Greater Fall River, Massachusetts. Catherine Cramer (cramer50@adelphia.
net) is a writer and communications manager for the Center for Ocean Sciences Education
Excellence–New England in Boston. Kim Frashure is a PhD fellow and Robert Chen is an
associate professor, both in the Department of Environmental, Earth, and Ocean Sciences at
the University of Massachusetts in Boston.
Explore carbon cycle at
www.scilinks.org.
Enter code SS010601
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Carbon
Cycle
J a n u a r y 2006
T
ity of the atmosphere increases and more heat is trapped.
As carbon dioxide levels increase, so does the average
temperature of Earth. Besides the obvious solution of cutting carbon dioxide emissions, which does have economic
drawbacks, the possibility of sequestering carbon dioxide
in long-term storage stages of the carbon cycle is an experimental option (see Figure 1). The following lessons
outline a classroom experiment that was developed to
introduce middle school learners to the carbon cycle. The
experiment deals with transfer of CO2 between liquid reservoirs and the effect CO2 has on algae growth (Figure 2).
It allows students to observe the influence of the carbon
cycle on algae growth, explore experimental design, collect data, and draw a conclusion.
Teaching the carbon cycle
This carbon cycle lesson is designed for a seventhgrade physical science class. In the activity, students
observe how different levels of carbon dioxide in the
atmosphere affect the growth of algae. Before getting
started, students review the steps in an experiment and
write an “If…, then…” prediction with a “because”
clause on how the carbon cycle might influence growth
of algae. For example, “If there is more CO2, then
there will be less algae growth, because too much CO 2
FIGURE 1
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will kill the algae.” Or, “ If CO 2 increases, then algae
growth increases, because it is used by algae during
photosynthesis to make sugar.”
Experimental design
This single experiment is set up by the teacher for multiple
classes to observe and analyze because
• the amount of equipment (Figure 3) required is impractical for multiple setups;
• the experiment is simple enough to set up for a science teacher, but too complex for a middle school
student to handle;
• there is a 0.4 molar potassium hydroxide (KOH) solution in the negative control that presents a safety
hazard to students; and
• the amount of algae transferred to each beaker needs to
be consistent, as large differences in algae settling time
could influence how much algae is in each group.
The students’ responsibilities during the experiment are
to make observations, collect data, reach a conclusion
on how different levels of CO2 affect algae growth, and
then apply that knowledge to determine how greenhouse
gases might be tied to global warming.
Causes of increased CO2 emissions and effects/results
CO2 is naturally
found in: respiration,
decomposition, natural
forest fires.
CO2 is human-made by:
burning fossil fuels for
transportation, electricity,
and human-made forest
fires for clearing land.
Contributes to the
greenhouse effect
causing increased
warming of the Earth.
Melting polar
ice caps and
alpine glaciers.
Rising sea levels
cause flooding,
changes in habitats for
plants and animals.
Change in weather
patterns increases
precipitation.
Wet places
become wetter.
Dry places
become drier.
Changes in
habitats for
plants and
animals
Heat waves
Increases the rate
of evaporation and
causes bursts of
increased rainfall.
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FIGURE 2
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Experimental design’s relationship
to photosynthesis
FIGURE 3
SUN
CO2 in the air
CO2 in the water
6 H2O in the water
6 CO2 in the water
6 O2 in water
Closterium algae
Sugar C6H12O6
6 CO2 + 6 H2O � C6H12O6 + 6 O2
Photosynthesis
The experiment consists of a set of three half-gallon containers partially filled with solutions that
establish different CO 2 levels. Inside each container
is a culture of actively growing freshwater Closterium
algae (see Figure 4). Each half-gallon container is a
closed system that prevents gas exchange with the
outside. The beaker of growing algae sits in a liquid
reservoir within the container. The reservoir solution
controls the amount of CO 2 in the air (Figure 5). The
Closterium algae is grown under a fluorescent light on
a 12-hour light/dark cycle.
The experiment consists of three groups:
(1) the normal control with a tapwater reservoir;
(2) the experimental group with a carbonated water reservoir; and
(3) the negative control with a 0.4 M KOH reservoir.
The tap water represents the natural condition of dissolved
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Materials needed
for the experiment
• two liters of distilled water
• one culture of actively growing Closterium
algae (Carolina Biological Supply #HT-152115, $ 4.95)
• tube of 50X Alga-Gro (Carolina Biological
Supply #HT-15-3751, $27.60)
• Schultz, 10-15-10 Plant Food Plus ($2.95)
• three half-gallon Rubbermaid screw-cap
containers
• four 600 mL Pyrex beakers (Ball canning
jars work as a substitute, but must be able
to fit inside the half-gallon container; plastic
containers may have impurities that inhibit
algae growth; beakers are cleaned and
rinsed twice with distilled or bottled water)
• 0.5 liter of carbonated water (seltzer water)
• 0.6 liter 0.4 Molar potassium hydroxide
solution (obtain from a chemistry teacher),
wear safety glasses when handling
• 500-mL graduated cylinder
• 50-mL graduated cylinder
• rubber band
• plastic wrap or parafilm
• glass-marking pen
• two fluorescent lights
• electric on-off timer
gases in the environment. The experimental group is a 1:2
mixture of bottled carbonated water to tap water, which is
enriched with CO2 gas. CO2 reacts with water to form carbonic acid (H2CO3).
CO2 (g) + H2O (l)  H2CO3 (aq)
Because 0.4 M KOH reacts with CO2 to form K2CO3 (s),
KOH effectively depletes CO2 from the atmosphere and
water of the closed system.
2 KOH (aq) + CO2 (g)  K2CO3 (s) + H2O (l)
Depending on the level of your students, you can explain
the reactions occurring inside each container, or simply
label them as Depleted CO2, Regular CO2, and High CO2.
Students should be instructed that as the algae culture
grows the cells will become more crowded and the culture
T
FIGURE 4
Growing closterium algae
1. Add 50X-Alga-Gro to 1 liter of distilled water to make
1X-Alga-Gro.
2. Transfer to the glass beaker 400 mL of 1X-Alga-Gro.
3. Add 1 drop of plant food.
4. Transfer half the volume of the Closterium algae culture
to the beaker.
5. Cover with plastic wrap and secure with a rubber band.
6. Punch six holes in the plastic wrap.
7. Place in a sunny window or under a fluorescent light for
about a week. As the culture grows, the green color will
intensify.
FIGURE 5
Experimental setup
1. Transfer into each half-gallon plastic container 600 mL
of one type of reservoir liquid:
• normal: 600 mL of tap water
• experiment: 400 mL of tap water and 200 mL of
carbonated water
• negative: 600 mL of 0.4 M potassium hydroxide
2. Label beakers Normal, Experiment, or Negative with a
marker.
3. Transfer 350 mL of 1X-Alga-Gro® to each beaker.
4. Transfer 50 mL of actively growing algae into each
beaker.
5. Resuspend the algae after each transfer.
6. Carefully insert beakers into half-gallon containers.
7. Close and wrap tightly with plastic wrap or parafilm.
half-gallon screwcap container
600 mL
beaker
or flask
reservoir
water
(tap water,
carbonated
water, or 0.4 M
KOH)
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will become a darker green. This is called cell density and
indicates a higher algae number from greater growth.
After the teacher has set up the experiment, students
observe the systems over the next two weeks and record
their observations in a data table (Figure 6).
At this point in the lesson, students start making connections between carbon dioxide levels and algae growth.
Specifically, they begin to realize that algae depend on
photosynthesis for survival, which is why algae live
within the sunlight zone in the ocean. To further student
understanding, the following equations for photosynthesis
and cell respiration are introduced and explained with
the use of guided questions.
6 CO2 + 6 H2O—Photosynthesis  C6H12O6 + 6 O2
C6H12O6 + 6 O2—Cell respiration  6 CO2+ 6 H2O
• What type of gas will collect in the container as photosynthesis occurs?
• How would photosynthesis be affected if too little or too
much carbon dioxide were present?
• How would respiration be affected if too little carbon
dioxide were present?
• In which container would you expect to find the greatest amount of algae growth? Why?
• In which container would you expect to find the smallest amount of algae growth? Why?
More advanced students can be introduced to the role
of carbon, hydrogen, and oxygen in the carbon cycle.
They can also study the atomic structure, electron orbitals, valence electrons, and the molecules of cellular
respiration and photosynthesis: carbon dioxide, glucose,
oxygen, and water. The relationship between the products and the reactants can also be discussed and the balancing of the equation examined.
Results
algae culture
During this experiment, students will observe that the
experimental group with carbonated water grows better than the normal control. For this reason, the experimental group has more algal cells and is a darker
green. The negative control grows less well than the
normal control; consequently, the resulting culture
has less algal cells and is a lighter green. These results demonstrate what would be predicted from the
photosynthesis reaction: When there is more carbon
dioxide, the algae grow better. When there is less carbon dioxide, the algae grow poorly. The negative control of 0.4 M KOH will show little detectable algae
growth. Students should compare these observations
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FIGURE 6
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Student data table
Day Observation
1
E
Depleted CO2
Regular CO2
High CO2
Darkness (1–10)
Growth (1–10)
3
Darkness (1–10)
Growth (1–10)
7
Darkness (1–10)
Growth (1–10)
10
Darkness (1–10)
Extensions
Growth (1–10)
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Darkness (1–10)
Growth (1–10)
to their predictions about how the CO 2 levels would
affect the algae growth.
As a follow-up, ask students the following questions to assess what they learned about the process:
• What is the reaction that this experiment demonstrates?
• What part of the reaction does the experiment test?
• What would you expect to be the outcome for the algae
in the experimental group?
• What would you expect to be the outcome for the algae
in the negative control?
• What is going on in each chamber of the experiment?
• Identify ecological/environmental conditions that are
similar to the conditions found in each container.
Conclusion
Through this experiment students learn that CO2 has a
positive effect on algae growth and, in fact, is essential
to the growth of algae. At the end of the activity, students should be able to make the connection that algae
growth is dependent on photosynthesis. Through a discussion of deforestation and consumption of fossil fuels,
students should be led to the concept of long-term carbon reservoirs being depleted and an increased amount
of carbon being put into the atmosphere. (One way to
remove more carbon dioxide from the atmosphere is to
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stimulate algae and plant growth.) Finally,
the concept of food chains built upon photoautotrophic organisms that use sunlight, water, and carbon dioxide to make sugar should
be introduced. The sugars then support the
energy demands of the herbivore heterotrophs and carnivore heterotrophs. Students
should be able to grasp the concept that
since most life is directly or indirectly dependent on photosynthesis, if photosynthesis
stopped due to sunlight being blocked, then
all life on Earth would be in jeopardy. However, if animal respiration stopped, which is
only one of several sources of carbon dioxide
for plants, plants could survive from carbon
dioxide released by volcanoes and the ocean.
J a n u a r y 200 6
You can supplement this activity by measuring the amount of oxygen released by the
algae and performing serial dilutions and
filtrations to further quantify the amount of
algae growth. By performing serial dilutions,
it is possible to determine the algae concentration per ml, so students can compare 1 × 107 cells
per mL to 1 × 109 cells per mL. This will allow them
to find the dilution where the algae cells limit out following dilution. Filtration allows all of the algae to be
captured on a solid substrate for densitometry or better
photographic documentation. These activities can be
found with the online version of this article, available
at www.nsta.org/middle. ■
Acknowledgments
This experiment grew out of collaborative work in the Ocean Science Education Institute (OSEI), which develops and implements
high-quality ocean science education for middle school students
through projects that connect with existing district curricula and
effective science educational practices. OSEI is a project of the
Center for Ocean Science Education Excellence–New England
(COSEE-NE), an NSF-funded partnership between the New
England Aquarium, the University of Massachusetts/Boston,
and the Woods Hole Oceanographic Institution (WHOI). The
OSEI format includes a five-day workshop, numerous classroom
visits, and two follow-up days. During the 2004–2005 school year,
researchers and Massachusetts middle school teachers, district
science coordinators, and facilitators teamed up to produce
districtwide, inquiry-based science curricula for middle school
students based on current ocean science research. To find out
more about OSEI and other COSEE-NE programs, please visit
our website at www.cosee-ne.net.