High School Science
Biology
Analyzing Ecosystems
SCoPE Site Lesson Plan
Title: Lesson 2— What Goes Around Comes Around
Abstract
In a previous lesson, students explored the relationships among members of a community by
setting up model aquarium ecosystems, and investigated the photosynthetic capacity of Elodea
and changes in biomass of producers and consumers.
In this lesson students consider the variables that may effect respiration to predict changes in
their model ecosystem related to the carbon cycle. With teacher guidance they generate new
questions and design and conduct experiments to explore how variations affect the rate of
photosynthesis. Students develop models for other cycles in nature.
Subject Area: Science
Grade Level and Course Title: High school Biology
Unit of Study: Analyzing Ecosystems
Content Expectations
B1.1A - Generate questions for investigations
B1.1C - Conduct scientific investigations
B1.1f - Predict results of changes in variables
B3.1B: Illustrate and describe the energy conversions that occur during photosynthesis and
respiration.
B3.3b - Describe environmental processes (e.g., the carbon and nitrogen cycles) and their role in
processing matter crucial for sustaining life.
B2.5f - Relate plant structures and functions to the process of photosynthesis and respiration
Key Concepts
carrying capacity
the carbon cycle
food web
Instructional Resources
Equipment/Manipulative
Baking soda (about 2 g per group)
Colored pencil (3 colors per student, erasable)
Elodea (2 sprigs per group)
Erasers
External light source (full spectrum)
Eye protection
Graduated cylinders (250 ml or greater 2 per group)
Meter stick
Razor blade (single edged, or scalpel, one for teacher only)
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High School Science
Biology
Analyzing Ecosystems
Water (aged)
Student Resource
The Microbial World: The Nitrogen cycle and Nitrogen fixation. Ed. Jim Deacon. University of
Edinburgh. 4 November 2003 <http://helios.bto.ed.ac.uk/bto/microbes/nitrogen.htm>.
Texley, Juliana, and Claudia Douglass. Unit 4 Lesson 2 Student Pages. Teacher-made material.
Lansing, MI: Michigan Department of Treasury, 2003.
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High School Science
Biology
Analyzing Ecosystems
Teacher Resource
Texley, Juliana, and Claudia Douglass. Grade 9 Unit 4 Teacher Background. Teacher-made
material. Lansing, MI: Michigan Department of Treasury, 2003.
Sequence of Activities
Advance Preparation: Put a fresh sprig of Elodea in a large graduated cylinder of aged water and
baking soda, as described in Lesson 1. Place the graduated cylinder with the Elodea in full light
before class time, so that there is a build up of oxygen bubbles on the stem.
Safety Precautions: Water can cause a light bulb to explode dangerously. If there is any chance
that drops of water could reach the light bulb eye protection is a must.
1. Ask students to recall to their observations of photosynthesis by Elodea (Anacharis) that
began in Lesson 1. Explain to them that you added a little baking soda to the water. Ask:
“What effect does the baking soda have on the water?” You may want to review the formula
for sodium bicarbonate (baking soda) at this time. [It increases the amount of carbon dioxide
in the water.] Ask: “What is happening within the Elodea?” [Photosynthesis.] “Where do the
bubbles on the cut end of the Elodea come from?” [They are traveling in the stem and out the
fresh cut. Note: As cold water from the tap warms, some dissolved oxygen will bubble out
and attach to stems and leaves. These bubbles are confusing. That is why the water should be
at room temperature before the demonstration begins.] Ask students to describe the process
of photosynthesis in their own words on the Student Page.
2. Ask students to explore their own ideas about how carbon cycles in through an aquatic
ecosystem. Give them these directions: “Draw a pond ecosystem. Add Elodea and as many
other organisms as you can. (You do not have to be an artist!) Next using a colored pencil,
draw a small number of carbon atoms in the Elodea plant. Then draw an arrow to show
where the carbon atoms go. Draw carbon in at least five other places in your diagram.
Connect the carbon atoms by arrows. Your last arrow should return to the Elodea plant.”
Give students time to express their preconceptions, and to discuss their drawings in groups.
Emphasize that they can erase their arrows and redraw them. Then lead a discussion which
helps students clarify the steps in the carbon cycle:
a) “Where is the carbon in the Elodea leaf?”[In organic molecules (sugar, starch, and
protein), and in carbon dioxide.]
b) “Is there carbon in the water?” [Yes, dissolved. This is a tricky question for
students. They may not realize that gases can dissolve in water.]
c) “When the plant dies, where does the carbon in the leaves go?” [It breaks down
and becomes food for animals, snails.]
d) “Where does the carbon in your food go?” [It forms tissues and is exhaled as
carbon dioxide into the air.]
e) “If the carbon becomes parts of animals, what happens next?” [Other animals up
the food chain eat them. Some of the carbon is released as a byproduct of
metabolism and is released into the air.]
f) “Where does the carbon dioxide in the air go?” [Some dissolves in water, some is
used by plants during photosynthesis.]
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Biology
Analyzing Ecosystems
As students complete their diagrams, provide them with the chemical formulas for carbon
dioxide,CO2, and glucose, C6H12O6.
3. Using another colored pencil to illustrate the role of energy in the carbon cycle. Solar
(electromagnetic) energy becomes stored chemical energy.
4. Facilitate a whole group discussion about the carbon cycle. Begin by asking: “Now that you
have diagrammed the carbon cycle, do you think that the Earth can get more carbon if we run
out?” [This is actually a misconception. While a miniscule amount of carbon could land on
Earth in a meteorite, the practical answer is “no.”] Carbon is released into the atmosphere as
carbon dioxide or methane gas and pools as oil organic matter.
5. Now that the students are acquainted with the carbon cycle, they are asked to design their
own experiment that will show the effect of one variable on the rate of photosynthesis. Write
the formula for photosynthesis on the board and review both the reactants and products. Ask:
“What factors affect the rate of photosynthesis?” [The amount of sunlight, the wavelength of
the light, and the amount of carbon dioxide and water available to the plant.] “We cannot
expect this particular plant to live outside of water, but we could vary the amount of carbon
dioxide or the amount of light available to the plant to determine their effects on the rate of
photosynthesis.” Divide the class into two groups (or more if you want to replicate the
experiment). One group will investigate the effect on light on the rate of photosynthesis. The
second group will compare the photosynthetic activity of plants with and without extra
carbon dioxide provided by adding baking soda (sodium bicarbonate) to the water. Next ask
the students to design for their experiment. They need to determine how they will measure
the rate of photosynthesis (they already know how to count the bubbles from the stem of an
Elodea plant) and they must control for all other variables. Students should write their
procedure on their Student Pages.
6. After you check the experimental procedures, give each group two graduated cylinders with
water, or more depending upon the supplies you have available. Students should put a sprig
of Elodea in each graduated cylinder. In each case, the Elodea sprigs should be about the
same length and from the same part of the plant (stem or end, etc.). Note: Do not provide too
many directions. This should be a semi-open inquiry. Students already know how to count
the bubbles from the stem of the Elodea. They should be able to complete a lab report, to
control other variables, and to establish a data collection technique without too much
direction. If necessary, you can suggest that students vary the light intensity by placing the
graduated cylinders various distances from the light source and vary the carbon dioxide
available for photosynthesis by altering the amount of baking soda added to the water.
7. Students can make this experiment more quantitative by including these options:
Using an indicator like bromthymol blue, you can visually get a qualitative
indication of decrease in carbonic acid in solution, and compare that to observed
oxygen bubbles. (It is often useful to review the action of this indicator by using it
in fresh and “flat” soda water or Sprite® for comparison.)
Using a pH probe, you can trace the rate of photosynthesis indirectly (measuring a
decrease in the carbonic acid in solution). This quantitative result can be graphed
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High School Science
Biology
Analyzing Ecosystems
opposite measures of oxygen released and the correlation (a negative one)
calculated.
Students can remove most dissolved gases from water (by boiling and then
cooling) and then add measured amounts of carbon dioxide for better
quantification.
Students can compare the effectiveness of various wavelengths of light on
photosynthesis. [In most plants, green wavelengths are least effective, but
accessory pigments can modify these results.]
7. Ask students to return to their drawing of a pond ecosystem. With another colored pencil, ask
them to add a shell. Ask what element the shell might add to the ecosystem. Then ask them to
find four other places in the pond where calcium might be found and connect them by
arrows. [Shells are calcium carbonate. Eggshells are 94% calcium carbonate, with
magnesium carbonate, calcium phosphate, and other organic matter. Calcium hydroxide is
sometimes dilute in acidic water. Calcium sediments are dissolved from limestone rocks in
chemical weathering and enter the soil and lake water.] Provide the following additional
information and allow the students to expand their drawings of the calcium cycle: “Calcium
is about 3.4% of the mass of the Earth’s crust, found in igneous rocks as calcium silicates and
in sedimentary and metamorphic rocks as calcium carbonates. Concentrations of Ca++ in
fresh water range from 0.01 to 0.1 millimoles, but is hundreds of times higher in salt water.
When water is warm, calcium carbonate precipitates easily and settles into the lake bottom.
Calcium is brought back to land in shells and fish skeletons. It is also moved through
geologic processes as sedimentary limestones are raised (like the white cliffs of Dover).”
8. Students may wish to explore the nitrogen cycle at the University of Edinburgh web site
shown in the Student Resources. This cycle is sometimes difficult for students because they
may not have the background to understand that the diatomic nitrogen which makes up about
three-quarters of Earth’s atmosphere is insoluble and almost useless to living things until
nitrogen-fixing bacteria or lightning act on it. The web site provides an age-appropriate
review. Students can use a third color of pencil to add the nitrogen cycle to their drawings.
Assessment
Students should be able to draw cycles for all nutrients studied in this lesson.
Application Beyond School
Students can further their knowledge of nutrient cycles by exploring how commercial fertilizers
make heavy use of urea or liquid nitrogen to increase agricultural yields.
Connections
Mathematics
When experimenting with various nutrient cycles, students graph experimental data and
summarize their results.
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