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Science Activities: Classroom Projects and Curriculum Ideas
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Overcoming Student Misconceptions about Photosynthesis: A
Model- and Inquiry-Based Approach Using Aquatic Plants
a
Andrew M. Ray & Paul M. Beardsley
a
a
Department of Biological Sciences, Idaho State University, Pocatello
Published online: 07 Aug 2010.
To cite this article: Andrew M. Ray & Paul M. Beardsley (2008) Overcoming Student Misconceptions about Photosynthesis: A Model- and
Inquiry-Based Approach Using Aquatic Plants, Science Activities: Classroom Projects and Curriculum Ideas, 45:1, 13-22
To link to this article: http://dx.doi.org/10.3200/SATS.45.1.13-22
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Overcoming Student Misconceptions
about Photosynthesis:
A Model- and
Inquiry-Based
Approach Using
Aquatic Plants
Andrew M. Ray and Paul M. Beardsley
P
Abstract. Even though photosynthesis is an obligatory part
of the science curriculum, research has shown that students
often have a poor understanding of it. The authors advocate
that classroom coverage of the topic of photosynthesis
should include not only its biochemical properties but also
the role of photosynthesis or photosynthetic organisms in
matter cycling and energy transfers in natural ecosystems.
The authors discuss several activities on photosynthesis
following the inquiry-based 5E (engage, explore, explain,
extend, and evaluate) learning model. The activities, which
incorporate different teaching styles to engage students with
different interests and modalities, highlight the dynamic
nature of photosynthesis, looking at the process across time
scales ranging from minutes to days. The activities provide
opportunities for hypothesis testing, use of experimental
controls, and application of summary statistics and statistical analysis. They also incorporate locally available aquatic
resources and provide opportunities to conduct experiments
in natural settings.
hotosynthesis is a biochemical process in which the
energy of sunlight is used to convert carbon dioxide
(CO2) into organic molecules. This vital process
supports nearly all ecosystems on earth. Photosynthesis
removes CO2 from the atmosphere and replenishes oxygen
(O2), resulting in the storage of photosynthetically derived
carbon as plant, algal, and bacterial biomass. The cumulative impact of photosynthetic organisms is responsible for
biochemical conversion of nearly 200 billion tons of CO2
into carbohydrates annually (Taiz and Zeiger 1998).
A useful understanding of photosynthesis is required
for any discussion of autotrophic organisms or the factors
that influence the distribution of life (e.g., light, water).
Additionally, the ability to produce carbohydrates through
photosynthesis represents a metabolic divide between the
plant kingdom and the animal and fungal kingdoms (Barker
and Carr 1989a). The life science content standards for
Grades 9–12 in the National Science Education Standards
(National Research Council 1996) state that coverage of the
biochemical properties of photosynthesis is an obligatory
part of the science curriculum, but education on photosynthesis should also help students achieve a better understanding of matter cycling and energy transfer in ecosystems.
Students with a thorough understanding of photosynthesis
should be capable of describing how plants, through photosynthesis, connect soil or water with the atmosphere
and use sunlight, CO2, nutrients, and water to produce
carbohydrates that become biomass. Equally important is
an understanding that photosynthetically derived carbohy-
Keywords: aquatic plants, 5E learning cycle, photosynthesis
ANDREW M. RAY is an affiliate faculty member in the Department
of Biological Sciences at Idaho State University in Pocatello.
PAUL M. BEARDSLEY, formerly an assistant professor in the
Department of Biological Sciences at Idaho State University, is
now a science educator with Biological Sciences Curriculum
Study in Colorado Springs, CO.
Copyright © 2008 Heldref Publications
13
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14
SCIENCE ACTIVITIES
drates represent energy that fuels ecosystem processes and
matter that is cycled among organisms. Coupling processes
like photosynthesis, consumption, and decomposition help
illustrate the constant exchange of matter among the atmosphere, hydrosphere, lithosphere, and biosphere. A basic
knowledge of energy transfer is also necessary for students
to develop a useful understanding of dominant patterns in
nature, such as energy pyramids.
Several assessments of students’ comprehension of photosynthesis suggest that conventional teaching methods do
not instill a useful understanding of the process (Amir and
Tamir 1994; Barker and Carr 1989b; Cañal 1999; Eisen
and Stavy 1988). These researchers, as well as Hershey
(2004), have identified several common misconceptions.
For example, the biochemical process of photosynthesis
found in most textbooks is described by the following simplified equation (Equation 1):
chlorophyll
Sunlight + 6CO2 + 12H2O
C6H12O6 + 6O2 + 6H2O
However, this equation is inaccurate. The combination
of the reactants shown and chlorophyll is insufficient for
photosynthesis; water needs are underestimated; and glucose (C6H12O6) is rarely the end product (Hershey 2004).
Hershey recommends that this simplified equation be abandoned and replaced by the following (Equation 2):
chloroplasts, light, mineral nutrients
H2O + CO2
O2 + (C6H10O5)n
water for transpiration or an aquatic environment
In the present article, we describe a series of activities
using aquatic plants that we designed to ensure students
have a thorough understanding of photosynthesis. We integrate the activities into the 5E (engage, explore, explain,
extend, and evaluate) learning cycle framework (Bybee
1997). These activities include multiple opportunities for
formative and summative assessments of student understanding of (1) photosynthesis, (2) controlled experimentation, (3) data presentation and analysis, (4) the interdependence among organisms, (5) the cycling of matter, and
(6) the flow of energy through living systems. We have
successfully used these exercises in high school courses,
but introductory activities are also appropriate for middle
school curricula.
Making Photosynthesis Relevant
Engage
It is easy for instructors to forget that many students do
not adequately understand the enormous impact of photosynthesis on their daily lives (see Carr 2001). This situation
Vol. 45, No. 1
is not helped by the fact that photosynthesis is often briefly
covered, either as a quick follow-up to teaching about respiration or a related concept when talking about the carbon
cycle. To increase students’ awareness of photosynthesis,
we begin our introduction to it by asking students to list
10 products that depend on photosynthesis with which they
interacted previously in their day. Most students write down
the plants they consumed, and some list other plant-derived
products with which they are familiar, such as cotton. We
place their responses in different categories (e.g., things we
eat, things we wear). We often suggest categories not mentioned by students (e.g., medicines, energy supplies such as
coal and oil).
Making Misconceptions Apparent: The
Importance of Carbon
Engage
The film Lessons from Thin Air in the Minds of Our Own
series (Harvard-Smithsonian Center for Astrophysics 1997)
pointed out that many students, including graduates from
elite American universities, struggle with the concept that
plant biomass is built largely with CO2 extracted from the
air. To expose the misconception that plants accumulate
biomass solely through water and minerals from the soil,
we present students with a seed and a large piece of wood
(overheads of these items substitute nicely in large lecture
classes). We then ask students to write down their answers
to the question, “What materials does a seed need to
develop into wood?” We tabulate the results on the board or
overhead. Typically, students recognize the need for water,
sunlight, and soil, but few understand the role of CO2. We
summarize the students’ results on the board with the heading “What most of us think plants need to build biomass”
while reserving further comment.
Explore
A discussion of Johannes Baptista van Helmont’s willow experiment in 1648 provides an opportunity to directly address the misconception that plants build biomass
mostly from materials in the soil, and it gives tremendous
insight into science as a process. In this activity, students
form small groups and are given van Helmont’s description of his experiment from the 1662 English translation
of his book Oriatrike, or Physick Refined (qtd. in Hershey
2003, 79; text in brackets added to aid students in reading
the passage):
But I have learned by this handicraft-operation that all Vegetables do immediately, and materially proceed out of the
Element of water onely [only]. For I took an Earthen vessel,
in which I put 200 pounds of Earth that had been dried in
a Furnace, which I moystened [moistened] with Rainwater,
and I implanted therein the Trunk or Stem of a Willow Tree,
weighing five pounds; and at length, five years being finished,
Spring 2008
SCIENCE ACTIVITIES
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the Tree sprung from thence, did weigh 169 pounds, and about
three ounces: But I moystened [moistened] the Earthen Vessel
with Rain-water, or distilled water (alwayes [always] when
there was need) and it was large, and implanted into the Earth,
and least the dust that flew about should be co-mingled with
the Earth, I covered the lip or mouth of the Vessel with an IronPlate covered with Tin, and easily passable with many holes. I
computed not the weight of the leaves that fell off in the four
Autumnes [Autumns]. At length, I again dried the Earth of the
Vessell, and there were found the same two hundred pounds,
wanting about two ounces. Therefore 164 pounds of Wood,
Barks, and Roots, arose out of water onely.
After reading the passage, we ask each group of students
to explain van Helmont’s experimental method, results, and
conclusions. We include prompts to encourage students to
think about controls and variables not measured (e.g., water
lost through transpiration) and to reflect on the importance
of soil nutrients to overall plant biomass.
Explain
We ask students to take into account van Helmont’s
results and revisit the previous list of materials that they
believed a seed needs to develop into wood. We then ask
the class to consider whether van Helmont’s conclusion that
only water is needed to produce plant biomass is justified
and, if not, why not.
Elaborate
We introduce students to the experiments and results of
John Woodward, Joseph Priestly, Jan Ingen-Housz, and
other famous scientists who have studied photosynthesis
by asking them to research their pioneering work. Their
experiments are widely cited in textbooks (Raven and
Johnson 2002) and on the Internet and collectively help
explain why van Helmont’s conclusions were incorrect.
For example, after completing his seminal work in 1771,
Joseph Priestly stated that plants are involved in “restoring
air” that has been “injured” by combustion and by animal
respiration (ctd. in Raven and Johnson, 186). Priestly’s
research demonstrated that sunlight and the green parts of
plants were needed to support the process of O2 production
while metabolizing CO2. These experiments build a deeper
understanding of the photosynthesis equation and reinforce
the tentative nature of scientific conclusions.
Explain
After providing the class with information on classic
experiments involving photosynthesis, we lead a discussion
aimed at developing a summary equation for photosynthesis. We start with the equation implied by van Helmont’s
experiments (Equation 3):
light, plant material
H2O → biomass
15
At this stage, we review the molecules of life and lead
students to the conclusion that biomass must be built from
organic macromolecules. We also remind students that
carbohydrates can be transformed into the other macromolecules. The general formula for a hexose (6-carbon
monosaccharide), (C6H10O5)n, may then be substituted for
biomass. We ask students to create photosynthesis summaries that are implied by the conclusions of each of the
experimenters listed in the Elaborate section. Ultimately,
we present the modern summary equation (Equation 2) for
photosynthesis suggested by Hershey (2004).
We have found it helpful, as suggested in the Lessons from Thin Air video (Harvard-Smithsonian Center for
Astrophysics 1997), to bring dry ice to class to reinforce the
concept that CO2 has mass.
Delving Deeper into the Photosynthesis
Equation: The Role of Light
Thus far, we have emphasized the biochemical process
of photosynthesis and introduced the concept of matter
cycling. The next series of activities illustrates the causal
relationship between light and photosynthesis, allowing students to visualize products of photosynthesis and beginning
a discussion of the importance of energy transfer. In this
exercise, students quantify photosynthetic rate as a function
of distance from a light source using bubble production as
an indicator of photosynthetic activity. It is interesting to
point out that this approach, developed by Jan Ingen-Housz
in 1779, was the standard method for quantifying photosynthesis of aquatic plants until the latter part of the 20th
century (Bowes 1989).
Materials
• Any submersed aquatic plant that is in good health and
appears capable of photosynthetic activity (i.e., not dried
or wilted). Plants can be harvested from local lakes or
streams, which may reduce activity costs. Plants such
as Canadian waterweed (Elodea canadensis) or coontail (Ceratophyllum demersum) are commonly found
throughout North America and would be appropriate for
this experiment. These or other plants, such as Brazilian
waterweed (formerly Anacharis; Egeria densa) and water
milfoil (Myriophyllum elatinoides), can be purchased at
aquarium supply stores or pet shops. We strongly discourage introducing aquatic plants purchased from
supply stores into local lakes or streams. Instead,
these materials should be discarded upon completion
of the experiment.
• Glass test tubes (20 × 150 mm) to represent experimental
microcosms
• Racks to hold test tubes
• A light source to represent the sunlight in Equation 2.
16
SCIENCE ACTIVITIES
Light fixtures outfitted with full-spectrum 65-watt bulbs
can be mounted on a ring stand to produce sufficient light
at the appropriate height. Common desk lamps available
in most classrooms can also be used.
• Large- to medium-sized drinking straws. Straws from
fast-food restaurants or the student cafeteria work well.
• Standard thermometer for measuring water temperatures
in test tubes
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Engage
At the beginning of class, pass out the test tubes with
plant segments (see Procedure) and ask students what they
observe in the tubes and how this relates to the previously
developed equation for photosynthesis. Students quickly
realize that the bubbles must contain oxygen (although the
bubbles also contain nitrogen; Hershey 2004). Lead a discussion to demonstrate that the rate of bubble formation is
a measure of the rate of photosynthesis.
Explore
In this activity, students measure the rate of photosynthesis at different light intensities (Buttner 2000). We manipulate light intensities by placing plants at different distances
from the light source. We have had students develop their
own protocol for this lab or have prescribed a protocol for
them, depending on the level of the students.
Procedure
Divide the test tubes into a treatment group and a control
group. An hour or more before class, place a 5-cm segment
of an aquatic plant into each treatment test tube. In separate
test tubes, place an inert object similar in dimension to the
plant segments (e.g., a 5-cm section of a drinking straw);
the test tubes with these plant surrogates act as controls.
If multiple plant species are available, add an additional
test tube for each additional species and place 5-cm cuttings of those species into their own test tubes. You should
attempt to have all plant clippings be as similar as possible
(i.e., taken from the same location on the stem of multiple
plants). Fill all test tubes with the same amount of tap
water, to within 2 cm of the top. Place all tubes at a known
distance from a light source and allow 15 min for the plant
to acclimate to the new environment. Make sure to plan
for enough test tubes to carry out this experiment using
multiple distances. We have successfully used the following distances to manipulate light levels: 15, 30, and 45 cm
from the light source. Fresh plants should be used for each
light intensity experiment (e.g., do not reuse plants that
were used to examine photosynthetic rates at 15 cm again at
30 cm or 45 cm).
This experiment can be conducted with the classroom
lights on; overhead lights generally do not induce bubbling
but allow students to see the experiment clearly. After the
Vol. 45, No. 1
test tubes containing plants or plant surrogates have been
exposed to the light for at least 15 min, students can begin
quantifying the rate of photosynthesis on the basis of the
number of bubbles that emerge from the plant and float to
the surface. Because the control tubes will collect bubbles,
it is important for students to count only the number of
bubbles that come from the plant or plant surrogate and rise
to the surface. On the basis of the rate of bubble production observed, students should determine over what period
of time (e.g., 15, 30, or 60 s) bubble production should
be measured; the greater the bubble production, the less
time necessary. If multiple lamps and test tube racks are
available, this experiment can be replicated by splitting the
class into groups of three or four and carrying out the same
measurements at each station.
Students should measure the temperature in all tubes at
the same time they count the number of bubbles. Increases in
temperature can influence rates of photosynthesis and have
been implicated in the spontaneous generation of bubbles from
nonphotosynthetic materials (Ganong 1906). Control test tubes
are necessary to demonstrate that, with the combination of
light and associated heat, bubbles may form at the surrogate’s
surface, but few, if any, of those bubbles will be released from
the surrogate and rise to the surface of the water.
Sample Results
We tested Canadian waterweed (Elodea canadensis)
and Eurasian watermilfoil (Myriophyllum spicatum) along
with a control (a drinking straw) at multiple distances from
a light source (15, 30, and 45 cm; see Figure 1). A single
individual monitored each test tube and its contents (plant
or straw) over three 30-s intervals. Because bubble production was monitored for each species and the control over
three sampling intervals, we were able to conduct amongspecies statistical comparisons for each distance from the
light source using a one-way analysis of variance (ANOVA).
Both the Elodea and Myriophyllum produced bubbles at all
distances during at least one of the observation periods. The
control that was 15 cm away from the light source produced
a single bubble. We detected differences among the plant
species, with Myriophyllum generating a greater number
of bubbles at distances of 15 cm (p < .001) and 30 cm (p =
.001); the control had significantly lower bubble production
than did either plant species at both of those distances. At a
distance of 45 cm, there were no significant differences in
the rate of bubble production between the two species and
the control (p = .118; see Figure 1).
Explain
Ask students to construct a graph in their lab notebook
that summarizes their data (e.g., Figure 1) and to present it
to the rest of the class, if time permits. Before the students
present their results, remind them of the summary equation
Spring 2008
SCIENCE ACTIVITIES
Elodea
Myriophyllum
Straw (control)
Number of Bubbles (per 30 sec)
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20
In fact, photosynthetically active aquatic plants are capable of
altering the pH of water by as much as 3 pH units (1000-fold
change in the availability of hydrogen ions) over the course
of a single day (Spencer, Teeri, and Wetzel 1994).
Materials
15
10
5
0
17
15
30
45
Distance from Light Source (cm)
FIGURE 1. Graph of photosynthetic rates of watermilfoil (Myriophyllum), Canadian waterweed (Elodea
canadensis), and control (plastic straw), as indicated by
number of bubbles emerging (per 30 sec) as a function
of distance (15, 30, or 45 cm) from light source. Bars
represent mean scores from three replicate observations; lines above bars represent 1 standard error over
the mean. Absence of bars for control treatments at
30 and 45 cm reflects the lack of bubble production (0
bubbles/30 sec) at these distances. At 15 and 30 cm
from light, Myriophyllum produced significantly more
bubbles than did Elodea, and both species produced
significantly more than did the control (p < .05). At 45
cm, the species did not differ significantly from each
other or from the control in bubble production.
of photosynthesis and ask that they present their conclusions in the context of that equation.
Elaborate
Students with an extensive background in statistics can
calculate means and standard deviations of bubble production for each species and the control. A one-way ANOVA
can be used to test for differences between each plant species and the control at each distance.
Detecting Changes in pH Associated with
Aquatic Plant Photosynthesis in Microcosms
In this exercise, students use aqueous indicator solutions
to detect changes in the pH of microcosms containing photosynthetically active plants. This experiment builds on the idea
that photosynthesis and respiration of aquatic plants influence O2 and CO2 concentrations in natural waters and consequently affect the pH (Kelly, Hornberger, and Cosby 1974).
• A pH indicator such as bromothymol blue. Depending on
the alkalinity of tap water in your area, 1M NaOH may
also be needed to adjust the pH (see Procedure).
• Parafilm-brand flexible plastic film to seal the test tubes.
Rubber test tube stoppers may be substituted.
• Submersed aquatic plants such as those used in the previous activity
• Glass test tubes (20 × 150 mm). All test tubes will be filled
with tap water at a level standardized among treatments.
• Test tube racks to secure test tubes in upright position
• A light source similar to that used in the previous exercise
• Large- to medium-sized drinking straws to act as plant
surrogates
Engage
In our experience, most students know that the major gas
that they (and other animals) exhale is CO2. The purpose
of this engage activity is to build on this facet of student
knowledge and create a deeper understanding of the impact
of photosynthesis on a microcosm.
Procedure
Fill all test tubes with tap water to within 2 cm of the
top. Add the necessary amount of bromothymol blue (see
instructions from the manufacturer) to provide a proper
indicator of the existing pH. The water in the test tubes
needs to be slightly basic, which will be reflected by the
blue color of the solution after the introduction of the indicator. If the water is not basic enough, add drops of 1M
NaOH until the solution turns blue (indicating a pH higher
than 7.6).
Ask students to blow CO2 into the test tube using a
straw. After the sustained introduction of CO2, the solution
will turn yellow. The yellow color indicates that the pH of
the solution is at or below 6.0. The change in pH occurs
because as CO2 is introduced, a small percentage of it reacts
with the water to form carbonic acid (H2CO3). Although
this is a very weak acid, it is sufficient to induce a color
change indicating the drop in pH.
Explore
At this point, introduce a 5-cm segment of plant material
into the treatment test tubes. Add the plant surrogate (e.g.,
straw) to the control test tubes. The plant and control should
be allowed to stabilize in the test tubes for approximately
15 min.
18
SCIENCE ACTIVITIES
In the first exercise, students documented the distances
from the light that resulted in high rates of photosynthesis by measuring bubble production. Ask students to
repeat this process using the test tubes with indicator
solution at only a single location. Encourage them to
consider the relationship between photosynthesis and
pH by monitoring the formation of bubbles, the color
indicator, and the presence of light. Depending on the
photosynthetic activity of the plant fragment and the pH
of the water in the test tube, a color change may take
30–60 min to occur.
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Explain
Ask groups of students to develop a hypothesis to
account for their observations. Tell them that they need
to be able to defend their hypothesis. Before the students
present their results, remind them of the summary equation of photosynthesis (Equation 2) and have them assist
in generating the equation for the reversible chemical
reaction through which CO2 and water form carbonic acid
(Equation 4):
H2O + CO2 ↔ H2CO3 (carbonic acid)
We recommend that their hypothesis include the common
elements of Equations 2 and 4.
Elaborate
Present students with the following hypothetical scenario:
A fisherman finds fish floating in a small local lake. An
industrial facility is blamed for discharging toxic materials
into the lake because the fish were found near the facility’s
outfall. The fisherman knows that the facility uses sulfuric
acid to remove impurities from materials used to make
widgets. The fisherman contacts the local newspaper and a
reporter promptly visits the site and documents the kill with
photographs. The paper does a story about the kill and interviews a professor from the local university who confirms that
acidic lakes tend to support fewer aquatic organisms than do
lakes with a neutral pH. The professor uses as an example
of how devastating acid rain has harmed aquatic life in lakes
in the Northeast United States, noting that many lakes have
been characterized as “dead” because of the radical changes
in pH. The Department of Environmental Quality contacts
the university and asks it to design a monitoring program
that measures the pH of the lake and evaluates how pH may
have influenced the death of fish. Because of the contentious
nature of the incident, it is possible that their findings will be
used in a future lawsuit.
Ask students to develop a sampling plan that assesses
the pH of the lake and design an experiment that describes
the reciprocal relationship between pH and aquatic life.
Encourage students to carefully consider how temporal and
spatial changes in pH may influence the recommendations
that emerge from their findings.
Vol. 45, No. 1
Monitoring Changes in Dissolved Oxygen and pH
in an Aquarium Containing Aquatic Plants
The final exercise allows students to use instrumentation
commonly available in a scientific classroom to document,
over multiple days, changes in the dissolved oxygen (DO)
concentrations and pH of aquaria containing either water
and aquatic plants or only water.
Materials
• Submersed or free-floating aquatic plants capable of
photosynthetic activity, as described previously. Because
macroalgae (e.g., species of Spirogyra or Cladophora)
have been shown to produce diurnal DO patterns from
photosynthesis that are similar to those produced by
vascular aquatic plants (Kelly, Moeslund, and Thyssen
1981), these algae work equally well for this part of the
experiment.
• Aquariums for experimental and control measures of
photosynthesis. One aquarium should contain aquatic
plants or algae. It is important that no large heterotrophic
organisms are present (e.g., fish or amphibians; these
are used successfully elsewhere to illustrate respiration
[Buttner 2000]). The other tank should be clear of plants
and attached algae (see Figure 2).
• Continuous monitoring device for recording changes in
DO and pH over an extended period (at least 48 hr). We
used YSI-brand data sondes to record changes in DO over
the course of a weekend. Most natural-resource agencies
or universities have instruments, usually requiring limited maintenance, that are capable of recording DO, pH,
and other water-quality parameters for extended periods
and at regular intervals (e.g., hourly). Alternatively, the
Vernier DO and pH sensors found in many classrooms
can be used to track changes in these parameters. The
Vernier probes can record the data automatically, like the
data sondes, or you can use them to take manual measurements to record in a laboratory book over the course
of the school day.
Engage
Show students authentic continuous data from a local
water-quality monitoring program. We presented students
with a graph that showed daily changes in DO and pH (see
Figure 3) from a local river (the Portneuf River) and asked
them to consider what in a river or lake could lead to such
dramatic changes in DO and pH. We reminded them that
pH is on a logarithmic scale, so a change of only one pH
unit actually reflects a change in the acidity or alkalinity of
a solution that is one order of magnitude larger. Students
quickly realized the cyclical nature of the changes that
occur daily. They made the connection that changes in pH
may reflect the daily photosynthetic activity of aquatic
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Spring 2008
SCIENCE ACTIVITIES
19
FIGURE 2. Aquariums used to monitor changes in dissolved oxygen and pH. We filled two 10-gallon aquariums with the
same volume of tap water. We added aquatic plants to one aquarium (left) and left the other without plants to act as a control (right). The gray cylinder in each aquarium is a YSI data sonde used to record dissolved oxygen and pH in each tank.
plants. We then led a discussion about how measuring the
changes in pH and DO represents a logical way to illustrate
daily patterns of photosynthesis in aquatic ecosystems.
set to record over a student-defined period of time; a cloudfree weekend works especially well. The daily changes in
both parameters can be tracked over multiple days.
Explore
Explain
For this portion of the learning model, two 10-gallon (or
larger) aquariums are needed.
Students should estimate the forecasted daylight hours
(or, if a light bank is used, set the timer) for each day over
the course of the study period and use this information when
explaining changes in DO and pH. DO concentrations and
pH can be plotted against time, and students should examine
patterns in each parameter over the course of the study period
for both control and treatment aquariums. This builds on the
information they learned in the previous exercises.
Procedure
One aquarium should contain luxuriant stands of submerged or free-floating aquatic plants (see Figure 2). For a
control tank, fill a second aquarium with water but no plants
or algae. Fill both aquariums to a standardized volume with
tap water and place them in a greenhouse or below an automated light bank. Water-quality instruments, such as YSI
data sondes, can be programmed to record measurements
of DO and pH at regular intervals over a defined period of
time. A data sonde should be placed in each aquarium and
Sample Results
We programmed YSI data sondes to record DO and pH
at 10-min intervals. We placed one sonde in an aquarium
containing luxuriant growth of four aquatic plants (Elodea
20
SCIENCE ACTIVITIES
Vol. 45, No. 1
DO (mg/L)
pH
8.2
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8.0
12
7.8
10
7.6
pH
Dissolved Oxygen (mg/L)
14
8
7.4
6
7.2
4
4th
5th
6th
7th
8th
9th
10th
11th
Date (in August 2005)
12th
13th
14th
15th
16th
FIGURE 3. Graph of dissolved oxygen (DO) concentrations (mg/L; solid line and left y-axis) and pH (dashed line and
right y-axis) in the Portneuf River, Idaho, over a 2-week period (August 4–16, 2005). DO and pH were measured at the
Siphon Road Bridge (see http://www.portneufriver.org) using a YSI data sonde. Data shown is considered provisional
and was provided by the Idaho Department of Environmental Quality.
canadensis, Myriophyllum spicatum, Lemna minor, and
Azolla mexicana) along with associated filamentous algae;
the aquarium was located in the greenhouse, and natural
sunlight entered through skylights. We placed a second
sonde in an aquarium containing only tap water (see Figure 2). The sondes were deployed at the close of school
on Friday and retrieved the following Monday. Both DO
(mg/L) and pH were plotted against time (see Figure 4). In
the aquarium containing plants, both parameters underwent
dramatic changes over the course of the weekend. DO concentrations changed by as much as 19 mg/L over a single
day, peaking at approximately 5:30 pm and precipitously
dropping thereafter. In contrast, the lowest oxygen concentrations occurred between 7:00 and 7:30 am, just after sunrise. pH levels underwent similar variations over the course
of the investigation, reflecting the direct impact of photosynthesis on pH. Maximum pH values corresponded to
maximum DO concentrations, and the two parameters were
strongly correlated (R = 0.939, p < .001). In the aquarium
lacking plants (i.e., the control), there was little variation
in either DO or pH over the course of the 60+ hours we
observed them (see Figure 4).
Elaborate
To expand on the concepts in this teaching model, we
asked students to make predictions about how DO and pH
in natural waters change over the course of a day, week,
season, and year. We required students to include biological, physical, and chemical factors that cause the variations
they described at each of the different time scales. Once
predictions were made, we used online real-time data from
the Portneuf River Monitoring Project (http://www.port
neufriver.org) to show students the changes in pH and DO
that occur in natural waters, using a locally relevant river.
We found archived data from multiple seasons and continuous data for single or multiple years to be extremely
valuable when demonstrating the relationships among day
length (photoperiod), DO, and pH. Water-quality monitoring programs are not unique to this river. For example,
the City of Indianapolis Department of Public Works has
been monitoring water quality using continuous-monitoring
instrumentation on the White River and its tributaries since
1997, and it also provides real-time access (Tice 2005). In
addition, investigations examining the impact of nuisance
aquatic plant growth on pH, DO, turbidity, and temperature
are actively underway on the Lower Yakima River in Washington State, and data from these studies and other studies
monitoring streams and rivers in near real time are available
on the U.S. Geological Survey’s Web site (http://waterdata
.usgs.gov/nwis/rt). We recommend contacting your local
Department of Environmental Quality and inquiring about
monitoring programs in your area that may have similar
data from a local lake or river.
Discussion
Many of the activities in this article will not be new to
teachers. However, we take a novel approach by incorporating several activities into a single teaching model following
the 5E learning cycle (Bybee 1997). We include activities
that use different teaching styles to engage students with
Spring 2008
SCIENCE ACTIVITIES
DO (mg/L)
With Plants
9.5
15
9.0
10
8.5
5
8.0
7.5
0
7.0
No Plants
20
10.0
9.5
15
9.0
10
8.5
5
8.0
pH
Dissolved Oxygen (mg/L)
10.0
pH
Dissolved Oxygen (mg/L)
20
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pH
7.5
0
0
6
12 18 24 30 36 42 48 54 60
Hours
7.0
FIGURE 4. Dissolved oxygen (DO) concentrations (mg/
L; solid line and left y-axis) and pH (dashed line and
right y-axis) plotted against time (in hours). DO and pH
were measured using a YSI data sonde.
different interests and learning modalities, which likely
achieves greater success in teaching about photosynthesis than do conventional approaches (McKeown 2003).
Moreover, we provide a teaching model that emphasizes
the role of autotrophs and photosynthesis in ecosystems by
integrating physical, chemical, and biological characteristics of ecosystems into individual activities. Others have
recommended a holistic strategy, considering ecosystemlevel ramifications as an alternative to existing strategies
of teaching photosynthesis (Eisen and Stavy 1988; Lin and
Hu 2003).
Students in biology and Advanced Placement biology
classes at public high schools in southeast Idaho used our
model to examine the process of photosynthesis as part
of a larger unit on plants. Students began by monitoring
the evolution of gaseous bubbles in the presence of an
artificial light source. We then introduced activities that
highlighted the relationships among O2, CO2, and pH in
natural waters and demonstrated to students how plants,
in the presence of sunlight, can influence the productivity
21
and chemistry of their environment. We found that using
authentic monitoring data collected from a local river was
especially helpful in illustrating the interconnectedness of
organisms and their environment and how primary producers alone affect their chemical environment through
the combination of photosynthesis and respiration. Considering these processes in tandem and tracking them over
the course of several days gave students the opportunity to
visualize patterns of energy flow common in simple ecosystems. Content knowledge that can be addressed from
these experiments includes the following:
• In many ecosystems, energy for life is derived from the
sun through photosynthesis.
• Trace energy flows through ecosystems in one direction,
from photosynthetic organisms to herbivores, carnivores,
and decomposers.
• The distribution and abundance of organisms and populations in ecosystems are limited by the availability of
matter and energy.
The combined use of these exercises highlights the
dynamic nature of photosynthesis and presents this across
three time scales: minutes, hours, and days. These exercises focus on the process of photosynthesis and contrast
with other activities that simply test for the presence
or absence of carbohydrates. These experiments can be
altered and the experimental design expanded, but, in their
current format, they provide opportunities for hypothesis
testing, use of experimental controls, and application of
summary statistics and statistical analysis. In addition,
by incorporating locally available plant material, these
experiments provide opportunities for students to conduct
research in natural settings.
Conclusion
Our use of a local aquatic ecosystem and authentic data
helped capture the attention of many students. We also found
that students enthusiastically participated in the exploratory
component of each of the activities. We are still assessing
the effectiveness of this approach to teaching photosynthesis, but our initial impressions are that students exposed to
the activities described here gained a deeper understanding
of photosynthesis and a greater appreciation of how plants,
as a consequence of photosynthesis, are actively involved
with energy transfer and matter cycling in ecosystems.
Acknowledgments
Andrew M. Ray was supported as a graduate teaching fellow
in the Idaho State University GK-12 Project, which is funded by
a National Science Foundation GK-12 Education Grant (DGE0338184). The authors thank Cara Sonnemann and Teri Mitton for
assistance in teaching these lessons; Rosemary Smith and Richard
Inouye for comments on a previous version of this manuscript;
and Greg Mladenka for assistance with data retrieval and equip-
22
SCIENCE ACTIVITIES
ment calibration. The Idaho Department of Environmental Quality
(IDEQ) provided the YSI instrumentation used in the classroom.
The IDEQ and participating members of the Portneuf River Monitoring Project provided provisional monitoring data.
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