The Power of Sunlight
Instructor Guide
Open-ended photosynthesis
Investigation for:
Grades 7-12 & College
Engagement Scenario:
Designing your Dream House
Tanner and Tracy had been saving to build their own house
ever since they met ten years earlier. Tanner was an
artist who worked in stained glass and had completed
several large commissions in local churches and businesses.
Tracy owned a florist shop and nursery and loved exotic
plants. Tracy envisioned a large family room with a
cathedral ceiling and a vast glass wall to provide plenty
of light for her to grow a variety of exotic tropical
houseplants. Tanner thought this was a great idea because
it would allow him to showcase is art in the stained glass
wall. The house, and particularly Tanner’s stained glass
wall, was a huge aesthetic success and were even featured
in an issue of Better Homes and Gardens. How well do you
think Tracy’s plants grew?
Studying Photosynthesis
During photosynthesis carbon dioxide and water are
converted in the chloroplasts of plants into carbohydrate
(sugar) and oxygen is released into the atmosphere. The
reactions of photosynthesis are complex. In fact, the
Nobel Prize was awarded to the botanist Melvin Calvin for
figuring out just part of the overall process: how carbon
dioxide (CO2) is converted into sugar (C6H12O6).
Nevertheless, the overall process can be summarized by a
simple equation:
CO2 + H2O + light energy
C6H12O6 + O2
chlorophyll
The light energy for this process is absorbed or captured
by pigments such as the chlorophylls. (There are two
different types of chlorophyll in land plants and three
other kinds in various algae!). This captured light energy
is then converted into a biologically usable chemical form
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of energy, the bonds of organic molecules such as sugar.
If excess sugar is produced, it can be stored temporarily
as chains of sugars: starch molecules. Through
photosynthesis in the green leaves, plants produce the food
they need to live and grow. During cellular respiration,
the food is converted into energy to be used by all parts
of a plant.
A variety of techniques can be used to examine if, and how
rapidly, photosynthesis is occurring. Examine the overall
equation above. What factors do you think could be
measured to determine if photosynthesis is occurring and
what would you expect to see?
In reactions like photosynthesis and respiration it is
often easiest to measure changes in the gases involved.
For instance, in a closed tube containing living green
plant tissue you would expect to see a decrease in CO2
and/or an increase in O2 if photosynthesis was occurring.
It is difficult to measure sugar production in living
tissues, but if excess sugar is converted into starch this
can be measured. The presence of starch can be determined
for an entire piece of plant tissue or peels or hand
sections can be made of tissue to examine starch
accumulation in individual cells.
Why do you think it is not practical to try to measure the
amount of water used by living cells undergoing
photosynthesis?
Different plants undergo photosynthesis at different rates,
even under exactly the same environmental conditions. This
is partially because of the different anatomical structure
of their photosynthetic tissues, but also because of
different physiological pathways that have evolved. The
rate of photosynthesis may also change in the same plant as
tissue ages. Do you think younger leaves near the tip,
middle-age leaves, or older leaves near the base of the
same plant will photosynthesize faster? You may want to
examine the same seedlings you used in your seed growth
experiments or you may want to use other plants growing in
your classroom, at home, or outdoors.
Demonstrating Photosynthesis (or respiration)
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A simple demonstration of photosynthesis (or respiration)
is to look for an indication of the use (or production) of
carbon dioxide, CO2. As CO2 dissolves in water it forms an
acid. As a result, the pH of the water will decrease if CO2
is produced and added to the water (respiration) or
increase if CO2 is used and removed from the water
(photosynthesis).
Add a small amount of the indicator dye Phenol Red to a
container of tap water until the water is distinctly red.
Pour some of the Phenol Red indicator solution into a
smaller jar and blow bubbles into it through a straw. The
exhaled air will contain excess CO2 and the solution will
begin to turn orange, then yellow. These two solutions can
now be used to demonstrate both photosynthesis and
respiration. Place identical pieces of living tissue (such
as germinating seeds or pieces of green stem or leaf tissue
from seedlings) into two small tubes, one containing “red”
and the other “yellow” Phenol Red. (Note that for a
demonstration it does not matter how much tissue you use,
but if you were going to quantify results you would want
the same amount of tissue in each treatment.)
If photosynthesis is occurring (faster than respiration),
then the red solution should stay red but the yellow
solution should begin to change back to red. If
respiration is occurring faster (for instance if the tubes
are covered with foil or kept in the dark) then the red
solution should begin to turn yellow but the yellow
solution should remain unchanged. Of course, the more
tissue you have in the tube, relative to the amount of
solution, the faster any change will be visible. Color
change is easiest to see if tissue is removed from the
solution, or some of the solution is removed from the
tubes, and colors are compared against a white background.
Designing a Photosynthesis Experiment
This photosynthesis investigation can be used at a variety
of levels, but you will want to fit the resources to your
students. A comprehensive Resource Manual, which describes
a variety of ways to design and analyze experiments, is
available to download from the program website. From this
manual geared toward highly independent learners, an
excerpt on measuring oxygen in leaf disks is provided here.
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Middle and High Schools: For middle and high school
students, leaf disk floatation experiments will be
accessible and require only low cost, simple materials.
Colleges: For college students, you may want to cut out
the Example of a Photosynthesis Experiment below so as not
to bias their own creativity. Students could also be
introduced to additional procedures if the equipment and
resources are available.
Example of a Photosynthesis Experiment
See: Emporia State University Team 6, Fall 2005
http://www.plantbiology.org/index.php?module=pagesetter&func=viewpub&ti
d=2&pid=11
The team’s question was whether
different colored areas of a
variegated Coleus leaf
photosynthesize at different rates?
The individual leaves of Coleus,
like some other ornamental plants,
have multiple colors in distinct
patterns. Does the whole leaf
photosynthesize at the same rate or
do different colored areas photosynthesize at different
rates? The team used the floating disk technique
(described below) and punched leaf disks from green and
purple segments of the same leaves. Their null hypothesis
was that there would not be a significant difference in the
rate of photosynthesis between these tissues. The green
and purple leaf sections should produce oxygen at the same
rate and therefore the disks should float at the same rate.
The team repeated their experiment twice and got similar
results both times. The green leaf disks tended to float a
little faster than the purple disks, but this difference
was not significant. The data generally supported their
null hypothesis, but this is not what they expected to
find. Intuitively they thought that the green leaf disks
should float much faster because, after all, chlorophyll is
a green pigment. They did not have time to pursue this
question, but perhaps if you replicate their experiment and
get similar results, you may want to pursue it further!
Leaf Floatation Technique
Leaves are the primary photosynthetic organs in plants.
Much of the internal volume of a leaf consists of air
spaces that are necessary for gas exchange. The principle
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of leaf disk flotation is to remove the air from these
intercellular spaces, a process called infiltration, and
replace it with a bicarbonate solution that serves as a
source of CO2. As photosynthesis occurs, O2 is produced
which forms ever increasing bubbles trapped in the spaces.
As more and more oxygen is produced, the leaf disks become
buoyant and eventually float. The rate of flotation is an
indirect measure of the rate of photosynthesis. The time
required for the disks to float is inversely proportional
to the rate of photosynthesis.
Basic Materials:
 Plants (anything from
seedling cotyledons to
mature leaves)
 Baking soda, water (at
room temperature)
 Stop clock or watch
 Cork borer, paper
punch, straw



Light source (light
banks or direct
sunlight)
Foil or other covering
to keep the light out
Plastic syringes
Procedure:
1) For most leaves, you can simply add bicarbonate to water
(0.02M sodium hydrogen carbonate) and use a water control.
To maintain the sodium bicarbonate solution at a pH of
6.8, a citrate-phosphate buffer can be used (see BUFFER and
OXYGEN in the comprehensive resource manual).
The amount of solution you make will depend on the size and
number of containers you use.
For instance, if you used
standard size Petri dishes with three repetitions for both
your experimental and control groups you would need at
least 300 ml of water, half of which would be used to make
bicarbonate solution. This would allow 50 ml of solution
for each dish. It would be better to make extra – say 500
ml – to be sure you have enough and to make your
calculations a bit easier (500 ml is ½ liter).
2) Select several leaves from your
plant of interest and use a cork
borer or paper punch to cut out
equal-sized leaf sections from
these leaves. Avoid getting major
veins in your leaf sections.
The number of disks you make
depends on the number of
repetitions you set up and how
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many disks you want per repetition. (Ten is a common
number per treatment simply to make calculations easier.)
Again, it is better to prepare more than you will
need. Some will infiltrate more easily than others. So it
is quicker to infiltrate a bunch, then pick out the number
you need from the “sinkers,” than to have to keep repeating
the infiltration procedure until all disks have sunk.
As soon as your disks are punched out, float half the
disks in a container of plain water and the other half in a
container of bicarbonate solution. You don’t want the leaf
tissue to dry out!
3) To infiltrate the leaf tissue:
Remove the plunger and half fill
syringe with bicarbonate solution.
Add the disks from the bicarbonate
solution container. Replace the
plunger, then turn the syringe right
side up.
Push the air out of the
syringe by pressing on the plunger.
Once the
excess air has been
removed, evacuate the tissue several
times by plugging the tip with your
finger and drawing the plunger to create
a vacuum.
You should see some bubbles of air
come out of the leaf. After holding the
vacuum for a few seconds, release the
vacuum either by removing your finger
from the tip of the syringe or by letting
go of the plunger handle. As the vacuum
is released, solution will replace some
of the air in the leaf.
Repeat this procedure several times until the disks
sink. Infiltrated disks (with their internal air replaced
by solution) should be uniformly dark green and appear
“water-logged.” Immediately cover these disks with foil so
that photosynthesis cannot occur until all disks are ready.
4) Repeat step 3 with the other half of the disks using the
control water in a syringe.
5) After enough disks have sunk, distribute the infiltrated
disks randomly between the three dishes for each treatment.
In each dish, the disks should be resting on the bottom.
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While you are preparing the sets of leaf disks, cover them
so that they will not begin to photosynthesize until you
are ready to start the experiment.
6) Place all six dishes containing the disks under the same
intensity of light. Record (in minutes) the time it takes
each leaf disk to float. Calculate the average for each
dish.
These are the baseline data your research team can now use
to investigate the effect of other environmental parameters
on the rate of photosynthesis. Some (but not all) effects
you might consider include:
 effect of light
 effect of CO2
quantity (intensity)
(bicarbonate)
 effect of light
concentration
quality (color)
 effect of temperature
 effect of pH
 response of different plants, such as C-3 and C-4
plants or plants adapted to shade or full sun
Teaching Tips and Strategies
Do you need buffer?
Sodium bicarbonate dissociates into carbonic acid when it
dissolves in water. This is a source of CO2 necessary for
photosynthesis but it also acidifies the solution, which
can affect the rate of photosynthesis. Ideally you want to
provide a buffer to keep the pH from changing in the
solution. However, if making the disks float, or not, is
the main thing you’re interested in, just use water.
What plant to use?
We’ve tried a wide variety of different plants—in fact,
that’s the first thing my student groups have to decide on.
Young pea seedlings, left over from seed germination, have
been some of the fastest responding leaf disks we’ve tried.
Dieffenbachia, dumbcane, is a common houseplant that
usually responds quickly. Some lab manuals suggest using
Bryophyllum or Kalanchoe, also common houseplants, or
spinach. Avoid using plants with “hairy” leaves.
Cutting disks
The write-up suggests using a cork borer to cut disks.
This makes it easy to cut uniform-sized disks and to let
groups choose different disk sizes. But, there is nothing
special about cork borers. You could use paper punches,
straws, or you could just cut out squares with a straight
edge and razor blade.
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Infiltrating the disks
Removing air from the internal leaf spaces and replacing it
with solution can be the most frustrating (but also the
most fun) part of this investigation. As mentioned above,
avoid using a plant with “hairy” leaves. These tend to
trap air making it difficult to get the leaves to sink,
even if internal tissues become infiltrated with solution.
Be sure to add a drop or two of dilute detergent to the
solution in the syringe to help reduce surface tension.
After pulling a vacuum for a few seconds, and then
releasing it, tap the syringe barrel a few times to jar
surface bubbles from the leaf disks. You also may have to
tap the barrel while the vacuum is drawn to get disks to
sink. If possible, have two syringes per group so students
can be evacuating both the bicarbonate treatment disks and
water control disks at the same time.
Floating the disks
We usually do the experiment in Petri dishes, but have used
finger bowls, test tubes, and baby food jars. The main
thing is that there is a sufficient volume that students
can easily tell if disks are floating or not. When the
disks are first poured from the syringe into the
containers, some disks may immediately float due to air
bubbles trapped on the surface. Tap them under the surface
a few times with a blunt probe or pencil and they will
often sink. If not, removed them (this is one reason to
infiltrate more disks than you think you’ll need - - to
have extras). With some leaves it’s made a difference if
the disks were “right-side up” or “upside down” sitting on
the bottom. I let students discover this when some disks
float much more rapidly than others in the same dish.
Occasionally disks adhere to the bottom of the dish
and do not float (perhaps because students initially pushed
them to the bottom to make them sink?) If this appears to
be happening in a dish (a few disks remain on the bottom
long after the others in that dish have floated) tap the
side of the dish a few times or gently tap the side of the
disk with a probe or dissecting needle. If it was stuck,
it will immediately float.
Some investigations geared for young learners (e.g.,
http://wwwsaps.plantsci.cam.ac.uk/worksheets/scotland/sunshade.htm or
http://www.fastplants.org/pdf/activities/exploring_photosyn
thesis.pdf - search="wisconsin fast plants
photosynthesis"), suggest recording the time the leaf disks
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take to float within the syringes.
the leaf disk number to 3 or 4.
This works but limits
Engagement Scenario
The introductory scenario raises the general question, what
do plants need to grow. It leads to discussion of the
properties of light and the absorption spectra of
chlorophyll pigments in plants. Once the overall process
of photosynthesis has been introduced, class discussion
questions might include: How do CO2 and water enter the
plant’s system? In a log of wood, where does all the mass
come from? How have plants (photosynthetic autotrophs)
changed the Earth’s atmosphere? How might changes in the
atmospheric concentration of CO2 affect plants?
Some Common Alternate Conceptions About Photosynthesis
Plant photosynthesize during the day and respire at night.
Plants obtain food from the soil.
Photosynthesis is the simple conversion of CO2 and water to
sugar and O2.
Plants are green because they absorb green light.
Only green plants are capable of photosynthesis.
Possible Assessment Ideas- Have students:
Make a concept map for photosynthesis linking at least 4
terms
Compare and contrast photosynthesis and respiration
Summarize what happens in the light dependent reactions of
photosynthesis
Web Resources and References
Photosynthesis animation by M. Tyree on TeachersFirst.com
http://www.teachersfirst.com/getsource.cfm?id=5955
Virginia Cooperative Extension’s Virtual Forest, with interactive
exploration of photosynthesis
http://www.ext.vt.edu/resources/4h/virtualforest/
Fox. M., Gaynor, J.J., and Shillcock, J. 1999. Floating spinach
disks- an uplifting demonstration of phototsynthesis.
Journal of College Science Teaching 28(3):210-212.
Hershey, D. R. 1995. Plant Biology Science Projects (Best
science projects for young adults). John Wiley and Sons,
New York.
Storey, R.D. 1989. Textbook errors and misconceptions in
biology: Photosynthesis. American Biology Teacher
51(5):271-274.
Tourtellotte, S.W. 1990. Biology and chemistry combine in
photosynthesis: An interdisciplinary focus on a natural
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occurrence. Journal of College Science Teaching 19(5):287291
Weyers, J.D.B., Högland, H-O., and McEwen, B. 1998. Teaching
botany on the sunny side of the tree: promoting
investigative studies of plant ecophysiology through
observations and experiments on sun and shade leaves.
Journal of Biological Education 32(3):181-190.
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National Science Education Standards
Unifying Concepts and Processes:
Systems, order, and organization
Evidence, models, and explanation
Constancy, change,
and measurements
Evolution and equilibrium
Form and function
Science as Inquiry (5-8, 9-12):
Abilities necessary to do scientific inquiry
Understanding about Scientific Inquiry
Life Science (5-8):
Life Science (9-12):
Populations of organisms can be
categorized by the function they
serve in an ecosystem. Plants and
some micro-organisms are producers-they make their own food. All
animals, including humans, are
consumers, which obtain food by
eating other organisms.
Decomposers, primarily bacteria and
fungi, are consumers that use waste
materials and dead organisms for
food. Food webs identify the
relationships among producers,
consumers, and decomposers in an
ecosystem.
Plant cells contain chloroplasts,
the site of photosynthesis. Plants
and many microorganisms use solar
energy to combine molecules of
carbon dioxide and water into
complex, energy rich organic
compounds and release oxygen to the
environment. This process of
photosynthesis provides a vital
connection between the sun and the
energy needs of living systems.
For ecosystems, the major source of
energy is sunlight. Energy entering
ecosystems as sunlight is
transferred by producers into
chemical energy through
photosynthesis. That energy then
passes from organism to organism in
food webs.
Science and Technology
Energy flows through ecosystems in
one direction, from photosynthetic
organisms to herbivores to
carnivores and decomposers.
The energy for life primarily
derives from the sun. Plants
capture energy by absorbing light
and using it to form strong
(covalent) chemical bonds between
the atoms of carbon-containing
(organic) molecules. These
molecules can be used to assemble
larger molecules with biological
activity (including proteins, DNA,
sugars, and fats). In addition, the
energy stored in bonds between the
atoms (chemical energy) can be used
as sources of energy for life
processes.
(5-8, 9-12):
Abilities of technological design
about science and technology
History and Nature of Science
endeavor
(5-8, 9-12):
Understandings
Science as a human
(5-8) Nature of Science
Nature of scientific knowledge
History of Science
Historical perspectives
(9-12)
See the complete NRC standards for content, teaching, and
assessment available online at
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http://www.nap.edu/readingroom/books/nses/overview.html content
See also the AAAS Benchmarks for Scientific Literacy
available online at
http://www.project2061.org/publications/bsl/online/bolintro
.htm
Credits
Marshall Sundberg,
Ph.D.
Valdine Mclean, M.S.
Claire Hemingway, Ph.D
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